Tropical and Boreal Forest – Atmosphere Interactions: A Review

Paulo Artaxo; Hans-Christen Hansson; Meinrat O. Andreae; Jaana Bäck; Eliane Gomes Alves; Henrique M. J. Barbosa; Frida Bender; Efstratios Bourtsoukidis; Samara Carbone; Jinshu Chi; Stefano Decesari; Viviane R. Després; Florian Ditas; Ekaterina Ezhova; Sandro Fuzzi; Niles J. Hasselquist; Jost Heintzenberg; Bruna A. Holanda; Alex Guenther; Hannele Hakola; Liine Heikkinen; Veli-Matti Kerminen; Jenni Kontkanen; Radovan Krejci; Markku Kulmala; Jost V. Lavric; Gerrit de Leeuw; Katrianne Lehtipalo; Luiz Augusto T. Machado; Gordon McFiggans; arco Aurelio M. Franco; Bruno Backes Meller; Fernando G. Morais; Claudia Mohr; William Morgan; Mats B. Nilsson; Matthias Peichl; Tuukka Petäjä; Maria Praß; Christopher Pöhlker; Mira L. Pöhlker; Ulrich Pöschl; Celso Von Randow; Ilona Riipinen; Janne Rinne; Luciana V. Rizzo; Daniel Rosenfeld; Maria A. F. Silva Dias; Larisa Sogacheva; Philip Stier; Erik Swietlicki; Matthias Sörgel; Peter Tunved; Aki Virkkula; Jian Wang; Bettina Weber; Ana Maria Yáñez-Serrano; Paul Zieger; Eugene Mikhailov; James N. Smith; Jürgen Kesselmeier
2022 | TELLUS B | 74 (24-163)

This review presents how the boreal and the tropical forests affect the atmosphere, its chemical composition, its function, and further how that affects the climate and, in return, the ecosystems through feedback processes. Observations from key tower sites standing out due to their long-term comprehensive observations: The Amazon Tall Tower Observatory in Central Amazonia, the Zotino Tall Tower Observatory in Siberia, and the Station to Measure Ecosystem-Atmosphere Relations at Hyytiäla in Finland. The review is complemented by short-term observations from networks and large experiments.

The review discusses atmospheric chemistry observations, aerosol formation and processing, physiochemical aerosol, and cloud condensation nuclei properties and finds surprising similarities and important differences in the two ecosystems. The aerosol concentrations and chemistry are similar, particularly concerning the main chemical components, both dominated by an organic fraction, while the boreal ecosystem has generally higher concentrations of inorganics, due to higher influence of long-range transported air pollution. The emissions of biogenic volatile organic compounds are dominated by isoprene and monoterpene in the tropical and boreal regions, respectively, being the main precursors of the organic aerosol fraction.

Observations and modeling studies show that climate change and deforestation affect the ecosystems such that the carbon and hydrological cycles in Amazonia are changing to carbon neutrality and affect precipitation downwind. In Africa, the tropical forests are so far maintaining their carbon sink.

It is urgent to better understand the interaction between these major ecosystems, the atmosphere, and climate, which calls for more observation sites, providing long-term data on water, carbon, and other biogeochemical cycles. This is essential in finding a sustainable balance between forest preservation and reforestation versus a potential increase in food production and biofuels, which are critical in maintaining ecosystem services and global climate stability. Reducing global warming and deforestation is vital for tropical forests.

Kinetics, SOA yields, and chemical composition of seconaary organic aerosol from beta-caryophyllene ozonolysis with and without nitrogen oxides between 213 and 313 K

Gao, LY; Song, JW; Mohr, C; Huang, W; Vallon, M; Jiang, F; Leisner, T; Saathoff, H
2022 | Atmos. Chem. Phys. | 22 (9) (6001-6020)
oxidation , sesquiterpene emissions
beta-caryophyllene (BCP) is one of the most important sesquiterpenes (SQTs) in the atmosphere, with a large potential contribution to secondary organic aerosol (SOA) formation mainly from reactions with ozone (O-3) and nitrate radicals (NO3). In this work, we study the temperature dependence of the kinetics of BCP ozonolysis, SOA yields, and SOA chemical composition in the dark and in the absence and presence of nitrogen oxides including nitrate radicals (NO3). We cover a temperature range of 213-313 K, representative of tropospheric conditions. The oxidized components in both gas and particle phases were characterized on a molecular level by a chemical ionization mass spectrometer equipped with a filter inlet for gases and aerosols using iodide as the reagent ion (FIGAERO-iodide-CIMS). The batch mode experiments were conducted in the 84.5 m(3) aluminium simulation chamber AIDA at the Karlsruhe Institute of Technology (KIT). In the absence of nitrogen oxides, the temperature-dependent rate coefficient of the endocyclic double bond in BCP reacting with ozone between 243-313 K is negatively correlated with temperature, corresponding to the following Arrhenius equation: k = (1.6 +/- 0.4) x 10(-15) x exp((559 +/- 97)/ T). The SOA yields increase from 16 +/- 5 % to 37 +/- 11 %, with temperatures decreasing from 313 to 243 K at a total organic particle mass of 10 mu g m(-3). The variation in the ozonolysis temperature leads to a substantial impact on the abundance of individual organic molecules. In the absence of nitrogen oxides, monomers C14-15H22-24O3-7 (37.4 %), dimers C28-30H44-48O5-9 (53.7 %), and timers C41_44H62_6609_11 (8.6 %) are abundant in the particle phase at 213 K. At 313 K, we observed more oxidized monomers (mainly C14-15H22-24O6-9, 67.5 %) and dimers (mainly C27-29H42-44O9-11, 27.6 %), including highly oxidized molecules (HOMs; C14H22O7,9C15H22O7,9C15H24O7,9), which can be formed via hydrogen shift mechanisms, but no significant timers. In the presence of nitrogen oxides, the organonitrate fraction increased from 3 % at 213 K to 12 % and 49 % at 243 and 313 K, respectively. Most of the organonitrates were monomers with Cis skeletons and only one nitrate group. More highly oxygenated organonitrates were observed at higher temperatures, with their signal-weighted O : C atomic ratio increasing from 0.41 to 0.51 from 213 to 313 K. New dimeric and timeric organic species without nitrogen atoms (C-20, C-35) were formed in the presence of nitrogen oxides at 298-313 K, indicating potential new reaction pathways. Overall, our results show that increasing temperatures lead to a relatively small decrease in the rate coefficient of the endocyclic double bond in BCP reacting with ozone but to a strong decrease in SOA yields. In contrast, the formation of HOMs and organonitrates increases significantly with temperature.

Ambio fit for the 2020s

Andersson, E; Boonstra, WJ; Castro, MD; Hughes, AC; Ilstedt, U; Jernelov, A; Jonsson, BG; Kalantari, Z; Keskitalo, C; Kritzberg, E; Katterer, T; McNeely, JA; Mohr, C; Mustonen, T; Ostwald, M; Reyes-Garcia, V; Rusch, GM; Bellamy, AS; Stage, J; Tedengren, M; Thomas, DN; Wulff, A; Soderstrom, B
2022 | Ambio | 51 (5) (1091-1093)

Fragmentation inside proton-transfer-reaction-based mass spectrometers limits the detection of ROOR and ROOH peroxides

Li, HY; Almeida, TG; Luo, YY; Zhao, J; Palm, BB; Daub, CD; Huang, W; Mohr, C; Krechmer, JE; Kurten, T; Ehn, M
2022 | Atmos. Meas. Tech. | 15 (6) (1811-1827)
chemical-ionization , chemistry , cyclohexene ozonolysis , energy-transfer , monoterpene , products , ro2 radicals , sulfuric acid , tof , voc emissions
Proton transfer reaction (PTR) is a commonly applied ionization technique for mass spectrometers, in which hydronium ions (H3O+) transfer a proton to analytes with higher proton affinities than the water molecule. This method has most commonly been used to quantify volatile hydrocarbons, but later-generation PTR instruments have been designed for better throughput of less volatile species, allowing detection of more functionalized molecules as well. For example, the recently developed Vocus PTR time-of-flight mass spectrometer (PTR-TOF) has been shown to agree well with an iodide-adduct-based chemical ionization mass spectrometer (CIMS) for products with 3-5 O atoms from oxidation of monoterpenes (C10H16). However, while several different types of CIMS instruments (including those using iodide) detect abundant signals also at "dimeric" species, believed to be primarily ROOR peroxides, no such signals have been observed in the Vocus PTR even though these compounds fulfil the condition of having higher proton affinity than water. More traditional PTR instruments have been limited to volatile molecules as the inlets have not been designed for transmission of easily condensable species. Some newer instruments, like the Vocus PTR, have overcome this limitation but are still not able to detect the full range of functionalized products, suggesting that other limitations need to be considered. One such limitation, well-documented in PTR literature, is the tendency of protonation to lead to fragmentation of some analytes. In this work, we evaluate the potential for PTR to detect dimers and the most oxygenated compounds as these have been shown to be crucial for forming atmospheric aerosol particles. We studied the detection of dimers using a Vocus PTR-TOF in laboratory experiments, as well as through quantum chemical calculations. Only noisy signals of potential dimers were observed during experiments on the ozonolysis of the monoterpene alpha-pinene, while a few small signals of dimeric compounds were detected during the ozonolysis of cyclohexene. During the latter experiments, we also tested varying the pressures and electric fields in the ionization region of the Vocus PTR-TOF, finding that only small improvements were possible in the relative dimer contributions. Calculations for model ROOR and ROOH systems showed that most of these peroxides should fragment partially following protonation. With the inclusion of additional energy from the ion-molecule collisions driven by the electric fields in the ionization source, computational results suggest substantial or nearly complete fragmentation of dimers. Our study thus suggests that while the improved versions of PTR-based mass spectrometers are very powerful tools for measuring hydrocarbons and their moderately oxidized products, other types of CIMS are likely more suitable for the detection of ROOR and ROOH species.

Influence of organic aerosol molecular composition on particle absorptive properties in autumn Beijing

Cai, J; Wu, C; Wang, JD; Du, W; Zheng, FX; Hakala, SM; Fan, XL; Chu, BW; Yao, L; Feng, ZM; Liu, YC; Sun, YL; Zheng, J; Yan, C; Bianchi, F; Kulmala, M; Mohr, C; Daellenbach, KR
2022 | Atmos. Chem. Phys. | 22 (2) (1251-1269)
air-pollution sources , black carbon , brown carbon , carbon light-absorption , chemical composition , mass spectrometry , matter , number size distributions , source apportionment , winter
Organic aerosol (OA) is a major component of fine particulate matter (PM), affecting air quality, human health, and the climate. The absorptive and reflective behavior of OA components contributes to determining particle optical properties and thus their effects on the radiative budget of the troposphere. There is limited knowledge on the influence of the molecular composition of OA on particle optical properties in the polluted urban environment. In this study, we characterized the molecular composition of oxygenated OA collected on filter samples in the autumn of 2018 in Beijing, China, with a filter inlet for gases and aerosols coupled to a high-resolution time-of-flight chemical ionization mass spectrometer (FIGAERO-CIMS). Three haze episodes occurred during our sampling period with daily maximum concentrations of OA of 50, 30, and 55 mu g m(-3). We found that the signal intensities of dicarboxylic acids and sulfur-containing compounds increased during the two more intense haze episodes, while the relative contributions of wood-burning markers and other aromatic compounds were enhanced during the cleaner periods. We further assessed the optical properties of oxygenated OA components by combining detailed chemical composition measurements with collocated particle light absorption measurements. We show that light absorption enhancement (E-abs) of black carbon (BC) was mostly related to more oxygenated OA (e.g., dicarboxylic acids), likely formed in aqueous-phase reactions during the intense haze periods with higher relative humidity, and speculate that they might contribute to lensing effects. Aromatics and nitro-aromatics (e.g., nitrocatechol and its derivatives) were mostly related to a high light absorption coefficient (b(abs)) consistent with light-absorbing (brown) carbon (BrC). Our results provide information on oxygenated OA components at the molecular level associated with BrC and BC particle light absorption and can serve as a basis for further studies on the effects of anthropogenic OA on radiative forcing in the urban environment.

Highly time-resolved chemical speciation and source apportionment of organic aerosol components in Delhi, India, using extractive electrospray ionization mass spectrometry

Kumar, V; Giannoukos, S; Haslett, SL; Tong, YD; Singh, A; Bertrand, A; Lee, CP; Wang, DS; Bhattu, D; Stefenelli, G; Dave, JS; Puthussery, JV; Qi, L; Vats, P; Rai, P; Casotto, R; Satish, R; Mishra, S; Pospisilova, V; Mohr, C; Bell, DM; Ganguly, D; Verma, V; Rastogi, N; Baltensperger, U; Tripathi, SN; Prevot, ASH; Slowik, JG
2022 | Atmos. Chem. Phys. | 22 (11) (7739-7761)
air pollution , aromatic-hydrocarbons , eesi-tof-ms , high-resolution , multilinear engine , particle composition , positive matrix factorization , road emission characteristics , urban , volatility
In recent years, the Indian capital city of Delhi has been impacted by very high levels of air pollution, especially during winter. Comprehensive knowledge of the composition and sources of the organic aerosol (OA), which constitutes a substantial fraction of total particulate mass (PM) in Delhi, is central to formulating effective public health policies. Previous source apportionment studies in Delhi identified key sources of primary OA (POA) and showed that secondary OA (SOA) played a major role but were unable to resolve specific SOA sources. We address the latter through the first field deployment of an extractive electrospray ionization timeof-flight mass spectrometer (EESI-TOF) in Delhi, together with a high-resolution aerosol mass spectrometer (AMS). Measurements were conducted during the winter of 2018/19, and positive matrix factorization (PMF) was used separately on AMS and EESI-TOF datasets to apportion the sources of OA. AMS PMF analysis yielded three primary and two secondary factors which were attributed to hydrocarbon-like OA (HOA), biomass burning OA (BBOA-1 and BBOA-2), more oxidized oxygenated OA (M0-00A), and less oxidized oxygenated OA (LO-OOA). On average, 40 % of the total OA mass was apportioned to the secondary factors. The SOA contribution to total OA mass varied greatly between the daytime (76.8 %, 10:00-16:00 local time (LT)) and nighttime (31.0 %, 21:00-04:00 LT). The higher chemical resolution of EESI-TOF data allowed identification of individual SOA sources. The EESI-TOF PMF analysis in total yielded six factors, two of which were primary factors (primary biomass burning and cooking-related OA). The remaining four factors were predominantly of secondary origin: aromatic SOA, biogenic SOA, aged biomass burning SOA, and mixed urban SOA. Due to the uncertainties in the EESI-TOF ion sensitivities, mass concentrations of EESI-TOF SOA-dominated factors were related to the total AMS SOA (i.e. MO-00A + LO-00A) by multiple linear regression (MLR). Aromatic SOA was the major SOA component during the daytime, with a 55.2 % contribution to total SOA mass (42.4 % contribution to total OA). Its contribution to total SOA, however, decreased to 25.4 % (7.9 % of total OA) during the nighttime. This factor was attributed to the oxidation of light aromatic compounds emitted mostly from traffic. Biogenic SOA accounted for 18.4 % of total SOA mass (14.2 % of total OA) during the daytime and 36.1 % of total SOA mass (11.2 % of total OA) during the nighttime. Aged biomass burning and mixed urban SOA accounted for 15.2 % and 11.0 % of total SOA mass (11.7 % and 8.5 % of total OA mass), respectively, during the daytime and 15.4 % and 22.9 % of total SOA mass (4.8 % and 7.1 % of total OA mass), respectively, during the nighttime. A simple dilution-partitioning model was applied on all EESI-TOF factors to estimate the fraction of observed daytime concentrations resulting from local photochemical production (SOA) or emissions (POA). Aromatic SOA, aged biomass burning, and mixed urban SOA were all found to be dominated by local photochemical production, likely from the oxidation of locally emitted volatile organic compounds (VOCs). In contrast, biogenic SOA was related to the oxidation of diffuse regional emissions of isoprene and monoterpenes. The findings of this study show that in Delhi, the nighttime high concentrations are caused by POA emissions led by traffic and biomass burning and the daytime OA is dominated by SOA, with aromatic SOA accounting for the largest fraction. Because aromatic SOA is possibly more toxic than biogenic SOA and primary OA, its dominance during the daytime suggests an increased OA toxicity and health-related consequences for the general public.

The SALTENA experiment: Comprehensive observations of aerosol sources, formation and processes in the South American Andes

Federico Bianchi; Victoria A. Sinclair; Diego Aliaga; Qiaozhi Zha; Wiebke Scholz; Cheng Wu; Liine Heikkinen; Rob Modini; Eva Partoll; Fernando Velarde; Isabel Moreno; Yvette Gramlich; Wei Huang; Markus Leiminger; Joonas Enroth; Otso Peräkylä; Angela Marinoni; Chen Xuemeng; Luis Blacutt; Ricardo Forno; Rene Gutierrez; Patrick Ginot; Gaëlle Uzu; Maria Cristina Facchini; Stefania Gilardoni; Martin Gysel-Beer; Runlong Cai; Tuukka Petäjä; Matteo Rinaldi; Harald Saathoff; Karine Sellegri; Douglas Worsnop; Paulo Artaxo; Armin Hansel; Markku Kulmala; Alfred Wiedensohler; Paolo Laj; Radovan Krejci; Samara Carbone; Marcos Andrade; Claudia Mohr
2021 | Bull. Amer. Meteor. Soc.

Insights into the molecular composition of semi-volatile aerosols in the summertime central Arctic Ocean using FIGAERO-CIMS

Siegel, K.; Karlsson, L.; Zieger, P.; Baccarini, A.; Schmale, J.; Lawler, M.; Salter, M.; Leck, C.; Ekman, A.; Riipinen, I.; Mohr, C.
2021 | Environ. Sci. Atmos. | 1 (4) (161-175)

The remote central Arctic during summertime has a pristine atmosphere with very low aerosol particle concentrations. As the region becomes increasingly ice-free during summer, enhanced ocean-atmosphere fluxes of aerosol particles and precursor gases may therefore have impacts on the climate. However, large knowledge gaps remain regarding the sources and physicochemical properties of aerosols in this region. Here, we present insights into the molecular composition of semi-volatile aerosol components collected in September 2018 during the MOCCHA (Microbiology-Ocean-Cloud-Coupling in the High Arctic) campaign as part of the Arctic Ocean 2018 expedition with the Swedish Icebreaker Oden. Analysis was performed offline in the laboratory using an iodide High Resolution Time-of-Flight Chemical Ionization Mass Spectrometer with a Filter Inlet for Gases and AEROsols (FIGAERO-HRToF-CIMS). Our analysis revealed significant signal from organic and sulfur-containing compounds, indicative of marine aerosol sources, with a wide range of carbon numbers and O : C ratios. Several of the sulfur-containing compounds are oxidation products of dimethyl sulfide (DMS), a gas released by phytoplankton and ice algae. Comparison of the time series of particulate and gas-phase DMS oxidation products did not reveal a significant correlation, indicative of the different lifetimes of precursor and oxidation products in the different phases. This is the first time the FIGAERO-HRToF-CIMS was used to investigate the composition of aerosols in the central Arctic. The detailed information on the molecular composition of Arctic aerosols presented here can be used for the assessment of aerosol solubility and volatility, which is relevant for understanding aerosol–cloud interactions.

Precursors and Pathways Leading to Enhanced Secondary Organic Aerosol Formation during Severe Haze Episodes

Zheng, Y; Chen, Q; Cheng, X; Mohr, C; Cai, J; Huang, W; Shrivastava, M; Ye, PL; Fu, PQ; Shi, XD; Ge, YL; Liao, KR; Miao, RQ; Qiu, XH; Koenig, TK; Chen, SY
2021 | Environ. Sci. Technol. | 55 (23) (15680-15693)
alpha-dicarbonyls , anthropogenic emissions , aqueous processing , aqueous-phase photooxidation , atmospheric aerosols , chemical composition , dicarboxylic acid , dicarboxylic-acids , fine particulate matter , haze , molecular composition , nitrated phenols , organic nitrates , oxocarboxylic acids , positive matrix factorization , soa
Molecular analyses help to investigate the key precursors and chemical processes of secondary organic aerosol (SOA) formation. We obtained the sources and molecular compositions of organic aerosol in PM2.5 in winter in Beijing by online and offline mass spectrometer measurements. Photochemical and aqueous processing were both involved in producing SOA during the haze events. Aromatics, isoprene, long-chain alkanes or alkenes, and carbonyls such as glyoxal and methylglyoxal were all important precursors. The enhanced SOA formation during the severe haze event was predominantly contributed by aqueous processing that was promoted by elevated amounts of aerosol water for which multifunctional organic nitrates contributed the most followed by organic compounds having four oxygen atoms in their formulae. The latter included dicarboxylic acids and various oxidation products from isoprene and aromatics as well as products or oligomers from methylglyoxal aqueous uptake. Nitrated phenols, organosulfates, and methanesulfonic acid were also important SOA products but their contributions to the elevated SOA mass during the severe haze event were minor. Our results highlight the importance of reducing nitrogen oxides and nitrate for future SOA control. Additionally, the formation of highly oxygenated long-chain molecules with a low degree of unsaturation in polluted urban environments requires further research.

When science and politics come together: From depletion to recovery of the stratospheric ozone hole This article belongs to Ambio’s 50th Anniversary Collection. Theme: Ozone Layer

2021 | Ambio | 50 (1) (31-34)

Photolytically induced changes in composition and volatility of biogenic secondary organic aerosol from nitrate radical oxidation during night-to-day transition

Wu, C; Bell, DM; Graham, EL; Haslett, S; Riipinen, I; Baltensperger, U; Bertrand, A; Giannoukos, S; Schoonbaert, J; El Haddad, I; Prevot, ASH; Huang, W; Mohr, C
2021 | Atmos. Chem. Phys. | 21 (19) (14907-14925)
alpha-pinene , carbonyl nitrates , chemical composition , evaporation kinetics , isoprene oxidation , mass-spectrometer , model , no3 , optical-properties , photolysis

Night-time reactions of biogenic volatile organic compounds (BVOCs) and nitrate radicals (NO3) can lead to the formation of NO3-initiated biogenic secondary organic aerosol (BSOANO(3)). Here, we study the impacts of light exposure on the chemical composition and volatility of BSOANO(3) formed in the dark from three precursors (isoprene, alpha-pinene, and beta-caryophyllene) in atmospheric simulation chamber experiments. Our study represents BSOANO(3) formation conditions where reactions between peroxy radicals (RO2 + RO2) and between RO2 and NO3 are favoured. The emphasis here is on the identification of particle-phase organonitrates (ONs) formed in the dark and their changes during photolytic ageing on timescales of similar to 1 h. The chemical composition of particle-phase compounds was measured with a chemical ionization mass spectrometer with a filter inlet for gases and aerosols (FIGAERO-CIMS) and an extractive electrospray ionization time-of-flight mass spectrometer (EESI-TOF). Volatility information on BSOANO(3) was derived from FIGAERO-CIMS desorption profiles (thermograms) and a volatility tandem differential mobility analyser (VTDMA). During photolytic ageing, there was a relatively small change in mass due to evaporation (< 5 % for the isoprene and alpha-pinene BSOANO3, and 12 % for the beta-caryophyllene BSOANO(3)), but we observed significant changes in the chemical composition of the BSOANO(3). Overall, 48 %, 44 %, and 60 % of the respective total signal for the isoprene, alpha-pinene, and beta-caryophyllene BSOANO(3) was sensitive to photolytic ageing and exhibited decay. The photolabile compounds include both monomers and oligomers. Oligomers can decompose into their monomer units through photolysis of the bonds (e.g. likely O-O) between them. Fragmentation of both oligomers and monomers also happened at other positions, causing the formation of compounds with shorter carbon skeletons. The cleavage of the nitrate functional group from the carbon chain was likely not a main degradation pathway in our experiments. In addition, photolytic degradation of compounds changes their volatility and can lead to evaporation. We use different methods to assess bulk volatilities and discuss their changes during both dark ageing and photolysis in the context of the chemical changes that we observed. We also reveal large uncertainties in saturation vapour pressure estimated from parameterizations for the ON oligomers with multiple nitrate groups. Overall, our results suggest that photolysis causes photodegradation of a substantial fraction of BSOANO(3), changes both the chemical composition and the bulk volatility of the particles, and might be a potentially important loss pathway of BSOANO(3) during the night-to-day transition.

Physical and chemical properties of aerosol particles and cloud residuals on Mt. Åreskutan in Central Sweden during summer 2014

Emelie Linnéa Graham; Paul Zieger; Claudia Mohr; Ulla Wideqvist; Tabea Hennig; Annica M. L. Ekman; Radovan Krejci; Johan Ström; lona Riipinen
2020 | Tellus Ser. B-Chem. Phys. Meteorol. | 72 (1) (1-16)

The size distribution, volatility and hygroscopicity of ambient aerosols and cloud residuals were measured with a differential mobility particle sizer (DMPS) and a volatility–hygroscopicity tandem differential mobility analyser (VHTDMA) coupled to a counterflow virtual impactor (CVI) inlet during the Cloud and Aerosol
Experiment at Åre (CAEsAR) campaign at Mt. Åreskutan during summer 2014. The chemical composition
of particulate matter (PM) and cloud water were analysed offline using thermo-optical OC/EC analysis and ion chromatography. The importance of aerosol particle size for cloud droplet activation and subsequent particle scavenging was clearly visible in the measured size distributions. Cloud residuals were shifted towards larger sizes compared to ambient aerosol, and the cloud events were followed by a size distribution
dominated by smaller particles. Organics dominated both PM (62% organic mass fraction) and cloud water (63% organic mass fraction) composition. The volatility and hygroscopicity of the ambient aerosols were representative of homogeneous aged aerosol with contributions from biogenic secondary organics, with
median volume fraction remaining (VFR) of 0.04–0.05, and median hygroscopicity parameter j of 0.16–0.24 for 100–300 nm particles. The corresponding VFR and j for the cloud residuals were 0.03–0.04 and 0.18–0.20. The chemical composition, hygroscopicity and volatility measurements thus showed no major
differences between the ambient aerosol particles and cloud residuals. The VFR and j values predicted based on the chemical composition measurements agreed well with the VHTDMA measurements, indicating the bulk chemical composition to be a reasonable approximation throughout the size distribution. There were
indications, however, of some more subtle changes in time scales not achievable by the offline chemical analysis applied here. Further, online observations of aerosol and cloud residual chemical composition are therefore warranted.

Contact information

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Geovetenskapens Hus,
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Arrheniuslaboratoriet, Svante Arrhenius väg 16, Stockholm (Unit for Toxicological Chemistry)

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Stockholm University
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