A method is presented to quantify the low-molecular-weight organic acids such
as
formic, acetic, propionic, butyric, pyruvic, glycolic, oxalic,
malonic, succinic, malic, glutaric, and methanesulfonic acid in the
atmospheric gas and particle phases, based on a combination of the Monitor for
AeRosols and Gases in ambient Air (MARGA) and an additional ion
chromatography (Compact IC) instrument. Therefore, every second hourly
integrated MARGA gas and particle samples were collected and analyzed by the
Compact IC, resulting in 12 values per day for each phase. A proper separation
of the organic target acids was initially tackled by a laboratory IC
optimization study, testing different separation columns, eluent compositions
and eluent flow rates for both isocratic and gradient elution. Satisfactory
resolution of all compounds was achieved using a gradient system with two
coupled anion-exchange separation columns. Online pre-concentration with an
enrichment factor of approximately 400 was achieved by solid-phase extraction
consisting of a methacrylate-polymer-based sorbent with quaternary ammonium
groups. The limits of detection of the method range between 0.5 ng m
Low-molecular-weight organic acids have been measured in the gas (Lee et al., 2009; Bao et al., 2012) and particle phases (Boreddy et al., 2017; Miyazaki et al., 2014; van Pinxteren et al., 2014) as well as in precipitation and cloud water (Sun et al., 2016; van Pinxteren et al., 2005). Next to known primary anthropogenic (Bock et al., 2017; Kawamura and Kaplan, 1987; Legrand et al., 2007) and biogenic sources (Falkovich et al., 2005; Stavrakou et al., 2012), organic acids are formed as secondary products by atmospheric oxidation processes (Lim et al., 2005; Tilgner and Herrmann, 2010; Hoffmann et al., 2016). However, there are still unknown sources of these short-chained compounds (Millet et al., 2015; Stavrakou et al., 2012).
Because of their hygroscopicity (Kawamura and Bikkina, 2016), the organic acids contribute to the acidity of precipitation, dew, fog and clouds (Lee et al., 2009; van Pinxteren et al., 2016). Atmospheric transport processes also lead to dry and wet deposition in remote areas, where they can have an influence on the sensitive ecosystem (Friedman et al., 2017; Himanen et al., 2012; Sabbioni et al., 2003).
Owing to the low concentrations and the high diversity of organic acids compared to inorganic compounds, a highly resolved and near-real-time quantification of organic acids is challenging. Studies on organic compounds in particulate matter (PM) were performed with filter measurements followed by off-line analysis with ion chromatography (IC) (Röhrl and Lammel, 2002; Granby et al., 1997; Legrand et al., 2007), gas chromatography coupled with mass spectroscopy (GC–MS) (Mochizuki et al., 2018; Miyazaki et al., 2014; Kawamura et al., 2012; Hu et al., 2018) or a flame ionization detector (GC–FID) (Deshmukh et al., 2018), capillary electrophoresis (CE) (Müller et al., 2005; van Pinxteren et al., 2009, 2014), or Raman spectroscopy (Kuo et al., 2011).
Gas-phase compounds were sampled for a few hours and analyzed off-line with coated filters and GC–MS (Limbeck et al., 2005), a denuder and GC–MS (Bao et al., 2012), a denuder and IC (Dawson et al., 1980), and a mist chamber and IC (Preunkert et al., 2007; Schultz Tokos et al., 1992).
Due to the long sampling time of filter and wet sampling techniques followed by laboratory analyses, these methods did not allow for a near-real-time quantification and the laboratory effort is huge. Recently, Stieger et al. (2018) showed that off-line filter analysis involves the risk of possible evaporation artifacts of volatile particulate compounds from the filter or the adsorption of gaseous compounds. Additionally, Boring et al. (2002) mentioned the difficulty of sampling very small particles using impaction techniques.
Over the last few years, new instruments have allowed online measurements with increased time resolution. Zander et al. (2010) and Pommier et al. (2016) quantified the vertical column of gaseous formic acid with ground-based Fourier-transform infrared spectroscopy (FTIR). However, the focus of the present work is on the ground-based detection of the carboxylic acids because of possible influences on the lower troposphere.
Gas-phase concentrations on the ground were monitored with a chemical
ionization mass spectrometer (CIMS) (Veres et al., 2011; J. M. Liu et al.,
2012; Crisp et al., 2014; Mungall et al., 2018). This instrument also enabled
airborne measurements of formic acid (Jones et al., 2014). Recently, Nah et
al. (2018b) assessed the use of sulfur hexafluoride (
As all organic acids are ionic, an application of the IC for the analysis is obvious. Boring et al. (2002) first described an instrument based on an IC system. The separation of the gas and particle phases was performed by the application of a parallel plate denuder and a particle collection system consisting of glass fiber filters. The filters were washed online with deionized water and the dissolved anions from the gas and particle phases, including formic, acetic and oxalic acid, were analyzed. The resulting time resolution from their example measurement period was approximately 30 min. A disadvantage in this study was the necessary exchange of the inserted glass fiber filters every 12 h. Fisseha et al. (2006) published results of formic, acetic, propionic and oxalic acid in Zurich, Switzerland, for 3 months in different seasons. These authors used a flattened denuder and an aerosol chamber under supersaturated conditions to quantify formate, acetate, propionate and oxalate in the gas and particle phases. The detection of other dicarboxylic acids (DCAs) was not possible due to co-elution with the carbonate peak and atmospheric concentrations of other monocarboxylic acids (MCAs) were mostly below the detection limit of the method. Lee et al. (2009) and Ku et al. (2010) sampled only gaseous compounds with a parallel plate denuder. While the first group analyzed C1–C3 MCAs within an hourly time resolution, the second group concentrated on the quantification of acetic acid every 10 min. Recently, Zhou et al. (2015) observed gaseous and particulate oxalate in their MARGA (Monitor for AeRosols and Gases in ambient Air) measurements in Hong Kong for 1 year. In this case, a pre-concentration column was installed instead of the injection loop, but the analysis of more carboxylic acids (CAs) was limited by the short separation column and thus separation efficiency.
Recently, Nah et al. (2018a) presented measurements of low-molecular-weight organic acids within the gas and particle phases with the use of a CIMS and a particle-into-liquid sampler (PILS) coupled with capillary high-pressure ion chromatography (HPIC). They received hourly concentrations of these compounds at a rural southeastern United States site for 1 month and were able to investigate the gas–particle partitioning.
Ullah et al. (2006) developed an online instrument to measure ionic species within the gas and particle phases. For the separation, they used a membrane denuder to collect the water-soluble gases, and a hydrophilic filter sampled the particles. In their IC analysis, it was possible to quantify formic and acetic acid every 40 min.
However, to the author's knowledge, online instruments properly quantifying a variety of low-molecular-weight organic acids (formic, acetic, propionic, butyric, glycolic, pyruvic, oxalic, malonic, succinic, malic, glutaric and methanesulfonic acid) within the gas and particle phases at a high time resolution do not exist yet.
The present study describes the instrumental development of an online-coupled pre-concentration and IC separation system to determine organic acids in the gas and particle phases as an extension of the MARGA. The MARGA has been reported to be a reliable field instrument for long-time measurements in Melpitz and other sites (Stieger et al., 2018, and references therein) and its upgrade with an additional IC separation allows for the analysis of all target compounds with a low risk of interferences from other species.
The developed setup was employed from November 2016 until October 2017 at the TROPOS research site in Melpitz. As a demonstration of a successful field application, the first tropospheric measurements will be presented. Data interpretation of the 1-year measurement campaign with a focus on the phase distribution and the investigation of primary and secondary sources will be published elsewhere.
Water-soluble chloride (
In addition to the two IC systems integrated into the MARGA, an additional one (930 Compact IC Flex, Metrohm, Switzerland; hereafter called Compact IC) together with an autosampler (robotic sample processor XL, Metrohm, Switzerland) is used for the determination of organic acids. The setup of the complete system is shown in Fig. 1. Therein, the different components that will be explained in the following are tagged. Comparable IC systems, for example from Thermo Scientific, were considered as possible alternatives. However, the liquid handling via the autosampler, especially the liquid flows from the MARGA to the necessary autosampler and the capacity of the autosampler, limited the use of other IC systems.
Setup of the IC system with (a) the first working station and (b) the second working station of the autosampler, (c) the 800 Dosino for the sample transportation, (d) the Compact IC, (e) the 10 mL sample loop, (f) the 800 Dosino for the gradient system and (g) an external six-way valve for the combination of MARGA and the IC system.
An autosampler with two working stations (a) and (b) has a sample plate with
120 slots for 12.5 mL vials with perforated plugs (polypropylene; Metrohm,
Switzerland). The slots are arranged in two circles. Additionally, one
working station is equipped with inner and outer sample needles (a) so that
the WRD and SJAC solutions can be pumped into the respective vial
simultaneously. After storage, the filled vials go to the second working
station consisting of a swing head with a further sample needle (b). To avoid
contamination, this sample needle is cleaned in a washing station with
ultrapure water after each suction. A commercial syringe pump (800 Dosino,
Metrohm, Switzerland; c) transports 10 mL of one sample from the autosampler
via a six-way injection valve within the Compact IC (d) to a sample loop (e)
with a velocity of 2 mL min
For gradient applications, a second 800 Dosino (f) was combined with the Compact IC. With an identical flow rate, a defined amount of a more highly concentrated eluent was added to the eluent flow in front of the eluent degasser. A trap column (Metrosep A Trap 1 100/4.0) cleans and ensures a complete mixing of both eluent solutions before the eluent is injected into the pre-concentration column. For the combination of the MARGA and Compact IC, an external six-way valve (Metrohm, Switzerland) is required (g). The complete setup and the time program for the gradient system is controlled by the Metrohm MagIC Net software (Metrohm, Switzerland).
The Compact IC is manually calibrated with three standard solutions twice a week, when the vials of the autosampler are replaced. The standard solutions are prepared in 50 mL flasks, stored in the refrigerator and renewed every 2 weeks. The concentrations of each inorganic and organic ion in the standard are given in Table 1.
Aqueous standard solution concentrations used for the calibration of the Compact IC.
Hydrogen peroxide (
The IC separation was developed in laboratory studies to ensure the best
separation efficiency of the target compounds formate, acetate, glycolate,
pyruvate, oxalate, malonate, succinate, malate and glutarate. The further
organic anions propionate, butyrate and methanesulfonate were later
identified in the first field applications and then included into the
standard solution. Due to their expected low concentrations, it was
considered important to pre-concentrate the ions online within the aqueous
MARGA sample streams from both the WRD and SJAC. Therefore, a
pre-concentration column was applied from the beginning of the optimization
studies, as described above. An enrichment factor of 400 was achieved by the
comparison of the peak areas of standard solutions applying a 20
First analyses were performed with an isocratic system and the separation
column Metrosep A Supp 16 250 mm with an eluent of 7 mM
Since, at this stage, a satisfying separation was not achieved, other
columns were additionally tested within the isocratic setup. An
anion-exchange column named Shodex IC SI-50 4E (Showa Denko Europe GmbH,
Germany) was included with an eluent of 3.2 mM
Overview of the varied flows and eluent compositions in the isocratic system using the Metrosep A Supp 16 250 mm column with their effects on separation and reference to the corresponding figures in the Supplement.
Possible improvements were investigated by changing the eluent flow and the
eluent composition, which are summarized in Table 2. The flow was increased
to 0.9 and 1.0 mL min
To combine the advantages of the different eluent compositions, a gradient
system was applied. Two different concentrated eluents were prepared. Within
the Compact IC, a highly concentrated eluent B (20 mM
Although the MCA separation was improved by applying the described gradient,
no baseline-separated pyruvate and
Changing the oven temperature was also considered. All measurements with the
Metrosep A Supp 16 250 mm were executed with a temperature of 65
To improve peak resolutions, the Metrosep A Supp 16 250 mm was extended with
an additional Metrosep A Supp 16 150 mm (Metrohm, Switzerland) column. An
even longer second column could not be chosen because of a system pressure
limitation of 20 MPa that would otherwise be exceeded. Due to the increased
back-pressure of the coupled columns, it was necessary to keep the oven
temperature at 65
The gradient profile was adjusted for this separation. First analyses were
performed with the described profile of Fig. 4 but the retention times were
not stable. The longer analysis time of 52.5 min and thus the shorter
regeneration time between the analyses led to a carryover of eluent B.
Therefore, other gradient profiles were tested and the best result was found
for starting with 100 % of eluent A. Afterwards, eluent B was slowly
increased to 40 % from
The described method allowed for the proper separation of all organic target anions, which is why this system was selected and applied for real atmospheric analyses.
All values of the limits of detection (LODs), linearity and precision for each
species are given in Table 3. The linearity of the calibration curve was
determined after Funk et al. (2005). For the linear calibration
function (
Type of calibration curve, LODs and the method precision for each ion.
Chromatogram of combined Metrosep A Supp 16 250 and 150 mm columns
with a gradient eluent. The concentrations of the standard solution are
50
The calibration of a nonlinear second-order function (
The residual standard deviation for the linear
To test each ion's linearity, the difference of the variances
The LODs for the Compact IC were estimated from mean
blank values plus 3 times the standard deviation (3
For the combination of the MARGA and the Compact IC, the liquid flows in the system had to be adjusted to achieve a high time resolution and to analyze the solutions as fast as possible after the sampling. As an overview, a schematic setup and a time diagram in Fig. 7 display the important steps for the CA analysis of the WRD and SJAC samples. Therein, the sampled airflow is described with green arrows. Syringe pumps within the MARGA collected the dissolved ions within the WRD (blue arrows) and SJAC (red arrows) solutions. This sampling required 1 h and yields 25 mL of sample solution in each of the two syringes.
In the second hour, the MARGA
syringe pumps transported the solutions to the IC system within the MARGA for analysis of the inorganic compounds, and
the solutions were transported to the autosampler and Compact IC where the
organic compounds were quantified. Thereby, the WRD solution was injected with a flow of
0.417 mL min
To achieve a pre-concentration and analysis of one sample in 1 h, the
transfer of analytes from the autosampler to the Compact IC and the
pre-concentration of the sample had to be performed within the remaining 7.5 min, as the final Compact IC analysis described previously needed
52.5 min. Therefore, the sample flows were increased to 4 mL min
In the following, the MARGA and the Compact IC analysis will be distinguished. Ions analyzed by the MARGA were measured by the original MARGA system, while ions from the Compact IC were measured by the added setup.
The original MARGA absorption solution in the denuder and SJAC contains
10 mg L
Because of the missing biocide
When measuring the gas and particle phases with a combination of WRD and
SJAC, the collection efficiency of gases and the particle penetration within
the denuder should be investigated. In the literature, experimentally
derived collection efficiencies are available for annular denuders that
correspond with the WRD within the MARGA. Wyers et al. (1993)
published an
Diffusion coefficients (D) calculated according to Fuller et al. (1966) and calculated annular denuder efficiencies (E) according to the equations of Winiwarter (1989), Possanzini et al. (1983), De Santis (1994) and Berg et al. (2010) for gases.
In the present study, the collection efficiencies of the annular WRD were
theoretically calculated for the different inorganic and organic acids
following different approaches suggested in the literature (Possanzini et
al., 1983; Winiwarter, 1989; De Santis, 1994; Berg et al., 2010). For each
approach, all equations for the denuder efficiency calculation are given in
the Supplement. Calculated denuder efficiencies are summarized in Table 4.
The calculated efficiencies according to Possanzini et al. (1983),
De Santis (1994) and Berg et al. (2010) are higher
than 99 %, indicating a nearly complete absorption of the investigated gases
within the WRD. Denuder efficiencies derived from the approach of
Winiwarter (1989) range between 95 % for glutaric acid and nearly
100 % for the inorganic gases as well as formic acid. However, compared to
the other three studies, Winiwarter (1989) did not consider the
geometry of an annular denuder in his approach and is therefore considered to
be less accurate. Regarding the efficiencies calculated after
Possanzini et al. (1983) and the highest formic acid gas-phase
concentrations of 7.58
Another method to evaluate the WRD collection efficiency of gases is the
comparison of measured compounds that are predominantly found in the
gas phase. The inorganic nitrite (
For the WRD particle collection, Wyers et al. (1993)
investigated possible particulate ammonium collection within the denuder. A
sampling of ammonium sulfate particles of 0.1 and 1
The three highest concentrations of the DCAs were compared with the gas-phase
concentrations measured in the field. The oxalate concentrations in the
particle phase ranged between 327 and 543 ng m
In conclusion, the calculated denuder efficiencies that are in agreement with the literature as well as low rates of denuder breakthrough and low particulate losses within the WRD approve the use of a coupled WRD–SJAC system as a valid method to separate the gas and particle phases for the sampling of the low-molecular-weight organic acids.
Both the MARGA and Compact IC determined the inorganic compounds in the gas and particle phases from the same aqueous solution but used different IC methods, including a different calibration, different sample enrichments, different separation columns, and different eluent compositions and profiles. For quality assurance, the inorganic ions were compared in the gas and particle phases for the complete 1-year field application of the extended MARGA system and the results are summarized numerically in Table 5 as well as graphically in Figs. S17 and S18.
Orthogonal regression parameters of the comparison of inorganic compounds measured by the MARGA and Compact IC in Melpitz for 1 year. Scatter plots are given in Figs. S17 and S18.
The
The HONO comparison revealed an obvious scattering (
To prove the suitability of the complete setup, 2 weeks of the 1-year measurement campaign are presented. Figure 8 displays the measured organic acids in the gas and particle phases from 3 to 14 May 2017. Included grey shaded periods display downtimes of both the MARGA and the Compact IC because of the MARGA cleaning procedure (12 May), blank measurements of the complete new MARGA setup (12 May) or measurements of calibration standards (5, 9, 12 May). Table 6 gives the percentage of data coverage, i.e., concentrations above LOD, for each organic acid in the gas and particle phases during the uptime periods.
Measured concentrations for gaseous formic, acetic, propionic, butyric, glycolic and pyruvic acid as well as for particulate glycolate, methanesulfonate and oxalate from 3 to 14 May 2017 for an example application. The shaded grey areas represent periods without data because of instrumental issues.
Very good data coverages were found for formate and acetate in both phases as well as for glycolate and methanesulfonate in the particle phase with percentage values of over 90 %. Table 6 indicates the dominance of non-glycolate MCAs in the gas phase while DCAs were predominantly detected in the particle phase. This finding is in agreement with the higher vapor pressures of MCAs (Howard and Meylan, 1997).
Data coverage for the organic species measured in the gas and particle phases from 3 to 14 May 2017 during instrument uptime periods shown for data above the LOD.
Box–whisker plot for the diurnal variation in gaseous
formic acid
For the calculations of mean concentrations, all values below LOD were
included and not-detected data were set to zero. Mean (maximum)
concentrations of 306 ng m
DCAs and methanesulfonate were rarely detected in the gas phase due to the
low vapor pressures of these compounds. Thus, an existence of these species
is more likely in the particle phase. However, malonate, succinate, malate
and glutarate were rarely or not at all detected in the particulate phase
(Table 6). Oxalate is the predominant DCA in the particle phase with a
percentage data coverage of 77.3 %. Interestingly, formic and acetic
acid were also detected in the particulate phase. Mean (maximum) concentrations of
31 (209), 30 (465),
34 (282), 26 (162)
and 18 ng m
As a comparison, Parworth et al. (2017) detected average glycolate
concentrations of 26.7 ng m
The comparison with the literature shows rather low concentrations of the
organic acids in the particle phase during the example application in the
field. A possible reason are the changeable weather conditions. The
temperature varied during the first 7 days between 0 and
15
The highest values for temperature and global radiation were observed during the daytime (Fig. S19) when elevated concentrations are expected. Diurnal cycles of formic acid and particulate oxalate are illustrated in Fig. 9a and b, respectively. Both compounds had the lowest concentrations in the early morning and increased in the afternoon until the maxima were reached in the evening following the observed average temperature. This trend is in agreement with previous studies (Lee et al., 2009; Millet et al., 2015; Khare et al., 1997; Nah et al., 2018b; Martin et al., 1991). During the night, concentration decreases due to deposition processes. Simultaneously, a decreasing surface temperature cools down the lower air layers, leading to an inversion layer that suppresses the vertical mixing. The increasing concentrations after sunrise are likely a result of downward mixing of enriched layers above the boundary layer (Khare et al., 1999). Biogenic emissions and photochemical processes lead to increasing concentrations during the daytime (Khare et al., 1999; Y. Liu et al., 2012).
Nah et al. (2018a) reached the same conclusion from their study. They
found higher concentrations during warm and sunny days caused by elevated
availability of biogenic precursors. During their 2-month measurement
campaign in late summer and autumn, the maximum temperature ranged on
average between 25 and 30
The application in the field demonstrates the suitability of the developed setup. The measured concentrations of low-molecular-weight organic acids in Melpitz are partly lower than concentrations, which can be found in the literature. The increase in the concentrations after 11 May 2017 indicates an influence of the increasing temperature and the available sunlight, which is needed for biogenic emissions or photochemical reactions of atmospheric precursors. Further in-depth analyses and detailed results of the 1-year measurements with the extended MARGA system will be presented elsewhere (Stieger et al., 2019).
An extension of the MARGA analysis was described to quantify online low-molecular-weight organic acids. Therefore, the MARGA was combined with a new setup consisting of an autosampler and a Compact IC with an internal pre-concentration. Laboratory optimizations of the Compact IC were performed to improve the separation of the target organic acids formate, acetate, propionate, butyrate, glycolate, pyruvate, oxalate, malonate, succinate, malate, glutarate and methanesulfonate. An upgrade to a gradient system and an extension of the Metrosep A Supp 16 column to a total length of 400 mm allowed for a satisfactory separation of all MCAs and DCAs with low limits of detection and precisions.
The example application of the system in May 2017 illustrated high concentrations of formic acid and oxalate in the late afternoon, indicating a photochemical formation by atmospheric precursors. Variations in the wind direction resulted in sudden changes in the concentrations, as was the case for methanesulfonate.
To the authors' knowledge, highly resolved data of low-molecular-weight organic acids are not available for rural central Europe. Before our investigation, a quantification of these acids in the particle phase was only possible with filter measurements resulting in a low time resolution and potential artifacts from adsorption or revolatilization. The results of the example application proved the suitability of the MARGA extension for field measurements. Compared to other online systems, the variety of quantifiable organic acids in the gas and particle phases is unique. The application of this online method reduces laboratory work and sampling artifacts by filter and impactor measurements. Additionally, obtaining information of the organic acids every second hour allowed for the investigation of diurnal cycles, improving the knowledge of their primary and/or secondary sources. For the investigation of tropospheric multiphase chemistry, simultaneous quantification of the gas- and the particle-phase concentrations promises interesting analyses of the phase distribution of each organic acid.
Data can be made available by the authors upon request.
The supplement related to this article is available online at:
HH provided the concept for the MARGA extension. BS performed the experimental development, the calculations, the combination in the field, and the measurements and wrote the paper. GS, DvP and HH contributed ideas and suggestions during the method development and the field measurements. AG helped during infrastructural issues in Melpitz. All authors provided additional input and comments during the preparation of the paper.
The authors declare that they have no conflict of interest.
We thank René Rabe for the support, especially in the field. The authors acknowledge financial support for this study and the deployment of the MARGA system from the German Federal Environment Agency (UBA) research foundation under contract no. 52436 as well as from the European Regional Development fund by the European Union under contract no. 100188826. This study is partly supported by ACTRIS-2 (Aerosol, Clouds, and Trace gases Research InfraStructure network) from the European Union's Horizon 2020 research and innovation program under grant agreement no. 654109. The publication of this article was funded by the Open Access Fund of the Leibniz Association. Edited by: Pierre Herckes Reviewed by: two anonymous referees