Particle nucleation from trace atmospheric vapours is important for climate since it gives rise to more than half of global cloud condensation nuclei. Sulfuric acid (H2SO4) has long been recognised to drive particle nucleation in the atmosphere and, more recently, highly oxygenated products of biogenic vapours-in particular monoterpenes such as α-pinene (C10H16)-have also been shown to nucleate under atmospheric conditions, without requiring additional vapours. This raises the question of whether a nucleation synergy exists between α-pinene oxygenated organic molecules (AP-OOM) and H2SO4, as has been suggested by early studies. Here we report new particle formation from AP-OOM and H2SO4 in the absence of base vapours such as ammonia (NH3), measured in experiments performed with the CERN CLOUD (Cosmics Leaving Outdoor Droplets) chamber at cool boundary layer temperatures of -10 °C and +5 °C. We find that AP-OOM nucleation rates increase strongly when H2SO4 concentrations exceed around 106 cm-3. The enhancement is synergistic and cannot be explained as a simple linear addition of independent chemical systems. Above this threshold, the nucleation rate depends approximately linearly on H2SO4 concentration, in contrast with the strong sensitivity to H2SO4 for H2SO4-NH3 nucleation. Nucleation rates are 10-100-fold higher in the presence of ions from galactic cosmic rays or from the CERN pion beam. Based on these measurements, we have parameterised a temperature-dependent H2SO4-AP-OOM nucleation rate in the absence of base vapours and implemented it in the EMAC (ECHAM/MESSy Atmospheric Chemistry) Earth system model. In comparison with a parameterisation developed in an earlier study [Riccobono et al., Science, 2014, 344, 717-721.], the new parameterisation indicates sharply reduced nucleation rates in the boundary layer over warm regions, and increased rates over northern boreal forests.

Dimethyl sulfide (DMS; CH3SCH3) from marine phytoplankton is a notable source of atmospheric sulfur1. Its oxidation products include sulfuric acid (SA; H2SO4) and methanesulfonic acid (MSA; CH3SO3H), which has a higher yield than SA below 10 °C (ref. 2). Although SA is known to drive the formation of new particles3, which may subsequently grow and act as cloud condensation nuclei (CCN), the role of MSA remains unclear4. Here, in experiments performed under atmospheric conditions at the CERN CLOUD (Cosmics Leaving OUtdoor Droplets) chamber, we show that MSA nucleates together with ammonia (NH3) below -10 °C, at rates comparable with SA-NH3. Moreover, MSA and SA nucleate synergistically below -10 °C, forming multi-acid molecular clusters with NH3. Even at ultralow NH3 levels, MSA drives particle growth at or near the kinetic limit below 9 °C and above 40% relative humidity (RH). Because MSA and SA generally coexist at similar concentrations in cool marine regions, our findings indicate that nucleation rates may be accelerated up to tenfold and growth rates up to twofold compared with SA-NH3 alone. Our global model simulations indicate that MSA can enhance CCN concentrations, especially in polar regions. We propose that MSA might be an important driver of biogenic particles in cool, pristine marine regions of both the present-day and pre-industrial atmospheres and yet is unaccounted for in global climate models5.

The formation and radiative properties of clouds in the marine boundary layer are highly sensitive to the number of cloud condensation nuclei (CCN), which largely originate from new particle formation. At present, most climate models only consider new particle formation from sulfuric acid (SA), in pristine marine regions produced via oxidation of dimethyl sulfide (DMS) emitted by phytoplankton. However, DMS oxidation also yields methanesulfonic acid (MSA) - often in higher amounts than SA under cool conditions (<10 °C) - yet MSA's role in NPF remains elusive. Here, we present results from the CERN CLOUD chamber at temperatures of -10 °C and +5 °C, demonstrating NPF from MSA and amines (dimethylamine, DMA, and trimethylamine, TMA). We isolated effects of MSA from SA by generating MSA from an evaporator at concentrations between 105 and 108 cm-3. We find MSA and DMA form particles at +5 °C but nucleation rates (J 1.7) are slow, reaching about 1 cm-3 s-1 at MSA concentrations of 5 × 107 cm-3. However, in the presence of low concentrations of SA (below 106 cm-3) and 2-15 pptv DMA, MSA at few 107 cm-3 strongly enhances SA-DMA nucleation, reaching up to 80 cm-3 s-1. Our measurements confirm MSA together with SA and DMA molecules in initial molecular clusters during nucleation. We find TMA less effective than DMA for new particle formation, likely resulting from steric hindrance of the additional methyl group. Our findings show that MSA can boost NPF rates by 1-2 orders of magnitude in pristine marine environments and should be incorporated in climate models.

Abstract. Laboratory experiments addressing complex phenomena such as atmospheric new-particle formation and growth typically involve numerous instruments measuring a range of key coupled variables. In addition to independent calibration, the combined dataset provides not just constraints on the parameters of interest but also on the critical instrument calibrations. Here we find good agreement between production and loss rates of sulfuric acid (H2SO4) in an experiment performed at the CERN CLOUD chamber involving oxidation of sulfur dioxide (SO2) in the presence of ammonia (NH3) at 58 % relative humidity, driving new-particle formation and growth of particles by H2SO4 + NH3 nucleation initiated by O3 photolysis via several light sources. This closure requires consistency across numerous parameters, including: the particle number and size distribution; their condensation sink for H2SO4; the particle growth rates; the concentration of H2SO4; and the nucleation coefficients for both neutral and ion-induced pathways. Our study shows that accurate agreement can be achieved between production and loss of condensable vapors in laboratory chambers under atmospheric conditions, with accuracy ultimately tied to particle number measurement (i.e. a condensation particle counter). This, in turn implies parameters such as the H2SO4 concentration and particle size distributions can be determined to a comparable precision.

Ammonia (NH3) fluxes were measured at a research farm in Southern Finland from 16 March to 22 May 2023 using the eddy covariance technique with a benzene chemical ionization mass spectrometer. NH3 emissions (0-0.02 & micro;gN m(-2) s(-1)) remained low for the primarily studied field until its fertilization with the solid fraction from solid-liquid separation of cattle manure on 27 April, which led to fluxes up to 1.4 & micro;gN m(-2) s(-1). The manure was incorporated into the soil by harrowing the next day, after which the fluxes declined rapidly and remained below 0.1 & micro;gN m(-2) s(-1). The NH3 emission factor (EF) from the solid fraction was 2.25% of applied total ammoniacal nitrogen (TAN) for the whole campaign, with 85% of the emissions during 27 April to 22 May occurring within 30 h. This EF lies at the low end of wide range of values reported for unincorporated solid cattle manure in literature (2.5-70%) depending on manure type and management. The comparatively low emissions in this study likely reflect the characteristics of the separated solid fraction and rapid post-application incorporation. Long-term background NH3 concentrations and fluxes were generally low and broadly linearly dependent on temperature, while no clear dependence on windspeed was observed. The measured background concentrations (0.3 +/- 0.2 & micro;g m(-3)) were similar to as reported for a nearby urban-background site in Finland (0.2 +/- 0.3 & micro;g m(-3)) but much lower than values observed at agricultural hotspots in Europe (>10 & micro;g m(-3)).

Isoprene-derived epoxydiols secondary organic aerosol (IEPOX SOA) and sulfate particles are major contributors to atmospheric particulate matter and play central roles in transforming gaseous emissions into secondary organic and inorganic aerosol. Although both species form through aqueous-phase reactions, the timing and lifecycle controls governing aqueous secondary organic aerosol (SOA) formation remain poorly constrained, in contrast to the relatively well-understood in-cloud formation of sulfate. Here, we combine single-particle measurements and a particle-resolved model of aerosol-cloud cycling to quantify processes governing aqueous sulfate and IEPOX SOA chemistry in aerosol particles and cloud droplets observed during the HI-SCALE field campaign, representing the first comparison of single-particle observations with particle-resolved simulations of cloud-processed aerosol. . We identify fundamental differences in the processes controlling their formation: sulfate production is governed by aqueous oxidation kinetics within cloud droplets, whereas IEPOX SOA forms rapidly during droplet evaporation and is strongly controlled by cloud-water acidity and the transient concentration of nucleophiles. These results demonstrate that sulfate and IEPOX SOA form at distinct stages of the aerosol–cloud life cycle and suggest that models neglecting droplet-scale concentration effects may substantially underestimate IEPOX SOA formation. More broadly, our findings indicate that evaporation-driven concentration dynamics during cloud cycling likely regulate other aqueous SOA pathways that depend on nucleophile availability.

Abstract. Biomass burning (BB) emits large amounts of pollutants in the particle and gas phases, with significant implications for air quality, human health and climate. Here, we investigate the emission of organic vapors from controlled burns of relatively understudied biomass fuels: woody plants and grasses from African savannah and European boreal forest surface using a high-resolution proton transfer reaction-mass spectrometer. To understand the effect of different oxidation regimes, organic vapors were aged in a 29 m3 Teflon chamber, where photochemical and dark aging were simulated. The average total primary emission factors (EFs) for organic vapors varied considerably with fuel type, ranging from 69 to 161 g kg-1. Photochemical aging led to substantial depletion of furanics, phenolics and oxygenated aromatics, accompanied by enhancements of carbonyl B compounds and O-containing compounds C<6 across experiments. In contrast, dark aging under low-NOx conditions produced minimal compositional changes. Hierarchical clustering of relative composition showed clear regime dependence, with regime-associated differences accounting for 73 % of the variance in group-level composition. Toluene and furan showed a strong negative correlation with secondary oxygenated volatile organic compounds (OVOCs), including anhydrides and small acids, consistent with their role as precursors. After 0.5 equivalent day of photochemical aging, organic vapors shifted to higher O/C (>0.70) and an increased fraction of CxHyOz (z≥3). These results highlight the integral role of OH·-driven photo-oxidation in governing the atmospheric evolution and composition of BB organic vapors and underscore the need for secondary organic aerosols (SOA) models to include non-traditional precursors.

Anthropogenic ammonia (NH3) emissions have significantly increased in recent decades due to enhanced agricultural activities, contributing to global air pollution. While the effects of NH3 on surface air quality are well documented, its influence on particle dynamics in the upper troposphere-lower stratosphere (UTLS) and related aerosol impacts remain unquantified. NH3 reaches the UTLS through convective transport and can enhance new particle formation (NPF). This modeling study evaluates the global impact of anthropogenic NH3 on UTLS particle formation and quantifies its effects on aerosol loading and cloud condensation nuclei (CCN) abundance. We use the EMAC Earth system model, incorporating multicomponent NPF parameterizations from the CERN CLOUD experiment. Our simulations reveal that convective transport increases NH3-driven NPF in the UTLS by one to three orders of magnitude compared to a baseline scenario without anthropogenic NH3, causing a doubling of aerosol numbers over high-emission regions. These aerosol changes induce a 2.5-fold increase in upper tropospheric CCN concentrations. Anthropogenic NH3 emissions increase the relative contribution of water-soluble inorganic ions to the UTLS aerosol optical depth (AOD) by 20% and increase total column AOD by up to 80%. In simulations without anthropogenic NH3, UTLS aerosol composition is dominated by sulfate and organic species, with a marked reduction in ammonium nitrate and aerosol water content. This results in a decline of aerosol mass concentration by up to 50%. These findings underscore the profound global influence of anthropogenic NH3 emissions on UTLS particle formation, AOD, and CCN production, with important implications for cloud formation and climate.

The Filter Inlet for Gases and AEROsols coupled with a Chemical Ionization Mass spectrometer (FIGAERO-CIMS) is a widely used method for determining the chemical composition of the molecular constituents of atmospheric organic aerosols (OA). This temperature-programmed desorption technique thermally desorbs OA in a linearly ramped desorption temperature, and the temperature at a detected molecule's peak desorption rate, Tmax⁡, is proportional to the molecule's volatility. Thereby, FIGAERO-CIMS also enables a direct measurement of the volatilities (saturation vapor pressures) of the OA constituents. A series of polyethylene glycols (PEGs) has been used to quantitatively connect FIGAERO measurement results (in particular, Tmax⁡) to volatilities (i.e., calibrate). However, available literature values of saturation pressure (Psat) or saturation mass concentration (C*) for these compounds only extend to PEG 9, which exhibits Tmax⁡ values around ∼ 90 °C, whereas Tmax⁡ values of OA constituents measured from lab-generated or ambient aerosols routinely reach up to 160 °C (Li et al., 2021; Masoud et al., 2022). To extend the region over which we can conveniently calibrate FIGAERO-CIMS, and hypothetically also other thermal desorption-based techniques for investigating OA composition and volatilities, we performed FIGAERO-CIMS calibration experiments using aerosol particles consisting of PEGs 5-15, which yielded Tmax⁡ values of up to ∼ 150 °C. We then set out to estimate the hitherto unknown Psat (C*) values of PEGs 10-15 by utilizing a suite of different Psat estimation methods: both measurement-independent methods (quantum chemistry-based calculations, molecular structure-based group contribution methods, and parametrizations based on molecular sum formulas) and fits of an explicit desorption model to our FIGAERO measurement results with C* and vaporization enthalpies as free parameters. We assess the respective suitability of each method and argue that we obtain the best estimates for PEG volatilities based on the fits to our measurements. We obtained log 10(C* (µg m−3)) values ranging from 0.51 ± 0.07 (PEG 6) to −9.2 ± 1.6 (PEG 14), agreeing with previous literature results on PEGs < 10. Within uncertainties, our results broadly continue the near log-linear relationship of C* with PEG mass for larger PEGs and also agree with some of the independent methods. Contrary to common assumptions in previous literature on FIGAERO results, we find that the relationship between log⁡10(C*(µgm-3)) and measured Tmax⁡ is not linear. We explore the consequences of this finding on the analysis of previously published FIGAERO-CIMS measurements of sesquiterpene-derived OA. Prospects for improving on our results in future work are discussed. We conclude that calibration experiments using aerosol containing PEGs up to ∼ PEG 15, with best-estimated saturation vapor pressures, provide promising opportunities for constraining the volatilities of aerosol constituents, down throughout the range of extremely low-volatility organic compounds (ELVOC, C* < 3 × 10−4 µg m−3), as detected not only via FIGAERO-CIMS but also other (online) temperature-programmed desorption techniques.

Isoprene (C5H8) is the non-methane hydrocarbon with the highest emissions to the atmosphere. It is mainly produced by vegetation, especially broad-leaved trees, and efficiently transported to the upper troposphere in deep convective clouds, where it is mixed with lightning NOx. Isoprene oxidation products drive rapid formation and growth of new particles in the tropical upper troposphere. However, isoprene oxidation pathways at low temperatures are not well understood. Here, in experiments at the CERN CLOUD chamber at 223 K and 243 K, we find that isoprene oxygenated organic molecules (IP-OOM) all involve two successive OH ∙ oxidations. However, depending on the ambient concentrations of the termination radicals ( HO 2 ∙ , NO ∙ , and NO 2 ∙ ), vastly-different IP-OOM emerge, comprising compounds with zero, one or two nitrogen atoms. Our findings indicate high IP-OOM production rates for the tropical upper troposphere, mainly resulting in nitrate IP-OOM but with an increasing non-nitrate fraction around midday, in close agreement with aircraft observations.

We present a "diagonal" Volatility Basis Set (dVBS) comparing gas-phase concentrations of oxygenated organic molecules (OOM) to their condensed-phase mass fractions. This permits closure of vapor concentrations with particle composition constrained by particle growth rates, allowing the contributions of quasi non-volatile condensation, equilibrium partitioning, and reactive uptake to be separated. The dVBS accommodates both equilibrium and dynamical (growth) conditions. Growth implies an association between gas and particle concentrations governed by a "condensation line" that is set by the particle growth rate, which fixes the total (excess) concentration of condensible vapors. The condensation line defines an infeasible region of high particle mass fraction and low gas concentration; under steady-state growth conditions, compounds cannot appear in this infeasible region without being formed by condensed-phase chemistry. We test the dVBS with observations from the CLOUD experiment at CERN using data from a FIGAERO I- Chemical Ionization Mass Spectrometer measuring vapors directly and particle composition via temperature programmed desorption from a filter. A dVBS analysis finds that data from an α-pinene + O3 run at 243 K are consistent with volatility driven condensation forming the large majority of particle mass, with no compounds clearly within the infeasible region.

The chain of chemical and physical processes leading to formation of new aerosol particles from gaseous precursors vapors is often called new particle formation (NPF). Although first observations of atmospheric NPF date back to more than a century ago, many aspects of the phenomenon and its importance on global climate remained unknown for a long time. Along with the development of more robust measurement techniques enabling continuous field measurements of particle size distributions down to the size of recently formed particles and their precursors vapors, NPF research has taken leaps forward in the past decades. In this article we review how the new measurement methods has enabled us to observe, analyze and classify atmospheric new particle formation events and how this has changed our understanding of the process and its significance in the atmosphere.

Isoprene is a reactive hydrocarbon emitted to the atmosphere in large quantities by terrestrial vegetation. Annual total isoprene emissions exceed 300 Tg a −1 , but emission rates vary widely among plant species and are sensitive to meteorological and environmental conditions including temperature, sunlight, and soil moisture. Due to its high reactivity, isoprene has a large impact on air quality and climate pollutants such as ozone and aerosols. It is also an important sink for the hydroxyl radical which impacts the lifetime of the important greenhouse gas methane along with many other trace gas species. Modeling the impacts of isoprene emissions on atmospheric chemistry and climate requires accurate isoprene emission estimates. These can be obtained using the empirical Model of Emissions of Gases and Aerosols from Nature (MEGAN), but the parameterization of this model is uncertain due in part to limited field observations. In this study, we use ground‐based measurements of isoprene concentrations and fluxes from 11 field sites to assess the variability of the isoprene emission temperature response across ecosystems. We then use these observations in a Metropolis‐Hastings Markov Chain Monte Carlo (MHMCMC) data assimilation framework to optimize the MEGAN temperature response function. We find that the performance of MEGAN can be significantly improved at several high‐latitude field sites by increasing the modeled sensitivity of isoprene emissions to past temperatures. At some sites, the optimized model was nearly four times more sensitive to temperature than the unoptimized model. This has implications for air quality modeling in a warming climate.

Diterpenes, found in trees, have been overlooked in atmospheric chemistry due to their low volatility and presumed negligible emissions. However, advances in sampling and analytical methods now allow for their detection in the gas phase. Here we quantify diterpene emission factors and evaluate their role in secondary organic aerosol formation. By incorporating all available emission data and laboratory-derived aerosol yields into the MONARCH chemistry transport model, we assessed their atmospheric contribution. Chamber experiments revealed kaurene yields of 1.8-17%, indicating efficient gas-to-particle conversion. Global emissions were estimated at 11.5 (0.1-94.3) Tg yr-(1), contributing 0.63 (0.005-5.19) Tg yr-(1) to secondary organic aerosol production and a burden of 0.008 (0.00007-0.07) Tg. This corresponds to 13%, 6.4%, and 19% of the aerosol burden from isoprene, monoterpenes, and sesquiterpenes, respectively. These results demonstrate that diterpenes are a previously underestimated, but potentially important, source of secondary organic aerosol, with implications for biosphere-atmosphere interactions.

Isoprene oxygenated organic molecules (IP-OOM) can nucleate new particles in the upper troposphere. These particles may grow into cloud condensation nuclei and influence the clouds and climate. However, little is known about the individual species driving growth and whether they undergo condensed-phase reactions. We conducted isoprene oxidation experiments at 223 and 243 K in the CLOUD chamber at CERN. Gas-phase concentrations were measured with chemical ionization mass spectrometers (NO3 --CIMS, Br--MION2-CIMS, and NH4 +-CIMS). Growth rates from 8 to 20 nm were measured by a Neutral Cluster and Air Ion Spectrometer. Particle-phase composition was measured by a filter sampling chemical ionization mass spectrometer. We use the diagonal volatility basis set (dVBS) analysis framework to compare gas- and particle-phase measurements and assess species and processes influencing growth. We find that kinetically limited condensation of a few species dominates particle composition and growth. Particle-phase processes, including oligomerization and organonitrate hydrolysis, do not influence the early growth. dVBS growth rate predictions can explain 90% of the measured growth, dominated by kinetic condensation of low-volatility species. Our findings indicate that initial growth of IP-OOM particles under cold, low-acid conditions may be controlled and modeled by the kinetically limited condensation of low-volatility compounds.

Global warming and climate change-induced rise in Earth’s temperature have increased the frequency of forest/wildfires over the past decade. Therefore, understanding the effect of fire emissions on aerosol-cloud interactions is crucial for improving Earth system models. We present observations from in-situ measurements of aerosol properties at the Puijo SMEAR IV station in eastern Finland and the Zeppelin Observatory in Ny-Ålesund, High Arctic. Both stations are frequently inside low-level clouds due to their topographic prominence. During the autumn of 2020, fire emissions from the same active fire region in south-eastern (SE) Europe reached both stations after ~2 - 8 days of atmospheric aging. This enabled us to investigate the changes in aerosol and cloud properties for clouds formed under the influence of aged fire emissions (referred to as the ‘fire’ period) and under cleaner conditions with no fire emission influence at these stations (‘non-fire’ period). The aerosol hygroscopicity parameter (κchem) was derived from the chemical composition data obtained from online aerosol mass spectrometers and was used to derive the number concentration of cloud condensation nuclei (NCCN) from the measured particle size distributions. At both stations, the aerosol number concentration in the accumulation mode and the cloud condensation nuclei concentration (NCCN) were higher during the fire period than during non-fire times. However, the aerosol hygroscopicity increased at Puijo but decreased a Zeppelin from the non-fire to fire period. At Puijo, in-situ measured cloud droplet number concentration (CDNC) was by a factor of ~7 higher when comparing fire to non-fire periods. This was in good agreement with the satellite observations (MODIS, Terra). At Puijo, the higher CCN concentrations during the fire period cause a depletion of the water vapor available for cloud droplet activation leading to larger observed activation diameters during cloud events despite the higher hygroscopicity of the aerosol particles. These observations show the importance of SE European fires for enhancing the CCN activity in Finland and the high Arctic. Results from this study emphasize the complex interplay between particle size and chemical composition, and how fires even from sources far away can have strong impacts in these remote regions.

Anthropogenic ammonia (NH 3 ) emissions have significantly increased in recent decades due to enhanced agricultural activities, contributing to global air pollution. While the effects of NH 3 on surface air quality are well documented, its influence on particle dynamics in the upper troposphere-lower stratosphere (UTLS) and related aerosol impacts remain unquantified. NH 3 reaches the UTLS through convective transport and can enhance new particle formation (NPF). This modeling study evaluates the global impact of anthropogenic NH 3 on UTLS particle formation and quantifies its effects on aerosol loading and cloud condensation nuclei (CCN) abundance. We use the EMAC Earth system model, incorporating multicomponent NPF parameterizations from the CERN CLOUD experiment. Our simulations reveal that convective transport increases NH 3 -driven NPF in the UTLS by one to three orders of magnitude compared to a baseline scenario without anthropogenic NH 3 , causing a doubling of aerosol numbers over high-emission regions. These aerosol changes induce a 2.5-fold increase in upper tropospheric CCN concentrations. Anthropogenic NH 3 emissions increase the relative contribution of water-soluble inorganic ions to the UTLS aerosol optical depth (AOD) by 20% and increase total column AOD by up to 80%. In simulations without anthropogenic NH 3 , UTLS aerosol composition is dominated by sulfate and organic species, with a marked reduction in ammonium nitrate and aerosol water content. This results in a decline of aerosol mass concentration by up to 50%. These findings underscore the profound global influence of anthropogenic NH 3 emissions on UTLS particle formation, AOD, and CCN production, with important implications for cloud formation and climate.

The apparent volatility of atmospheric organic aerosol (OA) particles is determined by their chemical composition and environmental conditions (e.g., ambient temperature). A quantitative, experimental assessment of volatility and the respective importance of these two factors remains challenging, especially in ambient measurements. We present molecular composition and volatility of oxygenated OA (OOA) particles in different rural, urban, and mountain environments (including Chacaltaya, Bolivia; Alabama, US; Hyytiälä, Finland; Stuttgart and Karlsruhe, Germany; and Delhi, India) based on deployments of a filter inlet for gases and aerosols coupled to a high-resolution time-of-flight chemical ionization mass spectrometer (FIGAERO-CIMS). We find on average larger carbon numbers (nC​​​​​​​) and lower oxygen-to-carbon (O : C) ratios at the urban sites (nC: 9.8 ± 0.7; O : C: 0.76 ± 0.03; average ±1 standard deviation) compared to the rural (nC: 8.8 ± 0.6; O : C: 0.80 ± 0.05) and mountain stations (nC: 8.1 ± 0.8; O : C: 0.91 ± 0.07), indicative of different emission sources and chemistry. Compounds containing only carbon, hydrogen, and oxygen atoms (CHO) contribute the most to the total OOA mass at the rural sites (79.9 ± 5.2 %), in accordance with their proximity to forested areas (66.2 ± 5.5 % at the mountain sites and 72.6 ± 4.3 % at the urban sites). The largest contribution of nitrogen-containing compounds (CHON) is found at the urban stations (27.1 ± 4.3 %), consistent with their higher NOx levels. Moreover, we parametrize OOA volatility (saturation mass concentrations, Csat) using molecular composition information and compare it with the bulk apparent volatility derived from thermal desorption of the OOA particles within the FIGAERO. We find differences in Csat values of up to ∼ 3 orders of magnitude and variation in thermal desorption profiles (thermograms) across different locations and systems. From our study, we draw the general conclusion that environmental conditions (e.g., ambient temperature) do not directly affect OOA apparent volatility but rather indirectly by influencing the sources and chemistry of the environment and thus the chemical composition. The comprehensive dataset provides results that show the complex thermodynamics and chemistry of OOA and their changes during its lifetime in the atmosphere. We conclude that generally the chemical description of OOA suffices to predict its apparent volatility, at least qualitatively. Our study thus provides new insights that will help guide choices of, e.g., descriptions of OOA volatility in different model frameworks such as air quality models and cloud parcel models.