The seasonal characteristics of cloud condensation nuclei (CCN) in the arctic lower troposphere

Jung, CH; Yoon, YJ; Kang, HJ; Gim, Y; Lee, BY; Strom, J; Krejci, R; Tunved, P
2018 | Tellus Ser. B-Chem. Phys. Meteorol. | 70 (1-13)
aerosol-particles , arctic aerosol , arctic troposphere , atmosphere , ccn , climate , cycle , ny-alesund , ocean , size distributions , summer aerosol , svalbard , zeppelin station
Cloud Condensation Nuclei (CCN) concentration and aerosol size distributions in the Arctic were collected during the period 2007-2013 at the Zeppelin observatory (78.91 degrees N, 11.89 degrees E, 474 masl). Annual median CCN concentration at a supersaturation (SS) of 0.4% show the ranges of 45 approximate to 81cm(-3). The monthly median CCN number density varied between 17cm(-3) in October 2007 and 198cm(-3) in March, 2008. The CCN spectra parameters C (83cm(-3)) and k (0.23) were derived. In addition, calculated annual median value of hygroscopicity parameter is 0.46 at SS of 0.4%. Particle number concentration of accumulation mode from aerosol size distribution measurements are well correlated with CCN concentration. The CCN to CN>10 nm (particle number concentration larger than 10nm in diameter) ratio shows a maximum during March and minimum during July. The springtime high CCN concentration is attributed to high load of accumulation mode aerosol transported from the mid-latitudes, known as Arctic Haze. CCN concentration remains high also during Arctic summer due to the source of new CCN through particle formation followed by consecutive aerosol growth. Lowest aerosol as well as CCN number densities were observed during Arctic autumn and early winter when aerosol formation in the Arctic and long-range transport into the Arctic are not effective.

Comparison of PM2.5 chemical composition and sources at a rural background site in Central Europe between 1993/1994/1995 and 2009/2010: Effect of legislative regulations and economic transformation on the air quality

Pokorna, P; Schwarz, J; Krejci, R; Swietlicki, E; Havranek, V; Zdimal, V
2018 | Environ. Pollut. | 241 (841-851)
atmospheric aerosol , back trajectory analysis , concentrations trends , elemental composition , ions , k-puszta , long range transport , particles , particulate matter , pollution hot-spot , positive matrix factorization , size , source apportionment , source identification , trace elements
From December 1993 to January 1995 and from October 2009 to October 2010, a total of 320 and 365 daily samples of the PM2.5 were collected at a rural background site (National Atmospheric Observatory Koetice) in Central Europe. The PM2.5 samples were analyzed for 29 and 26 elements respectively by Particle-Induced X-ray Emission (PIXE) and water-soluble inorganic ions by Ion Chromatography (IC) in 2009/2010. The Positive Matrix Factorization (PMF) was applied to the chemical composition of PM2.5 to determine its sources. The decreasing trends of almost all elements concentrations, especially the metals regulated by the EU Directive (2004/107/EC) are evident. The annual median ratios indicate a decrease in concentrations of the PM2.5 elements. The slight increase of K concentrations and Spearman's rank correlation coefficient r(s) 0.09 K/Se points to a rise in residential wood combustion. The S concentrations are nearly comparable (higher mean in 2009/2010, while the annual median ratio is under 1). The five major source types in the mid-1990s were ascribed to brown coal combustion, oil combustion, sea salt and dust - long-range transport, re-suspended dust and black coal combustion. The industrial combustion of brown and/or black coal (r(s) 0.75 Se/As, r(s) 0.57 Ga/Ge and r(s) 0.20 As/Zn) and oil (r(s) 0.72 V/Ni) of the regional origin dominated. In the 1990s, the potential source regions were the border area of Czech Republic, German and Poland (brown coal), the Moravia-Silesia region at the Czech-Polish border (black coal), and Slovakia, Austria, Hungary, and the Balkans (oil). In 2009/2010, the apportioned sources were sulfate, residential heating, nitrate, industry, re-suspended dust, and sea salt and dust long-range transport. The secondary sulfate from coal combustion and residential biomass burning (r(s) 0.96, K/K+) of local origin dominated. The declining trend of the elemental concentrations and change in the source pattern of the regional background PM2.5 in Central Europe between the mid-1990s and 2009/10 reflects the economic transformation and impact of stricter legislation in Central Europe. (C) 2018 Elsevier Ltd. All rights reserved.

Ground-based observation of clusters and nucleation-mode particles in the Amazon

Wimmer, D; Mazon, SB; Manninen, HE; Kangasluoma, J; Franchin, A; Nieminen, T; Backman, J; Wang, J; Kuang, CG; Krejci, R; Brito, J; Morais, FG; Martin, ST; Artaxo, P; Kulmala, M; Kerminen, VM; Petaja, T
2018 | Atmos. Chem. Phys. | 18 (17) (13245-13264)
aerosol-particles , air ion spectrometer , atmospheric nucleation , boreal forest , boundary layer , formation events , growth-rates , intermediate ions , neutral cluster , size distribution
We investigated atmospheric new particle formation (NPF) in the Amazon rainforest using direct measurement methods. To our knowledge this is the first direct observation of NPF events in the Amazon region. However, previous observations elsewhere in Brazil showed the occurrence of nucleation-mode particles. Our measurements covered two field sites and both the wet and dry season. We measured the variability of air ion concentrations (0.8-12 nm) with an ion spectrometer between September 2011 and January 2014 at a rainforest site (T0t). Between February and October 2014, the same measurements were performed at a grassland pasture site (T3) as part of the GoAmazon 2014/5 experiment, with two intensive operating periods (IOP1 and IOP2 during the wet and the dry season, respectively). The GoAmazon 2014/5 experiment was designed to study the influence of anthropogenic emissions on the changing climate in the Amazon region. The experiment included basic aerosol and trace gas measurements at the ground, remote sensing instrumentation, and two aircraft-based measurements. The results presented in this work are from measurements performed at ground level at both sites. The site inside the rainforest (T0t) is located 60 km NNW of Manaus and influenced by pollution about once per week. The pasture (T3) site is located 70 km downwind from Manaus and influenced by the Manaus pollution plume typically once per day or every second day, especially in the afternoon. No NPF events were observed inside the rainforest (site T0t) at ground level during the measurement period. However, rain-induced ion and particle bursts (hereafter, "rain events") occurred frequently (643 of 1031 days) at both sites during the wet and dry season, being most frequent during the wet season. During the rain events, the ion concentrations in three size ranges (0.8-2, 2-4, and 4-12 nm) increased up to about 10(4)-10(5) cm(-3). This effect was most pronounced in the intermediate and large size ranges, for which the background ion concentrations were about 10-15 cm(-3) compared with 700 cm(-3) for the cluster ion background. We observed eight NPF events at the pasture site during the wet season. We calculated the growth rates and formation rates of neutral particles and ions for the size ranges 2-3 and 3-7 nm using the ion spectrometer data. The observed median growth rates were 0.8 and 1.6 nm h(-1) for 2-3 nm sized ions and particles, respectively, with larger growth rates (13.3 and 7.9 nm h(-1)) in the 3-7 nm size range. The measured nucleation rates were of the order of 0.2 cm(-3) s(-1) for particles and 4-9 x 10(-3) cm(-3) s(-1) for ions. There was no clear difference in the sulfuric acid concentrations between the NPF event days and nonevent days (similar to 9 x 10(5) cm(-3)). The two major differences between the NPF days and nonevent days were a factor of 1.8 lower condensation sink on NPF event days (1.8 x 10(-3) s(-1)) compared to nonevents (3.2 x 10(-3) s(-1)) and different air mass origins. To our knowledge, this is the first time that results from ground-based sub-3 nm aerosol particle measurements have been obtained from the Amazon rainforest.

Identification of topographic features influencing aerosol observations at high altitude stations

Coen, MC; Andrews, E; Aliaga, D; Andrade, M; Angelov, H; Bukowiecki, N; Ealo, M; Fialho, P; Flentje, H; Hallar, AG; Hooda, R; Kalapov, I; Krejci, R; Lin, NH; Marinoni, A; Ming, J; Nguyen, A; Pandolfi, M; Pont, V; Ries, L; Rodriguez, S; Schauer, G; Sellegri, K; Sharma, S; Sun, J; Tunved, P; Velasquez, P; Ruffieux, D
2018 | Atmos. Chem. Phys. | 18 (16) (12289-12313)
1570 m a.s.l. , air-mass transport , convective boundary-layer , in situ measurements , long range transport , lower free troposphere , montsec southern pyrenees , particle formation , subtropical north-atlantic , western mediterranean basin
High altitude stations are often emphasized as free tropospheric measuring sites but they remain influenced by atmospheric boundary layer (ABL) air masses due to convective transport processes. The local and meso-scale topographical features around the station are involved in the convective boundary layer development and in the formation of thermally induced winds leading to ABL air lifting. The station altitude alone is not a sufficient parameter to characterize the ABL influence. In this study, a topography analysis is performed allowing calculation of a newly defined index called ABL-TopoIndex. The ABL-TopoIndex is constructed in order to correlate with the ABL influence at the high altitude stations and long-term aerosol time series are used to assess its validity. Topography data from the global digital elevation model GTopo30 were used to calculate five parameters for 43 high and 3 middle altitude stations situated on five continents. The geometric mean of these five parameters determines a topography based index called ABL-TopoIndex, which can be used to rank the high altitude stations as a function of the ABL influence. To construct the ABL-TopoIndex, we rely on the criteria that the ABL influence will be low if the station is one of the highest points in the mountainous massif, if there is a large altitude difference between the station and the valleys or high plains, if the slopes around the station are steep, and finally if the inverse drainage basin potentially reflecting the source area for thermally lifted pollutants to reach the site is small. All stations on volcanic islands exhibit a low ABL-TopoIndex, whereas stations in the Himalayas and the Tibetan Plateau have high ABL-TopoIndex values. Spearman's rank correlation between aerosol optical properties and number concentration from 28 stations and the ABL-TopoIndex, the altitude and the latitude are used to validate this topographical approach. Statistically significant (SS) correlations are found between the 5th and 50th percentiles of all aerosol parameters and the ABL-TopoIndex, whereas no SS correlation is found with the station altitude. The diurnal cycles of aerosol parameters seem to be best explained by the station latitude although a SS correlation is found between the amplitude of the diurnal cycles of the absorption coefficient and the ABL-TopoIndex.

Multi-year statistical and modeling analysis of submicrometer aerosol number size distributions at a rain forest site in Amazonia

Rizzo, LV; Roldin, P; Brito, J; Backman, J; Swietlicki, E; Krejci, R; Tunved, P; Petaja, T; Kulmala, M; Artaxo, P
2018 | Atmos. Chem. Phys. | 18 (14) (10255-10274)
atmospheric aerosols , basin , growth , isoprene epoxydiols , particle formation , secondary organic aerosol , sulfuric acid , trace gases , upper troposphere , wet season
The Amazon Basin is a unique region to study atmospheric aerosols, given their relevance for the regional hydrological cycle and the large uncertainty of their sources. Multi-year datasets are crucial when contrasting periods of natural conditions and periods influenced by anthropogenic emissions. In the wet season, biogenic sources and processes prevail, and the Amazonian atmospheric composition resembles preindustrial conditions. In the dry season, the basin is influenced by widespread biomass burning emissions. This work reports multi-year observations of high time resolution submicrometer (10-600 nm) particle number size distributions at a rain forest site in Amazonia (TT34 tower, 60 km NW from Manaus city), between 2008 and 2010 and 2012 and 2014. The median particle number concentration was 403 cm(-3) in the wet season and 1254 cm(-3) in the dry season. The Aitken mode (similar to 30-100 nm in diameter) was prominent during the wet season, while the accumulation mode (similar to 100-600 nm in diameter) dominated the particle size spectra during the dry season. Cluster analysis identified groups of aerosol number size distributions influenced by convective downdrafts, nucleation events and fresh biomass burning emissions. New particle formation and subsequent growth was rarely observed during the 749 days of observations, similar to previous observations in the Amazon Basin. A stationary 1-D column model (ADCHEM Aerosol Dynamics, gas and particle phase CHEMistry and radiative transfer model) was used to assess the importance of the processes behind the observed diurnal particle size distribution trends. Three major particle source types are required in the model to reproduce the observations: (i) a surface source of particles in the evening, possibly related to primary biological emissions; (ii) entrainment of accumulation mode aerosols in the morning; and (iii) convective downdrafts transporting Aitken mode particles into the boundary layer mostly during the afternoon. The latter process has the largest influence on the modeled particle number size distributions. However, convective downdrafts are often associated with rain and, thus, act as both a source of Aitken mode particles and a sink of accumulation mode particles, causing a net reduction in the median total particle number concentrations in the surface layer. Our study shows that the combination of the three mentioned particle sources is essential to sustain particle number concentrations in Amazonia.

How much of the global aerosol optical depth is found in the boundary layer and free troposphere?

Bourgeois, Q; Ekman, AML; Renard, JB; Krejci, R; Devasthale, A; Bender, FAM; Riipinen, I; Berthet, G; Tackett, JL
2018 | Atmos. Chem. Phys. | 18 (10) (7709-7720)
aerocom phase-ii , air pollution , caliop , calipso , cloud , models , ocean , satellite-observations , transport , vertical-distribution
The global aerosol extinction from the CALIOP space lidar was used to compute aerosol optical depth (AOD) over a 9-year period (2007-2015) and partitioned between the boundary layer (BL) and the free troposphere (FT) using BL heights obtained from the ERA-Interim archive. The results show that the vertical distribution of AOD does not follow the diurnal cycle of the BL but remains similar between day and night highlighting the presence of a residual layer during night. The BL and FT contribute 69 and 31 %, respectively, to the global tropospheric AOD during daytime in line with observations obtained in Aire sur l'Adour (France) using the Light Optical Aerosol Counter (LOAC) instrument. The FT AOD contribution is larger in the tropics than at mid-latitudes which indicates that convective transport largely controls the vertical profile of aerosols. Over oceans, the FT AOD contribution is mainly governed by long-range transport of aerosols from emission sources located within neighboring continents. According to the CALIOP aerosol classification, dust and smoke particles are the main aerosol types transported into the FT. Overall, the study shows that the fraction of AOD in the FT - and thus potentially located above low-level clouds - is substantial and deserves more attention when evaluating the radiative effect of aerosols in climate models. More generally, the results have implications for processes determining the overall budgets, sources, sinks and transport of aerosol particles and their description in atmospheric models.

Black carbon emission and transport mechanisms to the free troposphere at the La Paz/El Alto (Bolivia) metropolitan area based on the Day of Census (2012)

Wiedensohler, A; Andrade, M; Weinhold, K; Muller, T; Birmili, W; Velarde, F; Moreno, I; Forno, R; Sanchez, MF; Laj, P; Ginot, P; Whiteman, DN; Krejci, R; Sellegri, K; Reichler, T
2018 | Atmos Environ | 194 (158-169)
aerosol , black carbon , free troposphere , high-altitude station , network , particle-size spectrometers , pollution transport , range , traffic emissions , variability
Urban development, growing industrialization, and increasing demand for mobility have led to elevated levels of air pollution in many large cities in Latin America, where air quality standards and WHO guidelines are frequently exceeded. The conurbation of the metropolitan area of La Paz/El Alto is one of the fastest growing urban settlements in South America with the particularity of being located in a very complex terrain at a high altitude. As many large cities or metropolitan areas, the metropolitan area of La Paz/El Alto and the Altiplano region are facing air quality deterioration. Long-term measurement data of the equivalent black carbon (eBC) mass concentrations and particle number size distributions (PNSD) from the Global Atmosphere Watch Observatory Chacaltaya (CHC; 5240 m a.s.l., above sea level) indicated a systematic transport of particle matter from the metropolitan area of La Paz/El Alto to this high altitude station and subsequently to the lower free troposphere. To better understand the sources and the transport mechanisms, we conducted eBC and PNSDs measurements during an intensive campaign at two locations in the urban area of La Paz/El Alto from September to November 2012. While the airport of El Alto site (4040 m a.s.l.) can be seen as representative of the urban and Altiplano background, the road site located in Central La Paz (3590 m a.s.l.) is representative for heavy traffic-dominated conditions. Peaks of eBC mass concentrations up to 5 mu g m(-3) were observed at the El Alto background site in the early morning and evening, while minimum values were detected in the early afternoon, mainly due to thermal convection and change of the planetary boundary layer height. The traffic-related eBC mass concentrations at the road site reached maximum values of 10-20 mu g m(-3). A complete traffic ban on the specific Bolivian Day of Census (November 21, 2012) led to a decrease of eBC below 1 mu g m(-3) at the road site for the entire day. Compared to the day before and after, particle number concentrations decreased by a factor between 5 and 25 over the particle size range from 10 to 800 nm, while the submicrometer particle mass concentration dropped by approximately 80%. These results indicate that traffic is the dominating source of BC and particulate air pollution in the metropolitan area of La Paz/El Alto. In general, the diurnal cycle of eBC mass concentration at the Chacaltaya observatory is anti-correlated to the observations at the El Alto background site. This pattern indicates that the traffic-related particulate matter, including BC, is transported to higher altitudes with the developing of the boundary layer during daytime. The metropolitan area of La Paz/El Alto seems to be a significant source for BC of the regional lower free troposphere. From there, BC can be transported over long distances and exert impact on climate and composition of remote southern hemisphere.

Global analysis of continental boundary layer new particle formation based on long-term measurements

Nieminen, T; Kerminen, VM; Petaja, T; Aalto, PP; Arshinov, M; Asmi, E; Baltensperger, U; Beddows, DCS; Beukes, JP; Collins, D; Ding, AJ; Harrison, RM; Henzing, B; Hooda, R; Hu, M; Horrak, U; Kivekas, N; Komsaare, K; Krejci, R; Kristensson, A; Laakso, L; Laaksonen, A; Leaitch, WR; Lihavainen, H; Mihalopoulos, N; Nemeth, Z; Nie, W; O'Dowd, C; Salma, I; Sellegri, K; Svenningsson, B; Swietlicki, E; Tunved, P; Ulevicius, V; Vakkari, V; Vana, M; Wiedensohler, A; Wu, ZJ; Virtanen, A; Kulmala, M
2018 | Atmos. Chem. Phys. | 18 (19) (14737-14756)
aerosol size distribution , airborne measurements , atmospheric nucleation , black carbon , condensation nuclei production , formation events , free troposphere , high-altitude site , particulate matter , sulfuric acid
Atmospheric new particle formation (NPF) is an important phenomenon in terms of global particle number concentrations. Here we investigated the frequency of NPF, formation rates of 10 nm particles, and growth rates in the size range of 10-25 nm using at least 1 year of aerosol number size-distribution observations at 36 different locations around the world. The majority of these measurement sites are in the Northern Hemisphere. We found that the NPF frequency has a strong seasonal variability. At the measurement sites analyzed in this study, NPF occurs most frequently in March-May (on about 30 % of the days) and least frequently in December-February (about 10 % of the days). The median formation rate of 10 nm particles varies by about 3 orders of magnitude (0.01-10 cm(-3) s(-1)) and the growth rate by about an order of magnitude (1-10 nm h(-1)). The smallest values of both formation and growth rates were observed at polar sites and the largest ones in urban environments or anthropogenically influenced rural sites. The correlation between the NPF event frequency and the particle formation and growth rate was at best moderate among the different measurement sites, as well as among the sites belonging to a certain environmental regime. For a better understanding of atmospheric NPF and its regional importance, we would need more observational data from different urban areas in practically all parts of the world, from additional remote and rural locations in North America, Asia, and most of the Southern Hemisphere (especially Australia), from polar areas, and from at least a few locations over the oceans.

Arctic sea ice melt leads to atmospheric new particle formation.

Dall'Osto, M.; Beddows, DCS.; Tunved, P.; Krejci, R.; Strom, J.; Hansson, HC.; Yoon, YJ.; Park, KT.; Becagli, S.; Udisti, R.; Onasch, T.; O'Dowd, CD.; Simo, R.; Harrison, RM.
2017 | Sci Rep | 10

A new aerosol wet removal scheme for the Lagrangian particle model FLEXPART v10

Grythe, H.; Kristiansen, N.; Zwaaftink, CDG.; Eckhardt, S.; Strom, J.; Tunved, P.; Krejci, R.; Stohl, A.
2017 | Geosci. Model Dev. | 10 (1447-1466)

Pan-Arctic aerosol number size distributions: seasonality and transport patterns

Freud, E; Krejci, R; Tunved, P; Leaitch, R; Nguyen, QT; Massling, A; Skov, H; Barrie, L
2017 | Atmos. Chem. Phys. | 17 (13) (8101-8128)
air pollution , atmospheric aerosol , black carbon , cluster-analysis , long-term observations , marine boundary layer , northeast greenland , ny-alesund , particle formation , polar sunrise
The Arctic environment has an amplified response to global climatic change. It is sensitive to human activities that mostly take place elsewhere. For this study, a multi-year set of observed aerosol number size distributions in the diameter range of 10 to 500 nm from five sites around the Arctic Ocean (Alert, Villum Research Station - Station Nord, Zeppelin, Tiksi and Barrow) was assembled and analysed. A cluster analysis of the aerosol number size distributions revealed four distinct distributions. Together with Lagrangian air parcel back-trajectories, they were used to link the observed aerosol number size distributions with a variety of transport regimes. This analysis yields insight into aerosol dynamics, transport and removal processes, on both an intra- and an inter-monthly scale. For instance, the relative occurrence of aerosol number size distributions that indicate new particle formation (NPF) event is near zero during the dark months, increases gradually to similar to 40% from spring to summer, and then collapses in autumn. Also, the likelihood of Arctic haze aerosols is minimal in summer and peaks in April at all sites. The residence time of accumulation-mode particles in the Arctic troposphere is typically long enough to allow tracking them back to their source regions. Air flow that passes at low altitude over central Siberia and western Russia is associated with relatively high concentrations of accumulation-mode particles (N-acc) at all five sites - often above 150 cm(-3). There are also indications of air descending into the Arctic boundary layer after transport from lower latitudes. The analysis of the back-trajectories together with the meteorological fields along them indicates that the main driver of the Arctic annual cycle of N-acc, on the larger scale, is when atmospheric transport covers the source regions for these particles in the 10-day period preceding the observations in the Arctic. The scavenging of these particles by precipitation is shown to be important on a regional scale and it is most active in summer. Cloud processing is an additional factor that enhances the N-acc annual cycle. There are some consistent differences between the sites that are beyond the year-to-year variability. They are the result of differences in the proximity to the aerosol source regions and to the Arctic Ocean sea-ice edge, as well as in the exposure to free-tropospheric air and in precipitation patterns - to mention a few. Hence, for most purposes, aerosol observations from a single Arctic site cannot represent the entire Arctic region. Therefore, the results presented here are a powerful observational benchmark for evaluation of detailed climate and air chemistry modelling studies of aerosols throughout the vast Arctic region.

Microphysical explanation of the RH-dependent water affinity of biogenic organic aerosol and its importance for climate

Rastak, N; Pajunoja, A; Navarro, JCA; Ma, J; Song, M; Partridge, DG; Kirkevag, A; Leong, Y; Hu, WW; Taylor, NF; Lambe, A; Cerully, K; Bougiatioti, A; Liu, P; Krejci, R; Petaja, T; Percival, C; Davidovits, P; Worsnop, DR; Ekman, AML; Nenes, A; Martin, S; Jimenez, JL; Collins, DR; Topping, DO; Bertram, AK; Zuend, A; Virtanen, A; Riipinen, I
2017 | Geophys Res Lett | 44 (10) (5167-5177)
atmospheric aerosols , boreal forest , condensation nuclei activity , droplet activation kinetics , earth system model , hygroscopic growth , liquid phase-separation , mass-spectrometer , regional dust samples , southeastern united-states
A large fraction of atmospheric organic aerosol (OA) originates from natural emissions that are oxidized in the atmosphere to form secondary organic aerosol (SOA). Isoprene (IP) and monoterpenes (MT) are the most important precursors of SOA originating from forests. The climate impacts from OA are currently estimated through parameterizations of water uptake that drastically simplify the complexity of OA. We combine laboratory experiments, thermodynamic modeling, field observations, and climate modeling to (1) explain the molecular mechanisms behind RH-dependent SOA water-uptake with solubility and phase separation; (2) show that laboratory data on IP- and MT-SOA hygroscopicity are representative of ambient data with corresponding OA source profiles; and (3) demonstrate the sensitivity of the modeled aerosol climate effect to assumed OA water affinity. We conclude that the commonly used single-parameter hygroscopicity framework can introduce significant error when quantifying the climate effects of organic aerosol. The results highlight the need for better constraints on the overall global OA mass loadings and its molecular composition, including currently underexplored anthropogenic and marine OA sources. Plain Language Summary The interaction of airborne particulate matter ("aerosols") with water is of critical importance for processes governing climate, precipitation, and public health. It also modulates the delivery and bioavailability of nutrients to terrestrial and oceanic ecosystems. We present a microphysical explanation to the humidity-dependent water uptake behavior of organic aerosol, which challenges the highly simplified theoretical descriptions used in, e.g., present climate models. With the comprehensive analysis of laboratory data using molecular models, we explain the microphysical behavior of the aerosol over the range of humidity observed in the atmosphere, in a way that has never been done before. We also demonstrate the presence of these phenomena in the ambient atmosphere from data collected in the field. We further show, using two state-of-the-art climate models, that misrepresenting the water affinity of atmospheric organic aerosol can lead to significant biases in the estimates of the anthropogenic influence on climate.

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