Open AccessAtmosphericChemistryand PhysicsAtmos. Chem. Phys., 14, 4531–4538, 94/acp-14-4531-2014 Author(s) 2014. CC Attribution 3.0 License.Hydroxymethanesulfonic acid in size-segregated aerosol particles atnine sites in GermanyS. Scheinhardt, D. van Pinxteren, K. Müller, G. Spindler, and H. HerrmannLeibniz-Institut für Troposphärenforschung (TROPOS), Permoserstraße 15, 04318 Leipzig, GermanyCorrespondence to: H. Herrmann ([email protected])Received: 22 November 2013 – Published in Atmos. Chem. Phys. Discuss.: 10 December 2013Revised: 17 March 2014 – Accepted: 19 March 2014 – Published: 7 May 2014Abstract. In the course of two field campaigns, sizesegregated particle samples were collected at nine sites inGermany, including traffic, urban, rural, marine and mountain sites. During the chemical characterisation of the samples some of them were found to contain an unknown substance that was later identified as hydroxymethanesulfonicacid (HMSA). HMSA is known to be formed during the2 reaction of S(IV) (HSO 3 or SO3 ) with formaldehyde inthe aqueous phase. Due to its stability, HMSA can act as areservoir species for S(IV) in the atmosphere and is therefore of interest for the understanding of atmospheric sulfurchemistry. However, no HMSA data are available for atmospheric particles from central Europe, and even on a worldwide scale data are scarce. Thus, the present study now provides a representative data set with detailed information onHMSA concentrations in size-segregated central Europeanaerosol particles. HMSA mass concentrations in this data setwere highly variable: HMSA was found in 224 out of 738samples (30 %), sometimes in high mass concentrations exceeding those of oxalic acid. On average over all 154 impactor runs, 31.5 ng m 3 HMSA was found in PM10 , contributing 0.21 % to the total mass. The results show that theparticle diameter, the sampling location, the sampling season and the air mass origin impact the HMSA mass concentration. Highest concentrations were found in the particlefraction 0.42–1.2 µm, at urban sites, in winter and with eastern (continental) air masses, respectively. The results suggestthat HMSA is formed during aging of pollution plumes. Apositive correlation of HMSA with sulfate, oxalate and PMis found (R 2 0.4). The results furthermore suggest that thefraction of HMSA in PM slightly decreases with increasingpH.1IntroductionIn the course of two field campaigns (REGKLAM andGUAN, see Scheinhardt et al., 2013a, and Birmili et al.,2009), numerous size-segregated particle samples were collected at nine sites in Germany. During the chemical characterisation of the samples a fraction of the samples were foundto contain hydroxymethanesulfonic acid (HMSA, HO–CH2 –SO3 H).HMSA and other hydroxyalkanesulfonic acids (HASA;HO–CHR–SO3 H with R -(CH2 )n H) were suggested to beinvolved in atmospheric sulfur chemistry, notably regardingsulfate formation. Sulfate is one of the main constituentsof atmospheric aerosol particles. It is mainly formed fromthe oxidation of S(IV) by H2 O2 in the atmospheric aqueous phase and especially in cloud droplets. Dissolved S(IV)originates from gaseous SO2 , which, in central Europe, ismainly emitted by human activities, e.g. coal burning. SO2dissolution is described by a Henry’s law equilibrium, followed by the formation of hydrogen sulfite (HSO 3 ) or sulfite (SO2 ).However,invariousfieldstudies,ithas been3observed that S(IV) concentrations in the atmospheric aqueous phase were often higher than expected from Henry’s law(Richards et al., 1983; Munger et al., 1984). Rapidly, the re2 action of dissolved SO2 /HSO 3 /SO3 with dissolved aldehydes, yielding hydroxyalkanesulfonic acids, was identified asan explanation for this observation (Munger et al., 1984). Hydroxymethanesulfonic acid turned out to be the most important HASA since formaldehyde, its organic precursor, is themost common aldehyde in the atmospheric aqueous phase(Olson and Hoffmann, 1989; Takeuchi et al., 2001).Published by Copernicus Publications on behalf of the European Geosciences Union.

4532S. Scheinhardt et al.: Hydroxymethane acid in size-segregated aerosolFig. 1. Formation pathways of HMSA (Munger et al., 1984; Olsonand Hoffmann, 1989). All reactions are equilibrium reactions. Thecircle indicates an atmospheric droplet.An overview of the HMSA formation mechanism is givenin Fig. 1. HMSA formation is most rapid at pH 5, butHMSA decomposition as well is enhanced at higher pH. Thehighest stability is thus observed at acidic pH (Sorensen andAndersen, 1970; Munger et al., 1986). HMSA is comparablystable and not readily oxidised (Hoigné et al., 1985; Martin et al., 1989; Ojo et al., 2004). It therefore accumulatesin the atmosphere and can be found in high concentrations.Katagiri et al. (1996) found hydroxymethanesulfonate to bethe third most important anion in dew samples collected inJapan. It has been suggested that the high HMSA concentrations may act as a reservoir species for atmospheric S(IV)(Richards et al., 1983; Munger et al., 1984 and 1986; Voisinet al., 2000) and might thus impact S(IV) oxidation kinetics(McArdle and Hoffmann, 1983) by shielding S(IV) from thedirect oxidation by non-radical oxidants such as H2 O2 .The HMSA data set obtained here was evaluated becauseHMSA, on the one hand, is of great interest for the understanding of S(IV) oxidation processes but, on the otherhand, has only rarely been quantified in atmospheric particles (Suzuki et al., 2001). The aim of the present study isthus to investigate HMSA concentrations in size-segregatedatmospheric aerosol particles at selected sites in central Europe in order to identify the main factors determining HMSAmass concentrations. To this end, the influences of particlediameter, sampling location, sampling season and air massorigin are studied.Fig. 2. Sampling sites (see Scheinhardt et al., 2013b). See text forcoding explanation.2Material and methods2.1Particle samplingSize-segregated aerosol particle samples were collected intwo field campaigns in Germany in 2009 and 2010 as described by Scheinhardt et al. (2013b). Sampling was performed on ring-like aluminium foils using five-stage Bernerimpactors. Inside the impactors, particles are separated depending on their aerodynamic diameter, the 50 % cut-offs being 0.05–0.14–0.42–1.2–3.5–10 µm for impactor stages 1–5,respectively. Sampling was performed at nine sites in Germany (Fig. 2, see Birmili et al., 2009):1. Leipzig-Eisenbahnstraße (LE; 51.34 N, 12.37 E,119 m a.s.l.) is a traffic site located about half-way between Leipzig city centre and Leipzig-TROPOS (LT)in a street canyon aligned in the west–east direction.The air inlet is installed about 6 m above the groundlevel at the northern side of the canyon.2. Leipzig-TROPOS(LT;51.35 N,12.43 E,125 m a.s.l.) is an urban site located at the roofof TROPOS building, 4 km northeast of the city centreof Leipzig (530 000 residents). The air inlet is installed16 m above the ground level. The site is not directlyaffected by local particle sources like traffic.3. Dresden-Winckelmannstraße(DD;51.04 N, 13.73 E, 112 m a.s.l.) is an urban site in theAtmos. Chem. Phys., 14, 4531–4538,

S. Scheinhardt et al.: Hydroxymethane acid in size-segregated aerosolcity of Dresden (530 000 residents). The site is notdirectly affected by local particle sources like traffic.4. Augsburg (AU; 48.36 N, 10.91 E, 484 m a.s.l.) is anurban site located about 1 km southeast of the city centre of Augsburg (270 000 residents). There are no localparticle sources nearby.5. Melpitz (ME; 51.54 N, 12.93 E, 86 m a.s.l.) is awell-characterised rural site in the German lowlands(Spindler et al., 2010, 2012, 2013). It is surrounded bymeadows.6. Oberbärenburg (OB; 50.78 N, 13.72 E, 735 m a.s.l.)is a rural site 30 km south of Dresden. It is located in alow mountain range and is surrounded by forests.7. Bösel (BÖ; 53.00 N, 7.96 E, 16 m a.s.l.) is a ruralsite located 30 km southwest of Oldenburg (160 000residents), 80 km south of the North Sea coast.8. Schauinsland (SI; 47.91 N, 7.91 E, 1205 m a.s.l.) isa rural site located on the mountain with the samename, around 10 km southeast of Freiburg im Breisgau (220 000 residents), in the Black Forest.9. Hohenpeißenberg(HP;47.80 N,11.00 E,988 m a.s.l.) is a rural site located 50 km south ofMunich on top of a hill that rises about 300 m abovethe surrounding landscape.Sampling was started whenever the weather forecast predicted favourable meteorological conditions (i.e. no precipitation and a constant air mass origin throughout the samplingday). The sampling period was 24 h, corresponding to a sampled air volume of 108 m3 .Prior to use, the aluminium foils were pre-heated at 300 Cfor at least 2 hours in order to remove organic traces. Aftersampling, the foils were stored at 21 C until analysis.2.2Classification of sampling daysOut of the complete data set, only sampling days with unambiguous air mass origins were selected and chemicallyanalysed. This was the case for 30 sampling days, corresponding to 156 impactor runs or 738 Berner impactorfoils. Depending on the season and the air mass origin during sampling, sampling days were classified into six categories: Winter West (WiW), Winter North (WiN), WinterEast (WiE), Summer West (SuW), Summer North (SuN),and Summer East (SuE). This empirical categorisation hasproven to be successful in former studies in central Europe (Spindler et al., 2010, 2012, 2013) and considers differences in PM mass concentrations and compositions underdifferent meteorological conditions. Summer included sampling days between May and October and winter includeddays between November and April. The air mass originwas determined using 96-hour-backward trajectories NOAA HYSPLIT transport model (Hybrid Single Particle Lagrangian Integrated Trajectory Model; Draxler andHess, 1998; Backward trajectories were calculated for each sampling day andeach sampling site at 10:00 CET and 18:00 CET for arrivalheights of 200, 500 and 1000 m above the ground level. Anoverview of the sampling events is given in the Supplementary Material.2.3Weighing and chemical analysisThe determination of the collected particle mass was donegravimetrically using a microbalance (UMT-2, MettlerToledo, Switzerland) after at least 48 h of equilibration at(20 1) C and a relative humidity of (50 5) %. Afterweighing, the foils were cut with ceramic scissors and chemically analysed.Chemical analysis was performed after extraction of analiquot of an aluminium foil in 2 ml of deionised water( 18 M cm) following a standard protocol (10 min shaking, 10 min ultrasonic treatment, 10 min shaking). After extraction, the solution was filtered using syringe filters with apore size of 0.45 µm.Main inorganic ions (chloride, nitrate, sulfate) were determined from the aqueous extracts using ion chromatography with conductivity detection (ICS3000, Dionex). Cationswere separated applying a methylsulfonic acid eluent anda CS16 (3 mm) column, anions were separated applying aKOH eluent in a AS18 (2 mm) column. Calibration was donedaily, using a four-point standard (Fluka, Switzerland).HMSA and oxalate were determined from the aqueous extracts using a capillary electrophoresis method described byKramberger-Kaplan (2003) and Scheinhardt et al. (2013b).In brief, a background electrolyte consisting of 2 mM 5sulfosalicylic acid, 8 mM tris(hydroxymethyl)aminomethaneand 0.001 % hexadimethrine bromide at a pH of 8.2 wasused in an Agilent 7100 capillary electrophoresis system.An uncoated fused-silica capillary with an inner diameterof 75 µm and a total length of 80.5 cm (72 cm to the detector) was used. The capillary was maintained at 20 C.Separation of HMSA from other compounds was reachedwithin 13 min, applying a voltage of 30 kV (corresponding to a current of about 5 µA) following hydrodynamicsample injection with 750 mbar s (corresponding to 1 % ofthe capillary length). Indirect UV detection with a measurement wavelength of 260 nm (bandwidth: 20 nm), a referencewavelength of 208 nm (bandwidth: 36 nm) and a time resolution of 20 Hz was used for quantification. Migration timesand peak areas were very well reproducible (n 10, RSD0.08 % and 2.16 %, respectively). No significant blank valueswere found. The HMSA detection limit (LOD; three timesthe standard deviation of the background signal) was 1.15 µm(127 ppb) and the quantification limit (LOQ; LOQ 3 LOD)was 3.44 µm (382 ppb).Atmos. Chem. Phys., 14, 4531–4538, 2014

4534S. Scheinhardt et al.: Hydroxymethane acid in size-segregated aerosolHMSA concentrations above the LOD were found in 224of 738 samples. From these 224 samples, 157 had HMSAconcentrations between the LOD and the LOQ. Even thoughconcentrations in this range are less reliable, they were stillconsidered in this study, since the concentrations uncertainties associated with such concentrations are regarded lessproblematic than setting all those HMSA concentrations tozero. Thus, in the present study, all HMSA concentrationsabove the LOD are reported. In atmospheric units, the LODcorresponds to mass concentrations of about 6–7 ng m 3 (depending on the impactor stage and the extraction protocol).Values below the LOD were, however, taken as zero. TheHMSA quantification method in this study has a slightly better LOD than the method applied by Suzuki et al. (2001),who for the first time investigated particulate HMSA by 1 HNMR.The amount of organic and elemental carbon (OC/EC)was quantified using a thermographic method (C-mat 5500,Ströhlein, Germany) following VDI guideline 2465, Part 2with minor modifications. In the first step, an aliquot of analuminium foil was heated to 650 C in a nitrogen atmosphere. Under these conditions, organic compounds evaporated and were flushed towards a CuO catalyst. There theywere oxidised, forming CO2 that was subsequently quantified with a NDIR detector (OC). In the next step, after cooling to at least 75 C, the foil was again heated to 650 Cin an oxygen atmosphere oxidising EC, which was quantified as CO2 , too. Compared to other methods, this methodtends to overestimate EC and to underestimate OC (Schmidet al., 2001). However, due to the melting point of aluminium (660 C), the method is limited to a temperature of650 C, and thermo-optical methods applying the EUSAAR2, NIOSH or IMPROVE protocols cannot be applied. For details see Spindler et al. (2012).2.4Particle aqueous phase pHThe charge balance in our particle samples, considering main2 2 2 inorganic ions (NH 4 , Na , K , Ca , Mg , SO4 , NO3 ,Cl ) and organic ions (oxalate, malonate, tartronate, succinate, malate and hydroxymethanesulfonate), was generallyequalised and varied only within analytical errors. Contraryto recent studies from China (Cheng et al., 2011; Zhou et al.,2012), charge balances could thus not be applied to calculateparticle aqueous phase pH in this study. They were insteaddetermined applying measurements of the aqueous extracts’pH and a model that was able to calculate the particle liquidwater content. The particle aqueous phase pH was then calculated back from the pH of the aqueous extracts, assumingits dilution due to the extraction protocol mentioned above.A comparable approach was applied by Li et al. (1997) andKeene and Savoie (1998).The pH of the aqueous extracts was determined using amicro pH electrode (PHR 146S microelectrode, Lazar Research Laboratories, Los Angeles, USA). The particle waAtmos. Chem. Phys., 14, 4531–4538, 2014ter content was calculated using the E-AIM model (Wexlerand Clegg, 2002;, which has beenshown to give good agreement with measured data (Engelhart et al., 2011). In former studies, E-AIM Model III wasfound to be the most suitable E-AIM type for our samples(Scheinhardt et al., 2013b). The average relative humidityduring the measurement and the mass concentrations of the2 main constituents (NH 4 , Na , SO4 , NO3 , and Cl ) are themodel input parameters. The mass concentrations of H andOH were adjusted to ensure the particles’ charge neutrality.The formation of insoluble solids was enabled in the calculation. The model output provided the water content.33.1Results and discussionMass concentrations and contributions to PMThe HMSA mass concentrations determined in atmosphericsamples were highly variable within the set of 224 samples with HMSA above the detection limit (out of a total of738 samples, see Supplement). The highest observed valueof 625 ng m 3 was determined in a sample from Augsburgon December 16, 2009 under Winter-East conditions on impactor stage 2. Since this value was disproportionally high(more than 2.5 times higher than the second largest value), itwas identified as an outlier, most likely due to a local pollution episode at that site (analytical errors were excluded bymeans of repetition experiments). It was therefore regardedas being not representative and is thus not considered in thefollowing discussions. Concentrations below detection limitwere set to zero for all calculations.The data set was investigated regarding the influence ofsampling location, particle diameter and the meteorological conditions on HMSA mass concentrations (Fig. 3). Although some exceptions exist, urban sites generally exhibitedhigher HMSA mass concentrations than rural sites (Fig. 3a;average of the urban sites: 37.9 ng m 3 , average of the rural sites: 23.8 ng m 3 ). Moreover, rural lowland sites showhigher HMSA mass concentrations than mountain sites. Generally, HMSA mass concentrations in central Europe seemto be slightly higher than those in Japan (14.7 ng m 3 in urban aerosols, 1998–1999; Suzuki et al., 2001). To judge onthe relative contribution of HMSA to total PM, the fractionof HMSA in PM was calculated and compared for differentsample types (Fig. 4). We found HMSA to be enriched bya factor of 1.23 in urban samples (2.21 1.08 ‰ vs. 1.79 0.80 ‰, Fig. 4a). Even though this is not a statisticallysignificant difference, it is consistent with the precursors ofHMSA originating from anthropogenic emissions.Regarding the impact of the particle diameter, a strong dependency was observed, the highest HMSA mass concentrations being found on impactor stage 3 (Fig. 3b). A similarsize distribution was found by Suzuki et al. (2001). Sinceparticles of that size have the longest atmospheric

S. Scheinhardt et al.: Hydroxymethane acid in size-segregated aerosol4535Fig. 3. (a) HMSA mass concentrations in PM10 (sum of impactor stages 1–5) as a function of the sampling site. (b) HMSA mass concentrations as a function of particle size. (c) HMSA mass concentrations in PM10 (sum of impactor stages 1–5) as a function of the meteorologicalcategory. Boxes indicate the 25 %, 50 % and 75 % quartiles, whiskers indicate the minimum and maximum values. indicates the respectivemean value. The number of samples is given below each column. See text for discussion.Fig. 4. (a) Fraction of HMSA in PM10 (sum of impactor stages 1–5) as a function of the sampling site. (b) Fraction of HMSA in PM as afunction of the particle size. (c) Fraction of HMSA in PM10 (sum of impactor stages 1–5) as a function of the meteorological category. Thesymbols and numbers of samples are consistent with Fig. 3. See text for discussion.and can therefore be transported over long distances, this observation might suggest that HMSA was formed during theaging of particles, e.g. in pollution plumes, and/or in fog orcloud processing prior to the sampling time of the aerosolparticles. In fact, from its abundance in single particles withdiameter 0.7 µm and its coincidence with periods of fog orhigh relative humidity, HMSA has been suggested to represent a tracer for fog processing (Whiteaker and Prather, 2003;Healy et al., 2012). In contrast to impactor stage 3, shortlived particles (stages 1 and 5) contain only small amountsof HMSA. The relative contribution to PM is also largest instages 2 and 3 (Fig. 4b). However, this might be due to thelow absolute amounts of HMSA on the other stages, whichmakes the calculation of the HMSA/PM fraction on thosestages susceptible to errors.Regarding the air mass origin, highest HMSA concentrations in total PM10 (sum of impactor stages 1–5) were observed for eastern (continental) air mass origins (Fig. 3c;West: 23.9 ng m 3 , North: 7.7 ng m 3 , East: 51.0 ng m 3 ).This reflects the high HMSA precursor concentrations(VOCs, SO2 ) in continental air masses and shows that thehigh anthropogenic SO2 emissions in eastern Europe notonly promote the formation of main inorganic PM constituents such as sulfate, but might also enhance the formation of organic trace compounds such as HMSA. It can bepresumed that similar conclusions could be drawn from theanalysis of other sulfur-containing organic compounds, the fraction of HMSA in PM 10 it again appears thatHMSA is enriched in some samples (Fig. 4c).In summer, HMSA mass concentrations were lowerthan in winter (Fig. 3c; Summer: 23.9 ng m 3 , Winter:31.2 ng m 3 ), most likely again due to stronger anthropogenic emissions of HMSA precursors, e.g. from individualheating systems.It should be noted that, besides emissions, the meteorological conditions in winter and during eastern air massorigin (i.e. low mixing layer height, low temperatures, decreased turbulence, few precipitation) generally favour highPM loads, too. Clearly, this applies for HMSA as well as forall the other PM constituents and might partly explain theHMSA concentration differences observed between the seasons and air mass origins.3.2Correlations with other parametersIn the previous section, absolute HMSA mass concentrations were found to show dependencies generally resembling the behaviour of total PM (i.e. highest concentrationson impactor stage 3, at urban sites, in winter and with eastern air mass origins, respectively; see Spindler et al., 2010and 2012; Scheinhardt et al., 2013a). This is to some extent confirmed by correlating HMSA mass concentration andthe total PM mass concentration (Fig. 5a). The correlationof HMSA with sulfate (Fig. 5b) is comparably strong, confirming that both HMSA and sulfate are formed from theAtmos. Chem. Phys., 14, 4531–4538, 2014

4536S. Scheinhardt et al.: Hydroxymethane acid in size-segregated aerosolFig. 5. Correlations of HMSA mass concentrations with (a) the respective PM mass concentration and (b–f) the mass concentrations ofsulfate, oxalate, organic carbon, elemental carbon and potassium, respectively. In (f), R 2 is given with and without the consideration of anoutlier (0.96 ng m 3 K ). The charts comprise the complete data set (738 samples). See text for discussion.Fig. 6. (a) HMSA mass concentrations and (b) fractions of HMSA in PM as functions of the particle liquid phase pH. The charts compriseonly samples with HMSA concentration above the detection limit (224 samples). See text for discussion.same precursor (SO2 ) and under comparable conditions (secondary formation in the atmospheric aqueous phase). Oxalate, which is photochemically formed from higher organiccompounds, also correlates with HMSA (Fig. 5c). This alsomight be interpreted in a way that both substances are formedunder comparable conditions (secondary formation in the atmospheric aqueous phase via photochemical multiphase oxidation processes occurring in pollution plumes), even thoughboth substances have different precursors.Oxalate is one of the main contributors to the sum parameter OC, but the correlation of HMSA with OC is weak(Fig. 5d). This is probably due to the various sources ofthe different OC components, including, for example, directemissions. Traffic emissions, which are characterised by highEC (diesel soot) content, seem to affect HMSA concentrations only to a minor extent (Fig. 5e). The same is true forpotassium, a tracer for biomass burning (Fig. 5f). The combination of Fig. 4b–f suggests that HMSA is formed in sulfurrich aged air masses. Future discussions of HMSA formation should include direct SO2 and formaldehyde measureAtmos. Chem. Phys., 14, 4531–4538, 2014ments to identify possible impacts on HMSA concentrationsin more detail.3.3Impact of pHSince the formation and degradation kinetics of HMSA ispH-dependent, the impact of the particle aqueous phasepH on HMSA mass concentrations was investigated. In ourstudy, particle aqueous phase pH was found to vary betweenabout 0.5 and 2.5. These values are somewhat higher thanthose found in Los Angeles’ particles ( 2.4–0.2; Li et al.,1997) and lower than pH in marine particles (2.48–3.48;Keene and Savoie, 1998). The impact of pH on absoluteHMSA mass concentrations is weak (Fig. 6a; R 2 and slopeclose to zero). Interestingly, the fraction of HMSA in PMshows a small dependency and decreases with increasing pH.Although the scatter is large, this is in qualitative agreementwith the decreasing stability of HMSA at increasing pH. Inour samples, an empiric relationship of f 0.16 pH 0.72was found between pH 0 and 4, with f being the

S. Scheinhardt et al.: Hydroxymethane acid in size-segregated aerosolage of HMSA in PM10 . It has to be noted that HMSA formation is much more effective at pH levels higher than theones estimated for the samples of this study. Olson and Hoffmann, 1989, predict the formation rate to strongly increaseat pH above approx. 4.5. It is thus plausible to assume thatthe formation of HMSA might have taken place at higher pHin cloud and fog water. Upon cloud/fog dissipation, HMSAwill then be stabilised due to a much lower water content (i.e.higher apparent acidity) of the residual particles.4SummaryThe present study presents data from a unique data set regarding HMSA concentrations in size-segregated ambientaerosol particles. HMSA mass concentrations were found tobe highly variable. Highest concentrations were found in urban environments during winter and eastern advection onBerner impactor stage 3 (Dp 0.42–1.2 µm). The fractionof HMSA in PM generally showed similar trends. HMSAconcentrations correlated with sulfate (R 2 0.53), oxalate(R 2 0.46) and PM (R 2 0.42) mass concentrations. Correlations with EC (R 2 0.37), OC (R 2 0.23) and potassium (R 2 0.23/0.06) were also observed, but to a lesserextent. The fraction of HMSA in PM seems to be slightly influenced by pH, possibly due to its pH-dependent stability.Overall, the results are consistent with well-known aqueousphase formation of HMSA in polluted air masses from anthropogenic precursors SO2 and formaldehyde.Acknowledgements. This study has been supported by the German Federal Ministry of Education and Research under grantno. 01LR0802 (REGKLAM) and by the German Federal Environment Ministry under grant no. F&E 370343200 (GUAN). We thankall our project partners in GUAN and REGKLAM. We would alsolike to thank our technical staff – A. Dietze, S. Fuchs, A. Grüner, R.Rabe and A. Thomas – as well as two undergraduate students, A.Rau (Universität Leipzig) and E. Charlesworth (Seattle University),for whom DAAD RISE support is gratefully acknowledged. Inputof two anonymous reviewers to our paper is much appreciated aswell.Edited by: S. A. NizkorodovReferencesBirmili, W., Weinhold, K., Nordmann, S., Wiedensohler, A.,Spindler, G., Müller, K., Herrmann, H., Gnauk, T., Pitz, M.,Cyrys, J., Flentje, H., Nickel, C., Kuhlbusch, T. A. J.,Löschau, G., Haase, D., Meinhardt, F., Schwerin, A., Ries, L.,and Wirtz, K.: Atmospheric aerosol measurements in the German Ultrafine Aerosol Network (GUAN), Gefahrst. Reinhalt. L.,69, 137–145, 2009.Cheng, S. H., Yang, L. X., Zhou, X. H., Xue, L. K., Gao, X. M.,Zhou, Y., and Wang, W. X.: Size-fractionated water-soluble ions, pH and water content in aerosol on hazy days and the influences on visibility impairment in Jinan, China, Atmos. Environ.,45, 4631–4640, 2011.Draxler, R. R. and Hess, G. D.: An overview of the HYSPLIT 4modelling system for trajectories, dispersion and deposition,Aust. Meteorol. Mag., 47, 295–308, 1998.Engelhart, G. J., Hildebrandt, L., Kostenidou, E., Mihalopoulos, N.,Donahue, N. M., and Pandis, S. N.: Water content of agedaerosol, Atmos. Chem. Phys., 11, 911–920, doi:10.5194/acp-11911-2011, 2011.Healy, R. M., Sciare, J., Poulain, L., Kamili, K., Merkel, M., Müller,T., Wiedensohler, A., Eckhardt, S., Stohl, A., Sarda-Estève, R.,McGillicuddy, E., O’Connor, I. P., Sodeau, J. R., and Wenger, J.C.: Sources and mixing state of size-resolved elemental carbonparticles in a European megacity: Paris, Atmos. Chem. Phys., 12,1681–1700, doi:10.5194/acp-12-1681-2012, 2012.Hoigné, J., Bader, H., Haag, W. R., and Staehelin, J.: Rate constantsof reactions of ozone with organic and inorganic compounds inwater – III. Inorganic compounds and radicals, Water Res., 19,993–1004, 1985.Katagiri, Y., Sawaki, N., Arai, Y., Okochi, H., and Igawa, M.: Enhanced dissolution of SO2 into dewwater by forming hydroxyalkanesulfonate, Chem. Lett., 3, 197–198, 1996.Keene, W. C. and Savoie, D. L.: The pH of deliquesced sea-saltaerosol in polluted marine air, Geophys. Res. Lett., 25, 2181–2184, 1998.Kramberger-Kaplan, H. V.: Carbonsäuren und Dicarbonsäuren inatmosphärischen Mehrphasenprozessen, Ph.D. Thesis, Technische Universität Darmstadt, Darmstadt, Germany, 2003.Li, S. M., Macdonald, A. M., Strapp, J. W., Lee, Y. N., andZhou, X. L.: Chemical and physical characterizations of atmospheric aerosols over Southern California, J. Geophys. Res.Atmos., 102, 21341–21353, 1997.Martin, L. R., Easton, M. P., Foster, J. W., and Hill, M. W.: Oxidation of hydroxymethanesulfonic acid by Fenton’s reagent, Atmos. Environ., 23, 563–568, 1989.McArdle, J. V. and Hoffmann, M. R.: Kinetics and mecha

1. Leipzig-Eisenbahnstraße (LE; 51.34 N, 12.37 E, 119 ma.s.l.) is a traffic site located about half-way be-tween Leipzig city centre and Leipzig-TROPOS (LT) in a street canyon aligned in the west–east direction. The air inlet is installed about 6 m abov