In situ aerosol measurements were carried out with an aerosol sampling unit with two identical inlet systems attached to a proton-transfer-reaction time-of-flight mass spectrometer (PTR-TOF-MS, further referred to as “PTR-MS”) (Fig. 1a). The setup has been described in detail elsewhere (Holzinger et al., 2010a, 2013). In short, the air flow passes through 12 m long copper inlet tubes (ID = 6.5 mm), particles are humidified in a humidifier and then they are collected in a collection-thermal-desorption (CTD) cell. Afterwards, the cell is heated up in steps of 50 up to 350 ∘C and the emitted species are carried with a flow of nitrogen (ultrapure nitrogen, 5.7 purity, Air Products) into the PTR-MS. The PTR-MS was operated with the following settings: drift tube temperature, 120 ∘C, inlet tube temperature, 180 ∘C; ion source voltages, Us= 140 V, Uso= 92 V; E / N, 130 Td; extraction voltage at the end of the drift tube, Udx= 24 V. The intensity of the primary H3O+ ion signal (detected at m/z 21.023, H318O+) was typically higher than 5 × 105 counts per second.

After the measurements from the first inlet are finished, the valve system is switched automatically to allow aerosol measurements from the second inlet to start. Subsequent to the measurements from the second inlet, gas-phase measurements (not considered in the current paper) are carried out and then the measurement cycle starts over (see Fig. 1 in Holzinger et al., 2013).

Filter samples were analysed offline using comprehensive two-dimensional gas chromatography coupled to a time-of-flight mass spectrometer (GC × GC/TOF-MS, hereafter referred to as GC × GC). Details of the analysis method are described in Chan et al. (2013). In brief, filter punches (total area of 1.6 cm2) were thermally desorbed at 320 ∘C under helium (TDS3, Gerstel) to a two-dimensional gas chromatograph (Agilent 7890). Comprehensive GC × GC was performed using the Zoex thermal modulator interface, combining a 60 m × 0.25 mm × 0.25 µm non-polar capillary column (Rxi-5Sil MS, Restek) for the first-dimension separation (by volatility) with a medium-polarity second-dimension column (1 m × 0.25 mm × 0.25 µm, Rtx-200MS, Restek). The second-dimension column was maintained at 15 ∘C above the main oven temperature using a secondary oven. Effluent from the second column was analysed using a high-resolution (m/Δm ∼ 4000) time-of-flight mass spectrometer (Tofwerk, Thun, Switzerland) using 70 eV electron impact ionization. Peak detection and compound identification was performed using GC Image software (LLC). Around 1100 peaks were measured at above detection limits. Compounds were identified by confirmation with authentic standards, by mass spectral library search, or, in some cases, based on a unique ion (such as m/z 85 for gamma lactones, 217 for steranes) and its location in the two-dimensional chromatogram. Identification of otherwise unresolved branched and cyclic alkanes has also been done on these samples using soft ionization with vacuum ultraviolet radiation (Chan et al., 2013) but these alkanes are not included among the compounds discussed here.

Among the 1100 resolved peaks, the 153 compounds reported here were positively identified with the GC × GC technique, classified by compound groups: aromatic esters, benzofuranones, oxygenated polycyclic aromatic hydrocarbons, phthalates, 2-alkanones, 3-alkanones, alkanoic acids, alkyl esters, delta-lactones, gamma-lactones, nitrogen-containing aromatic compounds (N-aromatic), polycyclic aromatic hydrocarbons (PAHs), sulfur-containing compounds (S-compounds), amides, hopanes, alkanes, and several compounds were identified at a single m/z value (multiple). Another 31 compounds were classified into these compound groups without positive identification. Compound class nicknames are presented in the parentheses and further used in the article to refer to them.

The AMS measurements used in the current study has been described previously in detail (Hayes et al., 2013). In short, AMS allows for measurements of nonrefractory submicron aerosol (organic and inorganic) (DeCarlo et al., 2006). The operational principle of AMS can be presented briefly as follows. Air is sampled through a critical orifice with a consecutive focusing, acceleration, and separation of particles by size. Next, particles are vaporized at 600 ∘C, ionized by electron ionization (70 eV) and detected with a high-resolution time-of-flight mass spectrometer. Details of AMS operation and data analysis can be found in Hayes et al. (2013).

In this paper we present measurements of two types of standards: single compounds and a mixture of compounds. The following single compounds were measured: decanoic, pentadecanoic, and octadecanoic acids. Known quantities of each acid were first dissolved in ethanol, and then an aliquot of the solution containing 10 µg of a substance was placed on a quartz filter with a diameter of 5 mm. Next, 2 min were allowed to let most of the solvent evaporate before the filter was inserted in the oven, which is a part of the offline TD-PTR-MS system described in detail by Timkovsky et al. (2015) (Fig. 1b). Each measurement was repeated three times, and two blank filters were measured at the beginning and at the end of each measurement sequence. Blank filters were prepared by adding an aliquot of ethanol without a dissolved standard on a piece of filter.

A mixture containing 77 representative organic compounds and C8–C40 alkanes (this mixture is further called “multicomponent mixture”) was carefully prepared by dissolving respective compounds in deuterated acetone. An aliquot of the solution with 0.062 to 20 ng of the substances was placed on quartz filters. In this paper we focus only on acids contained in this standard (21 acids). Again, three filter replicas and two blank filters were measured with the offline TD-PTR-MS. Blank filters were prepared by adding an aliquot of deuterated acetone without a dissolved standard on a piece of filter.

The filter measuring procedure is described in detail by Timkovsky et al. (2015). In short, the sample is placed in the oven and allowed to stabilize for 2 min. Next, the temperature of the oven is increased stepwise from 100 to 350 ∘C in increments of 50 ∘C every 3 min. The desorbed compounds are carried by the 200 mL min-1 flow of nitrogen (ultrapure nitrogen, 5.7 purity, Air Products) into the PTR-MS. The operating conditions of the PTR-MS were the same as for the in situ TD-PTR-MS measurements (see Sect. 2.2.1).

Data evaluation was done with Interactive Data Language (IDL, version 8.1.0, ITT Visual Information Solutions) using custom-made routines described by Holzinger et al. (2010b) and Holzinger (2015). The initial mass lists consisted of 717 and 748 masses for multicomponent mixture and CalNex measurements respectively. Ions associated with primary ions and contaminations from the ion source were removed from the mass lists by filtering out m/z < 40 amu (except for m/z 31.017 and 33.033, corresponding to CH2OH+ and CH3OH2+ respectively). Inorganic ions (i.e. ions in the m/z range 40–50 amu that were matched with an inorganic formula) were also removed from the mass lists. Finally, the mass lists contained 653 and 726 masses for standard and CalNex measurements respectively. The mixing ratios of ions were calculated from the measured intensities by applying the same protonation reaction rate constant for all ions (3 × 10-9 cm3 s-1 molecule-1) (Holzinger et al., 2010b).

For the in situ data analysis, the initial mass spectra were first averaged to obtain data with a time resolution of 5 s. Second, the data were averaged over the measured temperature step (3 min each) and the data for all temperature steps were summed. Third, the resulted mixing ratios were converted to mass concentrations for individual ions by multiplying by ion molecular mass, volume of nitrogen used for desorption for one measurement cycle, and dividing by the volume of air sample from which aerosols were collected. Fourth, the data from inlet A and B were merged and averaged to match the filter sampling times. Fifth, the resulted mass concentrations were averaged over the whole comparison period (30–31 May) for the data presented in Sect. 3.2.2. The maximum total uncertainty of ∼ 54 % (mostly due to the uncertainty of the reaction rate coefficient) was calculated for these mass concentrations based on the method described by Timkovsky et al. (2015).

The same two initial steps were taken for the analysis of the offline TD-PTR-MS data. The obtained data with a 3 min resolution were processed according to the procedure described in Timkovsky et al. (2015). In short, the instrument background and blank corrected mass at a single temperature step (AT, in ng) was calculated according to Eq. (1): AT=(VMRi,0-VMRi,instrbgd-(VMRi,fb-VMRi,instrbgdfb))⋅Mi⋅Vnitrogen, where VMRi,0 is the uncorrected mixing ratio of the ion i, VMRi,fb is the mixing ratio of the ion i observed on the field blank, and VMRi,instrbgd and VMRi,instrbgd_fb are the instrument background mixing ratios of the ion i observed during the sample and field blank measurements respectively (all in nmol mol-1). Mi is the molecular weight of the ion i (minus 1 amu) and Vnitrogen is the volume of nitrogen used for desorption at a single temperature step in mol. As a next step, the six masses AT measured for the 50 ∘C intervals between 100 and 350 ∘C were summed to obtain the total mass of the substance which then compared with the known amount of the substance initially placed on the filters.

GC × GC data were analysed using GC Image (LLC). Peak volumes of quantification ions were used to calculate compound signal and then converted to the total ion signal based on ratios calculated from mass spectra in the NIST08 library. The total ion signals were then converted to on-column mass based on mass calibrations conducted using a representative set of commercially available organic compounds as external standards. For those compounds not commercially available, surrogate standards were assigned based on similarities in molecular structure. Deuterated internal standards were also used to correct for run-to-run variability in instrument response. Mass concentrations were then calculated based on the ratio of filter punch area to total filter area and sampling flow rate.

In order to match ions measured by the TD-PTR-MS with compounds spiked on the filters and those measured with the GC × GC technique (further referred to as “known compounds”) the following procedure was applied. First, we assumed that all known compounds were detected at their protonated mass or, in the case of oxygenated compounds, at the dehydrated fragment (i.e. protonated mass -18.010, the molecular weight (Mr) of the H2O fragment). Other fragmentation patterns are possible but not considered here. We matched the protonated and fragment masses with the ion masses detected by the TD-PTR-MS. A match was assigned if the difference between the protonated or fragment mass of the known compound and an ion detected with the PTR-MS was smaller than 250 ppm (corresponding to the mass resolution of the PTR-MS). By applying these rules several measured m/z values were consequently attributed to two or more different known compounds. In these cases, both the measured m/z values (protonated mass and fragment mass) and the known compounds were summed up and the sum signal was compared. Compounds were considered as not detected when either the detected amount by the TD-PTR-MS was negative after background subtraction or the abovementioned difference was above 250 ppm.

For example, 6H-Indolo[3,2,1-de][1,5]naphthyridin-6-one (C14H8N2OH+, m/z = 221.071) was detected at m/z 221.089 amu and its fragment was detected at m/z 203.087 amu. However, fluoranthene (C16H10H+, m/z = 203.086) and pyrene (C16H10H+, m/z = 203.086) were also detected at 203.087 amu. Consequently, 6H-Indolo[3,2,1-de][1,5]naphthyridin-6-one, fluoranthene and pyrene were considered as a single compound with Mr of 203.087 amu, and their measured concentrations were summed in both TD-PTR-MS and the GC × GC data.

Whenever more than one known compound was measured at the same mass, the most abundant known compound (based on the GC × GC measurements) was chosen to represent all of the measured signal. For example, phenaleno[1,9-bc]thiophene and anthraquinone were detected at the same m/z value (209.059 amu) with the TD-PTR-MS technique. The total averaged mass concentration of phenaleno[1,9-bc]thiophene and anthraquinone was 1.57 and 27.70 ng m-3 respectively (based on the GC × GC measurements). Thus, anthraquinone represents 95 % of the signal at that mass, and all of the signal at 209.059 amu was attributed to anthraquinone.

In the case where structural isomers were identified with the GC × GC technique, the corresponding GC × GC concentrations were summed. Mass concentrations of 22 alkanes (C14H30-C33H68) measured by the GC × GC were summed and all alkanes were considered as one compound with Mr of 113.133 amu as all alkanes are detected with the PTR-MS at the same set of masses (43.055, 57.070, 71.086 amu, and a few other masses). This resulted in the decrease of the GC × GC data set from 153 to 132 known compounds.

Applying these rules we were able to match 123 of the 132 distinguishable compounds measured with the GC × GC technique to the corresponding 64 ions measured with the TD-PTR-MS technique. The contribution of the unidentified nine compounds is minor (< 2 %) compared to the total mass concentration of the 123 compounds. While we applied rather relaxed rules when attributing detected m/z values to known compounds, we found that in practice the matches were much closer than 250 ppm: the median difference for 64 ions was 41 ppm.

In Fig. 3 we present the time series of total OA mass concentrations measured by the in situ TD-PTR-MS and the AMS instruments (further named “total OA_PTR” and “total OA_AMS” respectively), the total concentration of the 123 compounds measured by the GC × GC, and the total concentration of the corresponding 64 masses measured by the TD-PTR-MS (further named “123 compounds_GC × GC” and “64 masses_PTR” respectively) over 2 days. The 64 masses constitute 25 % of the total OA mass measured by the TD-PTR-MS.

In general, the total OA_PTR and the total OA_AMS correlate well with a correlation coefficient (r2) of 0.84. The average percentage of the total OA detected by the TD-PTR-MS is 33 %. Potential reasons for undetected OA by the TD-PTR-MS, i.e. fragmentation in the PTR-MS and thermal decomposition in the CTD cell, have been discussed in Holzinger et al. (2013).

A reasonable qualitative and quantitative correlation is observed between the 123 compounds_GC × GC and the 64 masses_PTR for the Los Angeles area (r2= 0.67). On average, the TD-PTR-MS detected 98 % of the total mass of the “123 compounds_GC × GC”. However, one can notice that the correlation between the 123 compounds_GC × GC and the 64 masses_PTR is better during the first than the second day of the measurements. This might relate to a different wind direction during the second day and to the fact that the TD-PTR-MS and the HiVol filter sampler were located ∼ 200 m apart during the campaign. Indeed, the prevailing wind directions were northeast on May 30 and northwest and west on 31 May, based on 48 h back trajectories using the model HYSPLIT (Draxler and Rolph, 2013; Rolph, 2013).

Figure 4 presents mass concentrations of compounds measured with the in situ TD-PTR-MS technique vs. corresponding mass concentrations measured with the GC × GC technique (referred to as “PTR” and “GC × GC” respectively) averaged over the whole comparison period with 1 : 1 line shown for reference. Compound classes are shown according to the scheme introduced in Sect. 2.2.2.

The PTR / (GC × GC) ratio indicates the ratio of the amount of a substance measured by TD-PTR-MS and by GC × GC respectively. The thin black lines above and below the 1 : 1 line in Fig. 4 mark the 0.25 < PTR / (GC × GC) < 2.0 boundaries. Figure 4 indicates that many compounds fall into the range of 0.25 and 2.0, especially for compounds with mass concentrations above ∼ 2 ng m-3 as measured by GC × GC. Therefore we conclude that in general, the concentrations of organic species measured with the two techniques agree reasonably well above this threshold.

An accuracy of 54 % for TD-PTR-MS and 40 % for GC × GC is consistent with boundaries of 0.4 and 3.0 for the PTR / (GC × GC) ratio. The upper boundary (2.0) suggested by Fig. 4 is smaller than the upper boundary (3.0) suggested by the stated accuracy levels (54 % for TD-PTR-MS and 40 % for GC × GC). This may indicate that the stated accuracy levels are an overestimate of the real instrumental accuracy. The lower boundary (0.25) suggested by Fig. 4 is somewhat lower than the value expected from the stated accuracies (0.4). This may be caused by condensation of semivolatile gas-phase compounds on the large surface of the quartz filters, which is a well-known sampling artifact and constitutes a positive bias of the GC × GC data.

For some compounds (such as hopanes and oxygenated PAHs), GC × GC detects less than the TD-PTR-MS. In general, for compounds with mass concentrations below 2 ng m-3, the TD-PTR-MS method yielded substantially higher mass concentrations. It should be noted that the 132 compounds measured with the GC × GC technique represent about 10 % of the total OA mass, with another 5–10 % associated with the unresolved complex mixture (Chan et al., 2013). There are likely additional species not quantified with the GC × GC, that are detected as a sum by PTR-MS at the corresponding m/z values.

At the same time, for alkanes and one amide substantially lower concentrations were detected by TD-PTR-MS (the corresponding points are located at a substantial distance from 1 : 1 line). For alkanes this can be explained by the fact that the main masses at which alkanes are detected (43.055, 57.070, 71.086 amu) were not considered because large contamination from the gas phase did not allow to quantify the condensed fraction. More complicated fragmentation in the PTR-MS can likely explain the lower concentrations found for the amide (N,N-dibutylformamide). For all compounds of the class of alkanoic acids (except for decanoic acid), the concentrations were measured to be lower by the TD-PTR-MS, which is consistent with a positive sampling artifact that is common to quartz filter collection. The latter will be discussed in the following section.

The four alkanoic acids shown in Fig. 4 as black crosses in a red oval are n-dodecanoic, n-tridecanoic, n-tetradecanoic, and n-hexadecanoic acids. These four compounds are among the most abundant species measured by the GC × GC (three among the seven compounds with the highest concentrations, see Fig. 4). To calculate the mass concentrations of the alkanoic acids measured with the in situ TD-PTR-MS technique, only the intensity of the parent ion signal was considered and multiplied by the calibration factor (1.45) developed for alkanoic acids (see Sect. 3.1). Even after applying this correction factor, the PTR / (GC × GC) ratios for the acids are below unity (Table 2). The semivolatile nature of the acids is a possible reason for this disagreement. If vapours of these acids condensed on the quartz filter during sampling an overestimation of the mass concentrations obtained with the GC × GC technique could be explained. This hypothesis is supported by the fact that positive artifacts have been shown to be more severe on filters with short air sampling duration. For example, Timkovsky et al. (2015) demonstrated substantial positive filter sampling artifacts on filters sampled for 24 h, which were much reduced with sampling durations of 48 and 72 h.

The fraction of the amount of a compound in the particle phase (Fp,i, amount in the particle phase divided by the total amount in the particle and the gas phase) can be calculated according to the procedure described by e.g. Yatavelli et al. (2014). Compounds for which Fp,i is significantly lower than unity are considered to be semivolatile. Fp,i is calculated based on the activity coefficient, vapour pressure, ambient temperature, and total OA mass concentration. We used the average activity coefficient calculated for alkanoic acids (1.6) based on Chandramouli et al. (2003). If a component has an activity coefficient above unity within a mixture, the component has a weaker interaction with other molecules in the condensed phase than with itself, and its effective vapour pressure is higher than the pure component vapour pressure. Vapour pressures for do-, tetra-, and hexadecanoic acids (Table 2) have been measured by Cappa et al., 2008. Assuming that the logarithm of the vapour pressure of an alkanoic acid has a linear dependency on the number of carbon atoms in the molecule (Goldstein and Galbally, 2007), we calculate the vapour pressure for tridecanoic acid to be 3.2 × 10-6 hPa using known vapour pressures of do-, tetra-, and hexadecanoic acids (Table 2). Using an ambient temperature of 25 ∘C and total OA of 9.4 µg m-3 (measured by the AMS), Fp,i was found to be substantially lower than unity for three out of the four acids (do-, tri-, and tetradecanoic acids, Table 2). This confirms their semivolatile nature and their potential to cause positive filter sampling artifacts when gas-phase molecules condense on the large surface of the quartz filters. This is also consistent with Sihabut et al. (2005) who observed a high contribution from gas-phase to particle-phase measurements on filters of alkanoic acids containing between 10 and 14 carbon atoms.

Since Fp,i for hexadecanoic acid is unity, it is expected to be fully in the particle phase and is not prone to positive filter sampling artifacts (Table 2). This is again consistent with Sihabut et al. (2005), who showed that only a little contribution to particle-phase measurements is observed from gas phase with filter sampling of alkanoic acids containing between 15 and 18 carbon atoms. The PTR / (GC × GC) ratio (0.53) for hexadecanoic acid is within the expected range (0.4–3.0) given by the combined accuracies of TD-PTR-MS and GC × GC. However, further study is needed to exclude the possibility that this low ratio may have resulted from a negative sampling artifact for the in situ TD-PTR-MS.

The full 2-day time series for the four alkanoic acids obtained with the TD-PTR-MS and the GC × GC method are presented in Fig. 5. The clear diurnal cycle detected by the TD-PTR-MS for all four acids is consistent with the diurnal variation of semivolatile compounds observed by the TD-PTR-MS (Holzinger et al., 2013) and the AMS (Hayes et al., 2013) during the same field campaign. The highest correlation coefficient (r, 0.69) between the TD-PTR-MS and GC × GC measurements is observed for hexadecanoic acid among the four acids (Table 2), which is the most abundant and least volatile compound within this compound group. It is mainly present in the particle phase and thus is not subject to a positive filter sampling artifact (Fig. 5d). Poor correlation is observed for tri- and tetradecanoic acid (Fig. 5b and c respectively) which may be caused by the semivolatile nature of the acids and potential measurement artifacts by the GC × GC technique. The latter may be due to the fact that the acids were not derivatized prior to GC × GC analysis. For dodecanoic acid a reasonable qualitative correlation is observed. However, quantitative agreement is poorer than for the other measured acids, which is likely caused by the relatively high volatility of the acid. As shown in Timkovsky et al. (2015), the larger concentrations measured by GC × GC may be caused by condensation of the gas-phase fraction of the acids during filter sampling. The poor correlation for tri- and tetradecanoic acid is currently not understood.