Ammonium sulfate (AS) particles are widely used for studying the
physical–chemistry processes of aerosols and for instrument calibrations.
Small quantities of organic matter can greatly influence the studied
properties, as observed by many laboratory studies. In this work,
monodisperse particles (200–500 nm aerodynamic diameter) were
generated by nebulizing various AS solutions and organic impurities were
quantified relative to sulfate using a high-resolution time-of-flight
aerosol mass spectrometer (HR-ToF-AMS). The organic content found in AS
solutions was also tentatively identified using a liquid
chromatography–tandem mass spectrometer (LC–MS). The results from both
analytical techniques were consistent and demonstrated that the organic
impurities contained oxygen, nitrogen, and/or sulfur, their molecular masses ranged from m/z 69 to 420, and they likely originate from the commercial AS
crystals. For AS particle sizes ranging from 200 to 500 nm, the total
mass fraction of organic compounds (relative to sulfate) ranged from 3.8 % to 1.5 %, respectively. An inorganic–organic mixture model suggested that the organic impurities were coated on the AS particle with a surface density of 1.1 × 10-3 g m-2. A series of tests were performed to remove the organic content (using pure N2 in the flow, ultrapure water in the solutions, and very high AS quality), showing that at least 40 % of the organic impurities could be removed. In conclusion, it is recommended
to use AS seeds with caution, especially when small particles are used, in
terms of AS purity and water purity when aqueous solutions are used for
atomization.
Introduction
Atmospheric aerosols are generally a complex mixture of inorganic and
organic compounds that have a strong impact on climate and human health
(IPCC, 2013; Pöschl and Shiraiwa, 2015). According to
the annual aerosol emission inventories, inorganic compounds account for the
majority of the mass (Andreae and Rosenfeld, 2008). Among them, ammonium sulfate (AS) is considered as one of the dominant components (Charlson et
al., 1992; Seinfeld et al., 2016). Because AS plays important roles in
physical and chemical atmospheric processes, it has been extensively used in
laboratory experiments to understand and reproduce these processes. In the
past 20 years, more than 200 articles have been published using AS as (seed)
particles for the study of optical properties, hygroscopic properties, phase
transition and viscosity, as well as chemical reactivity of aerosols (see
the detailed references in Sect. S1 in the Supplement). In all these studies, it was shown that the presence of organic matter, even at very low concentrations in AS particles, greatly influences these properties.
Among these studies, the study of aerosol hygroscopic properties represents the
major contribution (115 papers out of the 219 cited in Sect. S1). Ammonium sulfate aerosols are very often chosen as seed particles to study hygroscopic behavior of mixed organic–inorganic aerosols. Scanning various conditions of temperature and relative humidity (RH), hygroscopicity properties of aerosols were
investigated via measurements of hygroscopic growth, cloud condensation nuclei
(CCN) activity, and ice nuclei (IN) activity. It is well established that the
hygroscopic parameter kappa (κ) of pure AS particles is 0.53 [0.33–0.72] and 0.61 according to the growth factor derivation and the CCN
derivation, respectively (Clegg
et al., 1998; Koehler et al., 2006; Petters and Kreidenweis, 2007).
Laboratory studies show that the hygroscopic behavior of most water soluble
inorganic mixed aerosols is additive in nature, i.e., following the ZSR
(Zdanovskii, Stokes, and Robinson) assumption (Stokes and
Robinson, 1966). However, when organic compounds are present in AS aerosols,
their hygroscopic properties are more complex. Firstly, for water soluble
organic compounds such as short carbon chain (di)carboxylic acids, their
effects on AS aerosols are represented by their hygroscopicity and mass
fraction suggested by the ZSR assumption (Abbatt
et al., 2005; Brooks et al., 2004; Hämeri et al., 2002; Prenni et al.,
2003). Secondly, for less soluble compounds and complex mixtures, their
solubility significantly influences the hygroscopic behavior of AS aerosol.
At sub-saturation conditions, secondary organic aerosol (SOA) formed on AS
seed particles from α-pinene photo-oxidation lowers the hygroscopic
growth factor (HGF) of AS (Meyer
et al., 2009). Specifically, at RH below the deliquescence point, AS seeded
SOA do not follow the ZSR predictions because the solutions are highly
concentrated and thus non-ideal. When the insoluble organic compounds are
the dominant components of the aerosol, the water uptake on organic AS
particles is significantly slowed down and requires a longer residence time
to achieve thermodynamic equilibrium (Sjogren et al., 2007). The same
behavior has also been observed at super-saturation conditions, i.e., the
thick coating of insoluble organics compounds, such as stearic acids, act as
a shield preventing the interaction of AS and water, thus suppressing AS
hygroscopicity (Abbatt et al., 2005). Thirdly,
it was found that organic compounds could affect the hygroscopicity of AS by
lowering the surface tension, such as marine organic compounds at low
concentrations (Moore et al., 2008), or ozonolysis products of monoterpenes
(King et al., 2009; Wex et al., 2009; Engelhart et al., 2008). It has been shown that atmospheric surfactants could reduce the aerosol surface tension by a factor of 2 compared with water surface tension (72 mN m-1)
(Gérard
et al., 2019; Nozière et al., 2014; Sorjamaa et al., 2004). The presence
of small amounts of surface-active organic compounds may reduce the aerosol
surface tension, affecting its hygroscopic properties: according to some
models, a 10 % reduction in the surface tension results in a 30 %
increase in the hygroscopic parameter κ (Ovadnevaite et al., 2017; Petters and Kreidenweis, 2013). More recently, several models have shown that the inferred behavior of the surface tension strongly depends on the selected
modeling approach (Prisle,
2021; Vepsäläinen et al., 2022). In conclusion, organic compounds
can have a very complex impact on the hygroscopic growth and CCN activity of
aerosol particles which may go beyond the simple reduction of the value of a
single parameter in the Köhler equation, and numerous models are
currently under development on this issue.
Ammonium sulfate has also been widely used to understand the phase transition of
organic–inorganic compounds in single particles studies (49 papers out of
the 219 cited in Sect. S1). Under sub-saturated conditions, deliquescence of pure AS occurs at ∼ 80 % RH and efflorescence at ∼ 34 % RH. However, these phase transition behaviors of AS particles are
significantly influenced by organic compounds. For example, the presence of
humic acids decreases the deliquescence RH and increases the efflorescence
RH of AS aerosols (Badger et al., 2006). Furthermore, the presence of malonic acid on AS particle leads to a two-step deliquescence instead of a single step in the hygroscopic growth (Treuel et
al., 2009). Differently to the shift in deliquescence RH and/or
efflorescence RH, the presence of SOA completely removes the clear phase
transition of AS on the particle: this was clearly shown for SOA derived
from cycloalkene photolysis and monoterpenes photo-oxidation (Varutbangkul
et al., 2006). The change in phase transition behavior of AS particles also
depends on the quantity of organic compounds. During the hygroscopic growth,
for small organic or inorganic ratios (< 20 %), there is a
bulk-to-surface partitioning with a part of the organic material in
the solution, and another part as a film coated at the surface of the droplet
(Nandy and Dutcher, 2018; Smith et al., 2013), while high organic or inorganic ratios (> 80 %) induce liquid–liquid phase separation (Smith
et al., 2013; Saukko et al., 2015).
Due to their hygroscopic properties, AS aerosols easily provide an aqueous
environment, where reactions between NH4+, SO42-, and
water-soluble organic compounds can play an important role in the formation
of secondary aerosols (62 papers among the 219 cited in Sect. S1). Some of these reactions may explain the source of the so-called brown carbon, thus affecting the optical properties of particles (44 papers among the 219 cited in Sect. S1). Ammonium cations (NH4+) are in a pH-dependent equilibrium
with dissolved ammonia in the aqueous phase. NH3 can react with
carbonyl compounds such as glyoxal, methylglyoxal, glycolaldehyde,
hydroxyacetone, biacetyl, or unsaturated dialdehydes in Maillard-type
browning reactions to form light absorbing and oligomeric compounds such as
imidazoles or pyrazine-based compounds (Hensley
et al., 2021; Grace et al., 2020; Hawkins et al., 2018; Laskin et al., 2014;
Kampf et al., 2012). These reactions are of particular interest for the
atmosphere, as they have an impact on both health and climate. Their
aqueous-phase processes represent an important and rapid source of brown
carbon (Powelson
et al., 2014; De Haan et al., 2017; Jimenez et al., 2022). In addition,
SO42- is not inert in atmospheric water. Sulfate anions can react
with ⚫OH radicals forming sulfate radicals (SO4⚫-), an important atmospheric oxidant (Herrmann, 2003). Besides, sulfate radicals contribute to the formation of organosulfates (Nozière
et al., 2010; Brüggemann et al., 2020; Wach et al., 2019; Szmigielski,
2016). Organosulfates are ubiquitous compounds in SOA with important
implications in their physicochemical properties
(Shakya and Peltier, 2015). Other chemical pathways that lead to the formation of organosulfates might involve directly sulfate anions, i.e., their nucleophilic substitution to epoxides or tertiary nitrates (Hu et al., 2011; Darer et al., 2011), and the acid-catalyzed esterification of alcohol groups (Surratt et al., 2008; Linuma et al., 2007).
Due to the atmospheric representativity of AS particles and their well-known
physical and chemical properties, they are often chosen as a reference in
aerosol studies, as mentioned above, and for instrumentation calibration.
The presence of trace organic compounds in AS particles may induce
potentially important artifacts on the experimental results of physical and
chemical processes. Few laboratory studies show that organic compounds can
be present in synthetic AS particles. For example, 0.8 wt % of C was
observed in solutions where only ammonium sulfate was present as a solute in
the study of phase transitions of AS (Badger et al., 2006).
In another example, in a study of glyoxal uptake on AS particles monitored by
an aerosol mass spectrometer, Trainic
et al. (2011) found organic fragments on pure AS aerosols (purity not
mentioned), with a ratio of organic matter to sulfate (hereafter named [Org] / [Sulfate]) as high as 8 %. Scanning the
219 articles (mentioned in Sect. S1 in the Supplement) published over the past 20 years using AS aerosols in the laboratory, it appears that neither quantification nor
identification of these potential organic impurities were reported.
Furthermore, the majority of these studies (63 %) did not mention the
origin and purity of the AS used. As organic traces may significantly influence
the properties of AS particles, and thus bias experimental results, the
objectives of this work were to quantify organic traces in commercial AS
under conditions used in laboratory experiments, and when possible,
tentative identification was performed. Finally, recommendations are given
for purity improvements.
Method
Ammonium sulfate aerosol particles were generated by atomization of AS solutions at
concentrations ranging from 0.01 to 0.5 M. Ammonium sulfate aerosols were selected at four
sub-micrometer sizes to test the effect of particle size on the organic
content. This procedure allowed us to form quasi-monodisperse AS particles
at mass concentrations ranging from 5 to 30 µg m-3,
concentrations typically used for aerosol generation and/or calibration in
the literature. Their chemical composition was quantified online by a
high-resolution time-of-flight aerosol mass spectrometer (HR-ToF-AMS).
Additionally, aqueous solutions of AS were analyzed by liquid
chromatography–tandem mass spectrometry (LC–MS) to tentatively identify
organic content. All chemical compounds used in this work are listed in Sect. S2 in the Supplement.
Detection of organic traces in AS aerosol particlesAerosol generation and size classification
The experimental setup for quantifying organic content is
shown in Fig. 1. Aqueous solutions of AS (0.01–0.5 M) were nebulized by
atomization (using a 3076 atomizer, TSI) that generated droplets with
compressed air at 1 bar above atmospheric pressure and a flow rate of 1.8 L min-1. As a comparison with compressed air, pure N2 (Linde gas, 99.999 %) was used. The resulting droplets were dried by a Nafion™
dryer at relative humidity (RH) below 25 %, i.e., below the efflorescence
of AS aerosol. Dry AS particles then passed through a dilution system aimed
at controlling AS particle concentrations. It consisted of two parallel
pathways, one of which was connected to a HEPA filter. An aerodynamic
aerosol classifier (AAC, Cambustion) and an electrostatic classifier (3080, TSI) combined with a differential mobility analyzer (3081 long DMA) were
alternatively used to select monodisperse particles. The ratio of air sheath flow to sample flow was fixed at 10 for the two classifiers. To regulate the flow rate
through the AAC classifier, a mass flow controller (MFC) was connected to a
vacuum pump. Under this configuration, the sample flow through the AAC
varied from 0.4 to 1.4 L min-1, while the tests performed with the DMA
classifier were only done at one flow rate (0.4 L min-1). This setup
allowed the characterization of the aerosol particles by their sizes, as
well as their organic content as a function of particle sizes with respect
to the total mass of AS particles selected.
Experimental setup for the quantification of organic content in
monodisperse ammonium sulfate particles.
Experimental conditions for the investigation of AS aerosol particles
A series of experiments (Table 1) were performed to
quantify organic content on AS aerosol particles by scanning various
conditions of mass concentrations, particle size, and generation procedures.
In this work, four different particle diameters were used: the mobility
diameter (dm), the aerodynamic diameter (da), and the vacuum aerodynamic diameter (dva) were given by the SMPS, the AAC, and the AMS measurements, respectively; while the volume equivalent diameter
(dve) was used to determine the surface of AS particle. The relations between these diameters were described by DeCarlo et al. (2004). As organic compounds can be homogenously mixed in AS aerosol or remain at the particle surface inducing different behavior as a function of size (Jimenez
et al., 2009; Tervahattu et al., 2002), monodisperse AS particles were
selected with aerodynamic size (da) varied from 200 to 500 nm in each experiment. In experiment P1 (EXP P1), AS crystals (99.5 %, for analysis, from Acros Organics™ Fisher Scientific) were dissolved in Milli-Q water (18.2 MΩ cm, TOC < 2 ppb) at various concentrations (0.01–0.5 M). In EXP P2–P7, the aqueous phase
concentration of AS was fixed at the highest value (0.5 M) to study the
influence of other parameters. In EXP P2, the influence of the water quality
was tested. In EXP P3–P5, the influence of the purity of AS crystals was
tested. In EXP P3, AS crystals (99.5 %, for analysis, from Acros
Organics™ Fisher Scientific) were tentatively purified by an
accelerated solvent extractor (ASE 300, Dionex) using acetonitrile as
the extraction solvent. In EXP P4, AS crystals (99.5 %, for analysis, from
Acros Organics™ Fisher Scientific) were tentatively purified by recrystallization in Milli-Q water: AS crystals (20 g) were dissolved into boiling water (20 mL) reaching a concentration of 7.5 M and then
recrystallized by smooth cooling at room temperature. In EXP P5, high purity
AS (99.9999 % Suprapur® from Merck) was used. In EXP P6, the influence of the particle size selector was studied by intercomparison between the DMA and the AAC. Developed by
Tavakoli and Olfert (2013), the AAC is a
recent commercial aerosol classifier which selects the particle size by
centrifugal force instead of electrostatic force which is used in the DMA.
In the AAC, the aerodynamic diameter of selected particles is related to the
rotational speed of the concentric cylinder, the sheath flow rate, and the
sample flow rate. Rotational speed has been reported to influence the
geometric standard deviation (σgeo) of size distribution, i.e., the higher the rotational speed, the smaller the size and the larger the σgeo (Johnson
et al., 2018). In EXP P7, the effect of the rotational speed of the
concentric cylinder in the AAC was tested while maintaining a constant size
selection by regulating the MFC (Fig. 1), so that the sample flow through
the AAC varied from 0.4 to 1.4 L min-1.
Experiments to quantify organic traces on AS aerosol particles
under various conditions: 1 AS 99.5 %, for analysis, from Acros Organics™ Fisher Scientific; 2 purification by solvent (acetonitrile) extraction; 3 purification by recrystallization; 4 AS 99.9999 % Suprapur® from Merck. 5 Milli-Q water, 18.2 MΩ cm, TOC < 2 ppb; and 6 Fisher Chemical, LC–MS grade. Under each condition, duplicate experiments were performed for each selected particle size. The four sizes (da= 200, 300, 400, and 500 nm) selected with the AAC classifier correspond to the mobility sizes (dm= 130, 201, 269, and 336 nm).
ExperimentAS purity (%)WaterAS liquid concentration (M)Particle size (nm)Size selection by the AAC (sample flow = 0.4 L min-1) P199.51Milli-Q50.01, 0.02, 0.05, 0.1, 0.2, 0.5da= 200, 300, 400, 500P299.51Fisher60.5da= 200, 300, 400, 500P399.51 purified2Fisher60.5da= 200, 300, 400, 500P499.51 recryst3Fisher60.5da= 200, 300, 400, 500P599.99994Fisher60.5da= 200, 300, 400, 500Size selection by the DMA (sample flow = 0.4 L min-1) P699.51Milli-Q50.5dm= 122, 188, 250, 311, 375Size selection by the AAC (sample flow = 0.5, 1.0, 1.4 L min-1, corresponding to a rotation speed of 190, 285, and 369 rad s-1) P799.51Milli-Q50.5da= 300Effect of gas supplier (pure N2 instead of compressed air) P899.51Milli-Q50.5da= 200, 300, 400, 500Characterization of aerosol particles
The particles number size distribution was measured with a scanning mobility
particle sizer (SMPS) consisting of a DMA (3080, TSI) coupled with an
ultrafine condensation particle counter (CPC, 3776, TSI). A high-resolution
time-of-flight aerosol mass spectrometer (HR-ToF-AMS, Aerodyne Research) was
used to measure the bulk chemical composition of non-refractory sub-micron
particulate matter (DeCarlo et al., 2006). The
instrument was used under standard conditions (standard vaporizer at 600–650 ∘C and electron ionization at 70 eV) in V mode and in p-ToF mode. Each measurement point was averaged for an MS cycle of 1 min and a p-ToF cycle of 30 s. Calibrations using pure and dried particles of ammonium
nitrate and ammonium sulfate with a mobility diameter of 350 nm were carried
out every few days of operation to determine the ionization efficiency of
nitrate, ammonium, and sulfate named as IENO3, RIENH4, and RIESO4. The RIENH4 and RIESO4 values were related to
nitrate and were estimated experimentally as 3.3 and 1.8, respectively. The
standard recommended value for RIEorg of 1.4 was used for organic
compounds. Calibration in the p-ToF mode was carried out using pure ammonium
nitrate in the size range of 80–500 nm mobility diameter. The data treatment
has been performed with AMS Analysis Toolkit 1.63 and PIKA 1.23 under the
software Igor Pro 6.37. The selection of ions to fit in PIKA was derived
from the mass spectra produced by AS aerosols at da= 200 nm in EXP P1 and was rechecked in each experiment. To avoid any overestimation of the organic fraction, CHO+ and CO2+ fragments (at m/z 29 and 44) were corrected from the remaining gas phase in the AMS (Aiken
et al., 2007; Canagaratna et al., 2015). In addition, due to the very high
signals of the fragments NHx and SOx, other fragments located at
the tail of these main peaks (such as CH3+, CH4+,
CH2N+, C2H4+ …) were excluded from
the calculation of the total organic content to avoid any overestimation
of the organic fraction.
Seeking for organic traces in AS aqueous solutions
Experiments of characterization of organic traces in AS liquid
solutions. The elution gradient and ESI setup were the same for all experiments: 1 AS 99.5 %, for analysis, from Acros Organics™ Fisher Scientific; 2 AS 99.5 %
EMSURE® from Merck; 3 eluents: A = H2O with 0.1 % formic acid, and B as specified; 4 acetonitrile, 5 methanol, 6 methyl tert-butyl ether, 7 dichloromethane; L–L: liquid–liquid, S–L: solid–liquid.
Although the organic purity is not mentioned on commercial crystals (only
the inorganic content is indicated), two different brands of AS of the same
purity were analyzed to seek for potential organic traces and for comparison
purposes: one from Acros Organics™ Fisher Scientific (99.5 %, for analysis) that has been widely used in the literature, and EMSURE® from Merck (99.5 %, for analysis). The characterization experiments are shown in Table 2;
each one was systematically complemented by blank experiments, i.e.,
analysis of the water used for each AS solution.
In EXP C1 and C2, aqueous solutions of AS were injected into the LC–MS, and
two elution solvents were tested to optimize the chromatographic separation.
In EXP C3–C10, organic compounds were extracted from AS crystals by
different methods (solid–liquid and liquid–liquid extraction) using various
solvents, and pre-concentrated prior LC–MS analysis using both positive and
negative modes. For these extractions, an accelerated solvent extractor (ASE 300, Dionex) system was used under the following conditions: acetonitrile was the extraction solvent, the oven was set at 100 ∘C under 100 bars, the heat-up time and static time were 5 min each, and three extraction cycles were conducted. As an extension of EXP C6, EXP C9 was designed to perform MS/MS analysis using very high concentrations of AS to optimize the identification of organic traces, and analyses were performed in the positive mode only. For comparison purposes, another brand of AS of the same purity (99.5 %) was used in EXP C8 and C10 using two extraction methods.
Results and discussionOrganic content on AS aerosol particles
A significant quantity of organic matter was directly observed by the
HR-ToF-AMS in EXP P1. As an example, Fig. 2 shows the mass concentrations of total organic matter and sulfate and their ratios as a function of time during one size cycle performed in EXP P1 at AS concentration of 0.01 M. Figure 2 also provides background signals obtained with the two pure waters of different qualities used in EXP P1 and P2. In these background signals, the total mass concentrations of organic compounds and sulfate are 0.018 ± 0.009 and 0.002 ± 0.001 µg m-3, respectively, similar to the detection limits of the HR-ToF-AMS in the V mode
(DeCarlo et al., 2006). In the presence of AS, a
significant quantity of organic compounds was observed with concentrations
ranging from 0.15 to 0.33 µg m-3 when the aerodynamic size of the
particle varied from 500 to 200 nm, with a very good reproducibility in
the duplicate experiments, at each size. Furthermore, the evolution of the
mass ratio between total organic compounds and sulfate marked as
[Org] / [Sulfate] (Fig. 2) shows a clear correlation with particle size: this
ratio increases when the particle size decreases. The same observations were
obtained at all other AS concentrations investigated in EXP P1 and in the
other experiments. It was checked in EXP P8 that when replacing compressed air by
pure N2 (Linde Gas, 99.999 %), no significant differences were
observed from EXP P1 under the same conditions (Sect. S3 in the Supplement). Overall, it was
observed that [Org] / [Sulfate] varied from 1.5 % to 3.8 %, in the range of the values reported by studies mentioning the presence of organic compounds in laboratory experiments on AS particles (0.8 wt % of C in Badger et al., 2006, and up to 8 % in Trainic et al., 2011).
Mass concentrations of total organic matter, sulfate, and [Org] / [Sulfate] ratio as a function of time during one cycle performed in EXP P1 at an AS concentration of 0.01 M. Background signal obtained with pure water in EXP P1 and P2 is also shown for quality check and for comparison between the two types of water with different purity.
Unit-mass resolution spectra (averaged over 12 measurements,
converted from the high-resolution data) of AS aerosol particles (da= 200 nm) in EXP P1, performed at an AS concentration of 0.5 M.
The total mass concentration of organic compounds was calculated based on
all organic fragments, as described in the previous Sect. 2.1.3. Figure 3
shows the unit-mass resolution spectrum of AS aerosols selected by the AAC
at da= 200 nm.
SOx and NHx families represent sulfate and ammonium, respectively.
All the other fragments are between 1 and 3 orders of magnitude lower, but
detectable signals span for m/z up to 100. The sum of the signals of
NO2+ and NO+ fragments represent 30 % of the total organic
signal (Fig. S4-1 in the Supplement). To figure out the source of this non-negligible nitrate
signal, the AMS signal NO2+/ NO+ ratio was investigated and
showed significantly lower values in AS aerosol (0.46 in EXP1) compared with
ammonium nitrate particles used for calibration (1.18) (Fig. S4-2 in the Supplement). It is
thus suggested that the observed nitrate signal in AS aerosol is due to the
presence of organic nitrates (Kiendler-Scharr
et al., 2016). Concerning organic fragments, significant signals were
observed for fragments CxHy+ at m/z 27, 39, 41, 43, 55, 57,
67, and 69, for CxHyO+ at m/z of 28 and 30, for
CxHyO2+ at m/z 44, and for CxHyN+ at m/z 30,
58, 72, and 86. (Raw spectra of CHN fragments are shown in Sect. S5 in the Supplement.) The potential “Pieber effect” was also investigated to elucidate any positive artifact on the determined quantities of organic compounds. Indeed,
Pieber et al. (2016)
showed that inorganic salts like AS or ammonium nitrate can trigger the
CO2+ signal (m/z 44) due to reactions on the vaporizer of the AMS
instrument. In the present study, the observed CO2+ signal
represented 20 ± 5 % of the total organic signal. Thus, if all the
CO2+ fragments came from the interference signal of sulfate, the
maximum Pieber effect in this work would be 0.76 % relative to sulfate.
It was thus not considered in the results. The rest of the organic fragments
seem to represent organic molecules bearing various functionalities,
potentially comprising oxygen and nitrogen. Before further identification of
these compounds, Sects. 3.2 and 3.3 present the results and discussions
on the influence of the particle size and liquid AS concentrations on the
organic content.
The role of particle size and liquid AS concentrations on the quantity of organic content
Figure 4 shows [Org] / [Sulfate] measured in AS
aerosols at various nebulized solution concentrations as a function of
particle size. While no significant influence of AS aqueous phase
concentration has been found (within experimental uncertainty), a
significant influence of particle size is clearly observed on [Org] / [Sulfate].
[Org] / [Sulfate] in AS aerosols versus particle mobility size (dm) during EXP P1 scanning the six nebulized AS concentrations (0.01–0.5 M). The colored dots (with error bars) are the experimental data, while the red and black lines represent the calculated [Org] / [Sulfate] considering either surface coating of organic matter (in red from Eq. 3) or internal mixing (in black from Eq. 4).
To further understand how the organic matter was mixed with AS aerosols, two
series of calculations were performed, following two hypotheses on the
organic–inorganic mixing: (i) organic coating on the surface of monodisperse
AS particles, and (ii) internal mixing. To achieve this calculation, a
thorough study of the morphology of AS particles was performed using the
various size measurements (Appendix A). For monodisperse AS particles
selected by the AAC at a required aerodynamic diameter (da), its
mobility diameter (dm) and vacuum aerodynamic diameter (dva) were measured by SMPS and HR-ToF-AMS, respectively.
The calculations of [Org] / [Sulfate] using each of these two
hypotheses are detailed hereafter:
Hypothesis 1: AS aerosols were coated by organic compounds with a size-independent surface density ρorg,S (in g m-2).
For a non-spherical particle, the total surface of the particle was considered as
the surface of the volume equivalent particle. According to
DeCarlo et al. (2004), the relation between the volume
equivalent diameter (dve) and dais described as Eq. (1):da=dveρpρ01χCcdveCcda,where χ is the shape factor of aerosol particles and Cc is the Cunningham slip factor (Kim
et al., 2005), given by Eq. (2):CcKn=1+Kn1.165+0.483exp-0.997Kn.Kn(d)=2λg/d is the Knudsen number, and λg is the gas mean free path. In this
work, da was given by the AAC; χ was determined by dm and dva (Appendix A). Normal temperature and pressure (293.15 K at 1 atm) were applied for the calculation of dve. The surface of AS particles was estimated from the value of dve. Therefore, the prediction of surface coated organic compounds compared with sulfate mass is described in Eq. (3):
[Org]Sulfate=ρorg,SSveVveρASMSO4MAS=32ρorg,SdveρASMSO4MAS×100%,where Sve and Vve are the surface and volume of the volume equivalent AS particle, respectively. MSO4 and MAS are molar mass of SO42- and (NH4)2SO4, respectively. [Org] and
[Sulfate] are the mass concentrations of organic compounds and of sulfate in the particulate phase. Equation (3) shows that, in this case, [Org] / [Sulfate] decreases when the particle size increases.
Hypothesis 2: Organic compounds are homogeneously mixed in AS aerosols with a density ρorg,V (in g m-3).
In this case, the mass concentration of organic compounds compared with
sulfate is described by Eq. (4):[Org][Sulfate]=Vρorg,VVρASMSO4MAS=ρorg,VρASMSO4MAS×100%,where V is the total volume of the studied aerosol particle. In this case, [Org] / [Sulfate] is independent of the particle size.
The results of these two calculations are shown in Fig. 4 together with the
experimental results. The comparison clearly shows that the organic
compounds coat homogenously on the surface of AS particles with a surface
density of 1.1 × 10-3 g m-2. In addition, this result shows that the Nafion™ dryer was not efficient in removing these organic compounds, probably due to their low volatility.
Influence of AAC and DMA on the organic content
Particle size selection is important in aerosol science especially when
particle size influences the properties studied. In this work, both AAC and
DMA were used to provide monodisperse AS aerosols. As a recent commercial
instrument, AAC was used to select AS aerosols in most of the experiments.
The effects of the concentric cylinder rotation speed of the AAC were tested
in EXP P7. The results (Sect. S6 in the Supplement) demonstrate that the rotation speed does not affect [Org] / [Sulfate]. While the AAC selects particles using
centrifugal force, the DMA selects particles by their electromobility. In
the latter, the selected distribution is thus not exclusively monodispersed
as it contains double- and triple-charged particles at the corresponding
sizes. Correction of the multi-charge effect was successfully applied and is routine in aerosol size distribution determination
(Petters, 2018; Wiedensohler et al., 2012). However, Fig. 4
shows that the larger the particle, the lower the content of the organic compounds.
In this case, the multi-charged particles affect the organic quantification
using the DMA and the AMS, so corrections are needed. In this
work, we compared [Org] / [Sulfate] in AS particles selected by the DMA and by the AAC, respectively (Fig. 5).
Influence of instrumental conditions and instrument
inter-comparison: AAC selection (red squares, EXP P1), DMA selection (black
squares, EXP P6), and multi-charging correction (white squares). The red line
represents the calculation simulating organic coated compounds at the AS particle surface using Eq. (3) with a surface density of 1.1 × 10-3 g m-2 (selected from Fig. 4).
In Fig. 5 the white squares represent the multi-charging corrections considering the
hypothesis that organic compounds coat the surface of AS particles as shown
in Sect. 3.2. Details of the multi-charging corrections are given in Sect. S7 in the Supplement. Briefly, the corrected [Org] / [Sulfate] by the multi-charged modes are systematically higher than the non-corrected values because the correction considers the total amounts of organic compounds and sulfate. Using the DMA selection after correction, [Org] / [Sulfate] is clearly inversely related to the mobility size and in very good agreement with those results obtained using
the AAC selection. In conclusion, the intercomparison of the two instruments
shows that the influence of the instrument is negligible, and it also shows
that the selection by AAC leads to lower uncertainties on the y axis and
therefore more accurate results.
Tentative identification of organic content
From the HR-ToF-AMS mass spectra, the three main families present in AS
particles were CxHy, CxHyO(1–2), and
CxHyNO(0–2). Figure 6 shows the
proportion of these three groups relative to total organic compounds as a
function of AS particle size. Within the uncertainties, the proportion of
the three families remains constant and independent of the particle size,
which shows the stability of the organic compounds coated on AS particles.
Mass fractions of the three main sets of organic fragments present
in AS particles as measured by the HR-ToF-AMS during EXP P1. The error bars
represent the standard deviation from averaging multiple measurements.
LC/ESI+–MS base peak chromatogram of Milli-Q water, Fisher
water, and AS (99.5 %) solution of 1.5 M (EXP C1) with corresponding major ion masses associated with raw formulas. The bold ion masses and proposed raw formula were studied in MS–MS (EXP C9). Ions displayed in boldface were detected in their [M - H]- form in LC/ESI-–MS (in EXP C6–C8).
The identification of the corresponding organic compounds was limited by the
high fragmentation due to the electron impact ionization operated in the
HR-ToF-AMS instrument. Further identification was conducted by LC–MS with
liquid AS solutions. In EXP C1 and C2, an aqueous solution of ammonium
sulfate (99.5 %) at 1.5 M was injected and analyzed in the positive mode. The acetonitrile and methanol eluents showed similar results for compound separation. Figure 7 shows a chromatogram of an
ammonium sulfate solution (green line) as well as blanks of Milli-Q water
(red line) and Fisher water (blue line). In Milli-Q water, two ions were
present at retention times of 4.45 and 4.65 min, and their mass spectra were
attributed to nylon polymers (C6H11NO)n which are among the
frequently reported interfering compounds (Tran and Doucette, 2006; Keller et
al., 2008). However, no significant contamination was observed in the Fisher
water (LC–MS Grade). Figure 7 shows that the same ions found in Milli-Q
water were also present in the AS solution prepared in the same Milli-Q
water. Many other ions were detected in the AS solution, and their retention
times highly suggested that these molecules were organic. The proposed raw
formulas systematically contained carbon, hydrogen, oxygen, nitrogen, and
sometimes sulfur, consistent with the HR-ToF-AMS spectra. The double bond
equivalence (DBE) varies from 1 to 5 showing that the molecules are
unsaturated and potentially cyclic. Different to nylon polymers found in
Milli-Q water, these raw formulas are not referenced among the common
laboratory LC–MS contaminants (Keller et al., 2008).
The mass error was always well below 5 ppm and no raw formula with a sodium
adduct was suggested. Some of the m/z showed two different retention times,
implying potentially the presence of isomers. For example, at m/z 226.1918,
two peaks were detected, one at 2.89 min and the other at 3.11 min
as described in detail in Sect. S8 in the Supplement for the most intense ions. In the negative ESI mode, the signal was overwritten by the sulfate ion (detected at
[M + H]-: 96.9596, HSO4-) along the whole chromatogram, due
to the high ammonium sulfate concentrations used.
Mass spectra obtained from LC/ESI+–MS/MS
measurements of two detected compounds in EXP C9. The [M+H]+ ions
were detected at (a)m/z 340, using a collision energy of 25 eV, and (b)m/z 227, using a collision energy of 20 eV.
For further identification and for detection in the negative mode, AS
crystals and solutions at various concentrations were extracted using
different solvents and different methods (C3–C7 in
Table 2). In the positive mode, more than 60 % of
the molecules identified without the extraction step were also detected.
Compared with other solvents, acetonitrile was found to be the most efficient
extracting solvent. In the negative mode, the signal was systematically much
lower (ion current intensity < 103 counts s-1) and only molecules displayed in boldface in Fig. 7 were detected. Consequently, no further analysis was performed in the negative mode due to the low signal. In EXP C8 and C10, another brand of AS crystals of the same purity (99.5 %) was tested for comparison. Some common ions were detected in the extracts in the positive mode but at a much lower intensity (by a factor of ∼ 20; see Sect. S9 in the Supplement), thus demonstrating that the detected organic compounds were present in both AS crystals and that the level of organic contamination depends at least in part on the AS brand.
EMSURE® products are supposed to offer a high level of quality and appear to be less contaminated despite the same reported purity. Figure 8 shows the MS–MS measurements operated on two ions detected in EXP C9 (see Sect. S10 in the Supplement for the complete MS–MS results). These
results support the proposed raw formula, confirming that these organic
compounds contained oxygen and nitrogen and/or sulfur. They also show
recurrent fragments: for instance, fragment ion at m/z 96.082, most likely
described by the raw formula C6H10N, was found in all the MS–MS
spectra as well as the fragment ion at m/z 69.072 corresponding to
C5H9 which was also a fragment observed in the spectra from the
HR-ToF-AMS with high intensity.
These results confirm that the organic compounds present in AS are large
molecules: of the 20 most intense detected molecules, 12 contained 12
carbon atoms (C12), and the others contained C6 (2 molecules
bearing CxHyN1/2), C10 (1 molecule), C11 (1 molecule), C17 (1 molecule), C18 (2 molecules), and up to
C24 (1 molecule), with m/z ranging from 96 to 533. Apart from the 2
smallest molecules (C6), all of them bear mono- or poly-functional
groups with at least 2 heteroatoms (N and O are always present, and S is
found in 10 molecules) confirming their low volatility. Most of them
fragment following the same scheme, thus showing similar structures. No
structure is proposed here as there are many possible structures. The fact
that these molecules were largely detected in the positive mode and that
they all bear at least one N atom, and the fact that they were observed
specifically at the surface of the AS aerosol particles, could suggest that
they are cationic surfactants such as quaternary ammonium salts remaining
from the manufacturing processes. Ammonium sulfate is typically produced by
the reaction of gaseous ammonia with sulfuric acid, but the precise
manufacturing processes and raw materials of the suppliers are not known in
detail and therefore do not allow us to draw any conclusions for these
processes.
Removing organic traces from AS aerosol particles
To give recommendations on the use of AS for laboratory studies or
instrument calibrations with organic contaminations as low as possible, the
results of EXP P2–P5 using high-quality water, and AS or purified AS
crystals, are shown in Fig. 9 where [Org] / [Sulfate] is plotted versus particle size.
[Org] / [Sulfate] in AS aerosol particles as a function of mobility diameter in EXP P1–P5. The red lines represent the calculated
[Org] / [Sulfate] simulating surface coating of organic matter (from Eq. 3) for EXP P1, P4, and P5.
Figure 9 shows that, although decreasing with increasing purification,
organic traces remain present in all experiments. They seem to remain coated
at the surface of the particles, as shown by the good agreement between the
simulations and experimental data. According to the simulation results, a
reduction of 20 % of [Org] / [Sulfate] is observed when Fisher water (red squares) replaces Milli-Q water (pink triangles). This reduction might be due to the removal of nylon polymers found in Milli-Q water (Fig. 7). However, no significant difference is observed between EXP P2, P3, and P4, i.e., purification of AS crystals by acetonitrile or recrystallization is not efficient enough to remove organic traces. Finally, comparing EXP P5 (cyan triangles) and P2 (red squares), another 20 % reduction of organic
traces is observed when highly pure AS crystals are used (99.9999 %). In
this case, the organic traces are lower than 2.5 % on the AS particles
with dm= 130 to 336 nm (corresponding to da= 200 to 500 nm).
Conclusions and implications
Ammonium sulfate is one of the dominant components of atmospheric aerosol
and is widely used in laboratory experiments and for instrument calibration
purposes. However, the widely used commercial AS crystals may contain
interfering organic impurities. To answer questions related to the quantity
and the quality of organic impurities found in widely used commercial AS
products, a series of experiments were performed to quantify and identify
organic impurities on AS aerosol particles and in liquid solutions using an
HR-ToF-AMS and LC–MS. The results showed that, using 99.5 % purity AS,
up to 3.8 % of organic impurities were present related to sulfate mass.
This ratio was found to be independent of the concentration of nebulized AS
solutions but was inversely related to the particle size. The simulations of
AS organic mixtures showed a homogeneous organic coating on the surface of
AS particles with a constant surface density of 1.1 × 10-3 g m-2. Regarding the particle size selection system, the comparison between AAC and DMA showed consistent [Org] / [Sulfate], thus highlighting that the observed particle size effects were not due to
instrumental artifacts. Using LC–MS analysis of the organic content showed
up to 20 different stable, highly functionalized molecules with nitrate,
amine, and/or sulfate groups. The proposed raw formulas included mostly
C12 compounds with mono- or poly-functional groups with at least two
heteroatoms (N and O were always present, and S was found in 10 molecules)
suggesting low volatility. These nitrogenous molecules were abundantly
detected by LC–MS in AS solutions in the positive ESI mode, and in the
nebulized solutions, they were observed to be coated on the surface of AS
aerosol particles. Thus, it could be suggested that they might comprise
cationic surfactants such as quaternary ammonium salts remaining from the
manufacturing processes.
From this suggestion, the potential effects of organic impurities on CCN
activation of AS aerosols were investigated to determine the error made on
the critical supersaturation if one assumes 100 % AS, omitting the
presence of organic impurities. This estimation was performed with a simple
calculation using the κ-Köhler equation
(Petters and Kreidenweis, 2007) (see Sect. S11 in the Supplement
for details). For the estimation of the critical supersaturation in the
presence of organic impurities, two extreme hypotheses were explored. In
hypothesis 1, the organic fraction was considered soluble and
non-surface-active, whereas in hypothesis 2, it was considered extremely
surface-active, using the most powerful surfactant as a proxy. Following
each of these hypotheses, the supersaturation was calculated along the
droplet activation of an AS particle of 130 nm with and without organic
impurities (Fig. S11 in the Supplement). The results show that whereas the critical
supersaturation of AS aerosols is not significantly impacted by the presence
of organic impurities under hypothesis 1, it is highly impacted under
hypothesis 2, with a potential error of more than 70 %.
In view of these extreme results, and especially the very important error
observed for surface-active compounds, the potential quantity of
surface-active species contained in AS solutions (ammonium sulfate from Acros Organics, 99.5 % diluted
in Milli-Q water) was investigated (Sect. S12 in the Supplement). The results showed that AS solutions contain extremely low amounts of cationic and non-ionic surfactants, and an upper limit of [Org surfactant] / [Sulfate] mass ratio of 1 × 10-5 % was determined. Although this ratio is five
orders of magnitude lower than the total organic fraction detected in AS
particles, the potential effect of these surfactants on the surface tension
of 130 nm diameter AS particles was investigated. For such particles, the
obtained [Org surfactant] / [Sulfate] mass ratio induces a concentration of 6 × 10-7 mol L-1 of surfactants. At this very low
concentration, even the most surface-active molecules show a surface tension
similar to that of pure water as shown by surface tension isotherms
(Ekström et al., 2010; Frossard et al., 2019; Arabadzhieva et al., 2020). It was thus concluded that the CCN activity of AS particles with dm= 130 nm should not be significantly affected by the presence of the organic impurities. However, caution should be taken to keep their amounts as low as possible.
In this work, some efforts to remove these organic impurities have been
tentatively tested by purifying and recrystallizing AS crystals. Though no
significant difference was observed, it is likely that better results could
be obtained by increasing the number of purification and recrystallization
cycles, as very high purity AS crystals (99.9999 %) showed significantly
lower organic content. It is therefore recommended to use AS seeds with
caution, especially when small particles are used, in terms of AS purity and
water purity when aqueous solutions are used for atomization.
Morphological properties of AS aerosols
Following the method of Zelenyuk et al. (2006), the shape factor (χ) in any flow regime (continuous and transient) can be determined by the ratio of dva/dm in Eq. (A1):
dvadm=ρpρ01χ2Ccdvaχρp/ρ0Ccdm,
where ρp is the particle density, ρ0 is the standard
density noticed as 1 g cm-3, and Cc is the Cunningham slip factor (Kim et al., 2005), given by Eq. (A2):
CcKn=1+Kn1.165+0.483exp-0.997Kn.Kn(d)=2λg/d is the Knudsen number, and λg is the gas mean free path. In this
work, normal temperature and pressure (293.15 K at 1 atm) were applied for
the calculation of the shape factor of AS aerosols.
The shape factor (χ) of monodisperse AS aerosols selected by the AAC
was calculated according to Eq. (A1) and is shown in Table A1, together with
the aerodynamic diameter (da) selected in the AAC, the mobility diameter dm, and the vacuum aerodynamic diameter dva representing the
average mode of size distributions given by the SMPS and HR-ToF-AMS (p-ToF
mode), respectively. Table A1 shows that, within the uncertainty, the
selected AS particles own a size-independent shape factor of 1.06, which
falls within the interval of 1.03 and 1.07 reported by Zelenyuk et al. (2006).
Morphological properties of monodisperse AS aerosols selected by
the AAC (in EXP P1 and P2): aerodynamic diameter (da), mobility diameter (dm), vacuum aerodynamic diameter (dva), and shape factor (χ). The uncertainty represents the standard deviation of several measurements under the same selection criteria.
The data that are presented in this work are publicly available in the Zenodo public data repository at 10.5281/zenodo.6559283 (Wu et al., 2022).
The supplement related to this article is available online at: https://doi.org/10.5194/amt-15-3859-2022-supplement.
Author contributions
JW and AM provided the initial idea for this work. JW and BTR performed the
experiments and data analysis of AS aerosol characterization with the
HR-ToF-AMS. NB and SR carried out the measurements of AS solutions with the
LC–MS. JW, NB, and JLC proposed different purification processes of AS
aerosols. The organic–inorganic mixing model was provided firstly by JW and
improved by BR'M and AM. JW, NB, JMGS, and AM developed the structure of this
paper. JW summarized all contributions and expressed them in this paper. All
authors provided advice regarding improvements to this paper as well as to
the writing of the final version of the manuscript.
Competing interests
The contact author has declared that neither they nor their co-authors have any competing interests.
Disclaimer
Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
Acknowledgements
The authors acknowledge the support from the French National Research Agency
(ANR-PRCI), as well as the French program CNRS-LEFE-CHAT (Programme National – Les Enveloppes Fluides et l'Environnement – Chimie Atmosphrique). The
authors thank two colleagues, Jim Grisillon and Fabien
Robert-Peillard, from the laboratory LCE – Aix Marseille University, who
performed additional measurements for the quantification of surfactants in
an AS solution. Finally, the authors acknowledge the first anonymous
reviewer for her/his extremely thorough comments on the manuscript.
Financial support
This research has been supported by ANR-PRCI through the projects PARAMOUNT (grant no. ANR18-CE92-0038-02) and ORACLE (grant no. ANR-20-CE93-0008-01_ACT) and by CNRS-LEFE-CHAT through the project SURFACTs.
Review statement
This paper was edited by Hartmut Herrmann and reviewed by Angela Buchholz and one anonymous referee.
ReferencesAbbatt, J. P. D., Broekhuizen, K., and Pradeep Kumar, P.: Cloud condensation nucleus activity of internally mixed ammonium sulfate/organic acid aerosol particles, Atmos. Environ., 39, 4767–4778,
10.1016/j.atmosenv.2005.04.029, 2005.Aiken, A. C., DeCarlo, P. F., and Jimenez, J. L.: Elemental Analysis of
Organic Species with Electron Ionization High-Resolution Mass Spectrometry,
Anal. Chem., 79, 8350–8358, 10.1021/ac071150w, 2007.Andreae, M. O. and Rosenfeld, D.: Aerosol–cloud–precipitation
interactions. Part 1. The nature and sources of cloud-active aerosols,
Earth-Sci. Rev., 89, 13–41, 10.1016/j.earscirev.2008.03.001, 2008.Arabadzhieva, D., Tchoukov, P., and Mileva, E.: Impact of Adsorption Layer
Properties on Drainage Behavior of Microscopic Foam Films: The Case of
Cationic/Nonionic Surfactant Mixtures, Colloids Interfaces, 4, 53,
10.3390/colloids4040053, 2020.Badger, C. L., George, I., Griffiths, P. T., Braban, C. F., Cox, R. A., and Abbatt, J. P. D.: Phase transitions and hygroscopic growth of aerosol particles containing humic acid and mixtures of humic acid and ammonium sulphate, Atmos. Chem. Phys., 6, 755–768, 10.5194/acp-6-755-2006, 2006.Brooks, S. D., DeMott, P. J., and Kreidenweis, S. M.: Water uptake by
particles containing humic materials and mixtures of humic materials with
ammonium sulfate, Atmos. Environ., 38, 1859–1868,
10.1016/j.atmosenv.2004.01.009, 2004.Brüggemann, M., Xu, R., Tilgner, A., Kwong, K. C., Mutzel, A., Poon, H.
Y., Otto, T., Schaefer, T., Poulain, L., Chan, M. N., and Herrmann, H.:
Organosulfates in Ambient Aerosol: State of Knowledge and Future Research
Directions on Formation, Abundance, Fate, and Importance, Environ. Sci.
Technol., 54, 3767–3782, 10.1021/acs.est.9b06751, 2020.Canagaratna, M. R., Jimenez, J. L., Kroll, J. H., Chen, Q., Kessler, S. H., Massoli, P., Hildebrandt Ruiz, L., Fortner, E., Williams, L. R., Wilson, K. R., Surratt, J. D., Donahue, N. M., Jayne, J. T., and Worsnop, D. R.: Elemental ratio measurements of organic compounds using aerosol mass spectrometry: characterization, improved calibration, and implications, Atmos. Chem. Phys., 15, 253–272, 10.5194/acp-15-253-2015, 2015.
Charlson, R. J., Schwartz, S., Hales, J., Cess, R. D., Coakley, J. J.,
Hansen, J., and Hofmann, D.: Climate forcing by anthropogenic aerosols,
Science, 255, 423–430, 1992.Clegg, S. L., Brimblecombe, P., and Wexler, A. S.: Thermodynamic Model of
the System H+-NH4+-Na+-SO42--NO3--Cl--H2O at 298.15 K, J. Phys. Chem. A, 102, 2155–2171, 10.1021/jp973043j, 1998.
Darer, A. I., Cole-Filipiak, N. C., O'Connor, A. E., and Elrod, M. J.:
Formation and stability of atmospherically relevant isoprene-derived
organosulfates and organonitrates, Environ. Sci. Technol., 45, 1895–1902,
2011.DeCarlo, P. F., Slowik, J. G., Worsnop, D. R., Davidovits, P., and Jimenez,
J. L.: Particle Morphology and Density Characterization by Combined Mobility
and Aerodynamic Diameter Measurements. Part 1: Theory, Aerosol Sci.
Technol., 38, 1185–1205, 10.1080/027868290903907, 2004.DeCarlo, P. F., Kimmel, J. R., Trimborn, A., Northway, M. J., Jayne, J. T.,
Aiken, A. C., Gonin, M., Fuhrer, K., Horvath, T., Docherty, K. S., Worsnop,
D. R., and Jimenez, J. L.: Field-Deployable, High-Resolution, Time-of-Flight
Aerosol Mass Spectrometer, Anal. Chem., 78, 8281–8289,
10.1021/ac061249n, 2006.De Haan, D. O., Hawkins, L. N., Welsh, H. G., Pednekar, R., Casar, J. R.,
Pennington, E. A., de Loera, A., Jimenez, N. G., Symons, M. A., Zauscher,
M., Pajunoja, A., Caponi, L., Cazaunau, M., Formenti, P., Gratien, A.,
Pangui, E., and Doussin, J.-F.: Brown Carbon Production in Ammonium- or
Amine-Containing Aerosol Particles by Reactive Uptake of Methylglyoxal and
Photolytic Cloud Cycling, Environ. Sci. Technol., 51, 7458–7466,
10.1021/acs.est.7b00159, 2017.Ekström, S., Nozière, B., Hultberg, M., Alsberg, T., Magnér, J., Nilsson, E. D., and Artaxo, P.: A possible role of ground-based microorganisms on cloud formation in the atmosphere, Biogeosciences, 7, 387–394, 10.5194/bg-7-387-2010, 2010.Engelhart, G. J., Asa-Awuku, A., Nenes, A., and Pandis, S. N.: CCN activity and droplet growth kinetics of fresh and aged monoterpene secondary organic aerosol, Atmos. Chem. Phys., 8, 3937–3949, 10.5194/acp-8-3937-2008, 2008.Frossard, A. A., Gérard, V., Duplessis, P., Kinsey, J. D., Lu, X., Zhu,
Y., Bisgrove, J., Maben, J. R., Long, M. S., Chang, R. Y.-W., Beaupré,
S. R., Kieber, D. J., Keene, W. C., Nozière, B., and Cohen, R. C.:
Properties of Seawater Surfactants Associated with Primary Marine Aerosol
Particles Produced by Bursting Bubbles at a Model Air–Sea Interface,
Environ. Sci. Technol., 53, 9407–9417,
10.1021/acs.est.9b02637, 2019.Gérard, V., Noziere, B., Fine, L., Ferronato, C., Singh, D. K.,
Frossard, A. A., Cohen, R. C., Asmi, E., Lihavainen, H., Kivekäs, N.,
Aurela, M., Brus, D., Frka, S., and Cvitešiæ Kušan, A.:
Concentrations and Adsorption Isotherms for Amphiphilic Surfactants in PM1
Aerosols from Different Regions of Europe, Environ. Sci. Technol., 53,
12379–12388, 10.1021/acs.est.9b03386, 2019.Grace, D. N., Lugos, E. N., Ma, S., Griffith, D. R., Hendrickson, H. P.,
Woo, J. L., and Galloway, M. M.: Brown Carbon Formation Potential of the
Biacetyl–Ammonium Sulfate Reaction System, ACS Earth Space Chem., 4,
1104–1113, 10.1021/acsearthspacechem.0c00096, 2020.Hämeri, K., Charlson, R., and Hansson, H.-C.: Hygroscopic properties of
mixed ammonium sulfate and carboxylic acids particles, AIChE J., 48,
1309–1316, 10.1002/aic.690480617, 2002.Hawkins, L. N., Welsh, H. G., and Alexander, M. V.: Evidence for pyrazine-based chromophores in cloud water mimics containing methylglyoxal and ammonium sulfate, Atmos. Chem. Phys., 18, 12413–12431, 10.5194/acp-18-12413-2018, 2018.Hensley, J. C., Birdsall, A. W., Valtierra, G., Cox, J. L., and Keutsch, F. N.: Revisiting the reaction of dicarbonyls in aerosol proxy solutions containing ammonia: the case of butenedial, Atmos. Chem. Phys., 21, 8809–8821, 10.5194/acp-21-8809-2021, 2021.Herrmann, H.: Kinetics of Aqueous Phase Reactions Relevant for Atmospheric
Chemistry, Chem. Rev., 103, 4691–4716, 10.1021/cr020658q, 2003.Hu, K. S., Darer, A. I., and Elrod, M. J.: Thermodynamics and kinetics of the hydrolysis of atmospherically relevant organonitrates and organosulfates, Atmos. Chem. Phys., 11, 8307–8320, 10.5194/acp-11-8307-2011, 2011.Iinuma, Y., Müller, C., Berndt, T., Böge, O., Claeys, M., and
Herrmann, H.: Evidence for the Existence of Organosulfates from â-Pinene
Ozonolysis in Ambient Secondary Organic Aerosol, Environ. Sci. Technol., 41,
6678–6683, 10.1021/es070938t, 2007.IPCC: Climate Change 2013 – The Physical Science Basis: Working Group I
Contribution to the Fifth Assessment Report of the Intergovernmental Panel
on Climate Change, edited by: Stocker, T. F., Qin, D., Plattner, G.-K., Tignor, M., Allen, S. K., Boschung, J., Nauels, A., Xia, Y., Bex, V., and Midgley, P. M., Cambridge University Press, Cambridge, 10.1017/CBO9781107415324, 2013.Jimenez, J. L., Canagaratna, M. R., Donahue, N. M., Prevot, A. S. H., Zhang,
Q., Kroll, J. H., DeCarlo, P. F., Allan, J. D., Coe, H., Ng, N. L., Aiken,
A. C., Docherty, K. S., Ulbrich, I. M., Grieshop, A. P., Robinson, A. L.,
Duplissy, J., Smith, J. D., Wilson, K. R., Lanz, V. A., Hueglin, C., Sun, Y.
L., Tian, J., Laaksonen, A., Raatikainen, T., Rautiainen, J., Vaattovaara,
P., Ehn, M., Kulmala, M., Tomlinson, J. M., Collins, D. R., Cubison, M. J.,
E, Dunlea, J., Huffman, J. A., Onasch, T. B., Alfarra, M. R., Williams, P.
I., Bower, K., Kondo, Y., Schneider, J., Drewnick, F., Borrmann, S., Weimer,
S., Demerjian, K., Salcedo, D., Cottrell, L., Griffin, R., Takami, A.,
Miyoshi, T., Hatakeyama, S., Shimono, A., Sun, J. Y., Zhang, Y. M., Dzepina,
K., Kimmel, J. R., Sueper, D., Jayne, J. T., Herndon, S. C., Trimborn, A.
M., Williams, L. R., Wood, E. C., Middlebrook, A. M., Kolb, C. E.,
Baltensperger, U., and Worsnop, D. R.: Evolution of Organic Aerosols in the
Atmosphere, Science, 326, 1525–1529,
10.1126/science.1180353, 2009.Jimenez, N. G., Sharp, K. D., Gramyk, T., Ugland, D. Z., Tran, M.-K., Rojas,
A., Rafla, M. A., Stewart, D., Galloway, M. M., Lin, P., Laskin, A.,
Cazaunau, M., Pangui, E., Doussin, J.-F., and De Haan, D. O.:
Radical-Initiated Brown Carbon Formation in Sunlit Carbonyl–Amine–Ammonium
Sulfate Mixtures and Aqueous Aerosol Particles, ACS Earth Space Chem., 6,
228–238, 10.1021/acsearthspacechem.1c00395, 2022.Johnson, T. J., Irwin, M., Symonds, J. P. R., Olfert, J. S., and Boies, A.
M.: Measuring aerosol size distributions with the aerodynamic aerosol
classifier, Aerosol Sci. Technol., 52, 655–665,
10.1080/02786826.2018.1440063, 2018.Kampf, C. J., Jakob, R., and Hoffmann, T.: Identification and characterization of aging products in the glyoxal/ammonium sulfate system – implications for light-absorbing material in atmospheric aerosols, Atmos. Chem. Phys., 12, 6323–6333, 10.5194/acp-12-6323-2012, 2012.Keller, B. O., Sui, J., Young, A. B., and Whittal, R. M.: Interferences and
contaminants encountered in modern mass spectrometry, Anal. Chim. Acta, 627,
71–81, 10.1016/j.aca.2008.04.043, 2008.Kiendler-Scharr, A., Mensah, A. A., Friese, E., Topping, D., Nemitz, E.,
Prevot, A. S. H., Äijälä, M., Allan, J., Canonaco, F.,
Canagaratna, M., Carbone, S., Crippa, M., Dall Osto, M., Day, D. A., De
Carlo, P., Di Marco, C. F., Elbern, H., Eriksson, A., Freney, E., Hao, L.,
Herrmann, H., Hildebrandt, L., Hillamo, R., Jimenez, J. L., Laaksonen, A.,
McFiggans, G., Mohr, C., O'Dowd, C., Otjes, R., Ovadnevaite, J., Pandis, S.
N., Poulain, L., Schlag, P., Sellegri, K., Swietlicki, E., Tiitta, P.,
Vermeulen, A., Wahner, A., Worsnop, D., and Wu, H.-C.: Ubiquity of organic
nitrates from nighttime chemistry in the European submicron aerosol,
Geophys. Res. Lett., 43, 7735–7744, 10.1002/2016GL069239,
2016.Kim, J. H., Mulholland, G. W., Kukuck, S. R., and Pui, D. Y. H.: Slip
Correction Measurements of Certified PSL Nanoparticles Using a Nanometer
Differential Mobility Analyzer (Nano-DMA) for Knudsen Number From 0.5 to 83,
J. Res. Natl. Inst. Stand. Technol., 110, 31–54,
10.6028/jres.110.005, 2005.Kind, T. and Fiehn, O.: Seven Golden Rules for heuristic filtering of
molecular formulas obtained by accurate mass spectrometry, BMC
Bioinformatics, 8, 105, 10.1186/1471-2105-8-105, 2007.King, S. M., Rosenoern, T., Shilling, J. E., Chen, Q., and Martin, S. T.: Increased cloud activation potential of secondary organic aerosol for atmospheric mass loadings, Atmos. Chem. Phys., 9, 2959–2971, 10.5194/acp-9-2959-2009, 2009.Koehler, K. A., Kreidenweis, S. M., DeMott, P. J., Prenni, A. J., Carrico, C. M., Ervens, B., and Feingold, G.: Water activity and activation diameters from hygroscopicity data - Part II: Application to organic species, Atmos. Chem. Phys., 6, 795–809, 10.5194/acp-6-795-2006, 2006.Laskin, J., Laskin, A., Nizkorodov, S. A., Roach, P., Eckert, P., Gilles, M.
K., Wang, B., Lee, H. J., and Hu, Q.: Molecular Selectivity of Brown
Carbon Chromophores, Environ. Sci. Technol., 48, 12047–12055,
10.1021/es503432r, 2014.Meyer, N. K., Duplissy, J., Gysel, M., Metzger, A., Dommen, J., Weingartner, E., Alfarra, M. R., Prevot, A. S. H., Fletcher, C., Good, N., McFiggans, G., Jonsson, Å. M., Hallquist, M., Baltensperger, U., and Ristovski, Z. D.: Analysis of the hygroscopic and volatile properties of ammonium sulphate seeded and unseeded SOA particles, Atmos. Chem. Phys., 9, 721–732, 10.5194/acp-9-721-2009, 2009.Moore, R. H., Ingall, E. D., Sorooshian, A., and Nenes, A.: Molar mass,
surface tension, and droplet growth kinetics of marine organics from
measurements of CCN activity, Geophys. Res. Lett., 35, L07801,
10.1029/2008GL033350, 2008.Nandy, L. and Dutcher, C. S.: Phase Behavior of Ammonium Sulfate with
Organic Acid Solutions in Aqueous Aerosol Mimics Using Microfluidic Traps,
J. Phys. Chem. B, 122, 3480–3490, 10.1021/acs.jpcb.7b10655,
2018.Nozière, B., Dziedzic, P., and Córdova, A.: Inorganic ammonium salts
and carbonate salts are efficient catalysts for aldol condensation in
atmospheric aerosols, Phys. Chem. Chem. Phys., 12, 3864–3872,
10.1039/B924443C, 2010.Nozière, B., Baduel, C., and Jaffrezo, J.-L.: The dynamic surface
tension of atmospheric aerosol surfactants reveals new aspects of cloud
activation, Nat. Commun., 5, 3335, 10.1038/ncomms4335, 2014.Ovadnevaite, J., Zuend, A., Laaksonen, A., Sanchez, K. J., Roberts, G.,
Ceburnis, D., Decesari, S., Rinaldi, M., Hodas, N., Facchini, M. C.,
Seinfeld, J. H., and O' Dowd, C.: Surface tension prevails over solute
effect in organic-influenced cloud droplet activation, Nature, 546,
637–641, 10.1038/nature22806, 2017.Petters, M. D.: A language to simplify computation of differential mobility
analyzer response functions, Aerosol Sci. Technol., 52, 1437–1451,
10.1080/02786826.2018.1530724, 2018.Petters, M. D. and Kreidenweis, S. M.: A single parameter representation of hygroscopic growth and cloud condensation nucleus activity, Atmos. Chem. Phys., 7, 1961–1971, 10.5194/acp-7-1961-2007, 2007.Petters, M. D. and Kreidenweis, S. M.: A single parameter representation of hygroscopic growth and cloud condensation nucleus activity – Part 3: Including surfactant partitioning, Atmos. Chem. Phys., 13, 1081–1091, 10.5194/acp-13-1081-2013, 2013.Pieber, S. M., El Haddad, I., Slowik, J. G., Canagaratna, M. R., Jayne, J.
T., Platt, S. M., Bozzetti, C., Daellenbach, K. R., Fröhlich, R.,
Vlachou, A., Klein, F., Dommen, J., Miljevic, B., Jiménez, J. L.,
Worsnop, D. R., Baltensperger, U., and Prévôt, A. S. H.: Inorganic
Salt Interference on CO2+ in Aerodyne AMS and ACSM Organic Aerosol
Composition Studies, Environ. Sci. Technol., 50, 10494–10503,
10.1021/acs.est.6b01035, 2016.Pöschl, U. and Shiraiwa, M.: Multiphase Chemistry at the
Atmosphere–Biosphere Interface Influencing Climate and Public Health in the
Anthropocene, Chem. Rev., 115, 4440–4475,
10.1021/cr500487s, 2015.Powelson, M. H., Espelien, B. M., Hawkins, L. N., Galloway, M. M., and De
Haan, D. O.: Brown Carbon Formation by Aqueous-Phase Carbonyl Compound
Reactions with Amines and Ammonium Sulfate, Environ. Sci. Technol., 48,
985–993, 10.1021/es4038325, 2014.Prenni, A. J., DeMott, P. J., and Kreidenweis, S. M.: Water uptake of
internally mixed particles containing ammonium sulfate and dicarboxylic
acids, Atmos. Environ., 37, 4243–4251,
10.1016/S1352-2310(03)00559-4, 2003.Prisle, N. L.: A predictive thermodynamic framework of cloud droplet activation for chemically unresolved aerosol mixtures, including surface tension, non-ideality, and bulk–surface partitioning, Atmos. Chem. Phys., 21, 16387–16411, 10.5194/acp-21-16387-2021, 2021.Saukko, E., Zorn, S., Kuwata, M., Keskinen, J., and Virtanen, A.: Phase
State and Deliquescence Hysteresis of Ammonium-Sulfate-Seeded Secondary
Organic Aerosol, Aerosol Sci. Technol., 49, 531–537,
10.1080/02786826.2015.1050085, 2015.Seinfeld, J. H., Bretherton, C., Carslaw, K. S., Coe, H., DeMott, P. J.,
Dunlea, E. J., Feingold, G., Ghan, S., Guenther, A. B., Kahn, R., Kraucunas,
I., Kreidenweis, S. M., Molina, M. J., Nenes, A., Penner, J. E., Prather, K.
A., Ramanathan, V., Ramaswamy, V., Rasch, P. J., Ravishankara, A. R.,
Rosenfeld, D., Stephens, G., and Wood, R.: Improving our fundamental
understanding of the role of aerosol-cloud interactions in the climate
system, P. Natl. Acad. Sci. USA, 113, 5781–5790,
10.1073/pnas.1514043113, 2016.Shakya, K. M. and Peltier, R. E.: Non-sulfate sulfur in fine aerosols across
the United States: Insight for organosulfate prevalence, Atmos. Environ.,
100, 159–166, 10.1016/j.atmosenv.2014.10.058, 2015.Sjogren, S., Gysel, M., Weingartner, E., Baltensperger, U., Cubison, M. J.,
Coe, H., Zardini, A. A., Marcolli, C., Krieger, U. K., and Peter, T.:
Hygroscopic growth and water uptake kinetics of two-phase aerosol particles
consisting of ammonium sulfate, adipic and humic acid mixtures, J. Aerosol
Sci., 38, 157–171, 10.1016/j.jaerosci.2006.11.005, 2007.Smith, M. L., You, Y., Kuwata, M., Bertram, A. K., and Martin, S. T.: Phase
Transitions and Phase Miscibility of Mixed Particles of Ammonium Sulfate,
Toluene-Derived Secondary Organic Material, and Water, J. Phys. Chem. A,
117, 8895–8906, 10.1021/jp405095e, 2013.Sorjamaa, R., Svenningsson, B., Raatikainen, T., Henning, S., Bilde, M., and Laaksonen, A.: The role of surfactants in Köhler theory reconsidered, Atmos. Chem. Phys., 4, 2107–2117, 10.5194/acp-4-2107-2004, 2004.Stokes, R. H. and Robinson, R. A.: Interactions in Aqueous Nonelectrolyte
Solutions. I. Solute-Solvent Equilibria, J. Phys. Chem., 70, 2126–2131,
10.1021/j100879a010, 1966.Surratt, J. D., Gómez-González, Y., Chan, A. W. H., Vermeylen, R.,
Shahgholi, M., Kleindienst, T. E., Edney, E. O., Offenberg, J. H.,
Lewandowski, M., Jaoui, M., Maenhaut, W., Claeys, M., Flagan, R. C., and
Seinfeld, J. H.: Organosulfate Formation in Biogenic Secondary Organic
Aerosol, J. Phys. Chem. A, 112, 8345–8378, 10.1021/jp802310p, 2008.Szmigielski, R.: Evidence for C5 organosulfur secondary organic aerosol
components from in-cloud processing of isoprene: Role of reactive SO4 and SO3 radicals, Atmos. Environ., 130, 14–22,
10.1016/j.atmosenv.2015.10.072, 2016.Tavakoli, F. and Olfert, J. S.: An Instrument for the Classification of
Aerosols by Particle Relaxation Time: Theoretical Models of the Aerodynamic
Aerosol Classifier, Aerosol Sci. Technol., 47, 916–926,
10.1080/02786826.2013.802761, 2013.Tervahattu, H., Hartonen, K., Kerminen, V.-M., Kupiainen, K., Aarnio, P.,
Koskentalo, T., Tuck, A. F., and Vaida, V.: New evidence of an organic layer
on marine aerosols, J. Geophys. Res.-Atmos., 107, AAC 1-1–AAC 1-8,
10.1029/2000JD000282, 2002.Trainic, M., Abo Riziq, A., Lavi, A., Flores, J. M., and Rudich, Y.: The optical, physical and chemical properties of the products of glyoxal uptake on ammonium sulfate seed aerosols, Atmos. Chem. Phys., 11, 9697–9707, 10.5194/acp-11-9697-2011, 2011.Tran, J. C. and Doucette, A. A.: Cyclic polyamide oligomers extracted from
nylon 66 membrane filter disks as a source of contamination in liquid
chromatography/mass spectrometry, J. Am. Soc. Mass Spectrom., 17, 652–656,
10.1016/j.jasms.2006.01.008, 2006.
Treuel, L., Pederzani, S., and Zellner, R.: Deliquescence behaviour and
crystallisation of ternary ammonium sulfate/dicarboxylic acid/water
aerosols, Phys. Chem. Chem. Phys., 11, 7976–7984,
10.1039/B905007H, 2009.Varutbangkul, V., Brechtel, F. J., Bahreini, R., Ng, N. L., Keywood, M. D., Kroll, J. H., Flagan, R. C., Seinfeld, J. H., Lee, A., and Goldstein, A. H.: Hygroscopicity of secondary organic aerosols formed by oxidation of cycloalkenes, monoterpenes, sesquiterpenes, and related compounds, Atmos. Chem. Phys., 6, 2367–2388, 10.5194/acp-6-2367-2006, 2006.Vepsäläinen, S., Calderón, S. M., Malila, J., and Prisle, N. L.: Comparison of six approaches to predicting droplet activation of surface active aerosol – Part 1: moderately surface active organics, Atmos. Chem. Phys., 22, 2669–2687, 10.5194/acp-22-2669-2022, 2022.Wach, P., Spólnik, G., Rudziñski, K. J., Skotak, K., Claeys, M.,
Danikiewicz, W., and Szmigielski, R.: Radical oxidation of methyl vinyl
ketone and methacrolein in aqueous droplets: Characterization of
organosulfates and atmospheric implications, Chemosphere, 214, 1–9,
10.1016/j.chemosphere.2018.09.026, 2019.Wex, H., Petters, M. D., Carrico, C. M., Hallbauer, E., Massling, A., McMeeking, G. R., Poulain, L., Wu, Z., Kreidenweis, S. M., and Stratmann, F.: Towards closing the gap between hygroscopic growth and activation for secondary organic aerosol: Part 1 – Evidence from measurements, Atmos. Chem. Phys., 9, 3987–3997, 10.5194/acp-9-3987-2009, 2009.Wiedensohler, A., Birmili, W., Nowak, A., Sonntag, A., Weinhold, K., Merkel, M., Wehner, B., Tuch, T., Pfeifer, S., Fiebig, M., Fjäraa, A. M., Asmi, E., Sellegri, K., Depuy, R., Venzac, H., Villani, P., Laj, P., Aalto, P., Ogren, J. A., Swietlicki, E., Williams, P., Roldin, P., Quincey, P., Hüglin, C., Fierz-Schmidhauser, R., Gysel, M., Weingartner, E., Riccobono, F., Santos, S., Grüning, C., Faloon, K., Beddows, D., Harrison, R., Monahan, C., Jennings, S. G., O'Dowd, C. D., Marinoni, A., Horn, H.-G., Keck, L., Jiang, J., Scheckman, J., McMurry, P. H., Deng, Z., Zhao, C. S., Moerman, M., Henzing, B., de Leeuw, G., Löschau, G., and Bastian, S.: Mobility particle size spectrometers: harmonization of technical standards and data structure to facilitate high quality long-term observations of atmospheric particle number size distributions, Atmos. Meas. Tech., 5, 657–685, 10.5194/amt-5-657-2012, 2012.Wu, J., Brun, N., González-Sánchez, J. M., R'Mili, B., Temime Roussel, B., Ravier, S., Clément, J.-L., and Monod, A.: Raw data of article “Substantial organic impurities at the surface of synthetic ammonium sulfate particles”, Zenodo [data set], 10.5281/zenodo.6559283, 2022.Zelenyuk, A., Cai, Y., and Imre, D.: From Agglomerates of Spheres to
Irregularly Shaped Particles: Determination of Dynamic Shape Factors from
Measurements of Mobility and Vacuum Aerodynamic Diameters, Aerosol Sci.
Technol., 40, 197–217, 10.1080/02786820500529406, 2006.