Secondary organic aerosols (SOA) contribute significantly to cloud condensation nuclei (CCN), which depend on particle size distribution (PSD), chemical composition and the hygroscopicity parameter (kappa). Simulating SOA and CCN in chemical transport models relies on parameterizations, which need to be evaluated and improved against process-level models as a benchmark. Here, we simulated SOA concentration, chemical composition, PSD, kappa, and CCN in alpha-pinene ozonolysis, a classical system for SOA studies, using a process-level box model PyCHAM with near-explicit chemical mechanisms. We assessed how CCN, chemical composition, PSD and kappa can be modelled against measurements and evaluated the influence of these factors on CCN simulation. The model well simulated SOA mass concentration but overestimated O : C and H : C ratios, suggesting a possible lack of particle-phase chemistry. Highly oxygenated molecules (HOMs) contributed substantially to SOA mass. Simulated kappa closely agreed with measurements at moderate supersaturation (0.37 %) but was overestimated at low supersaturation (0.19 %) and underestimated at high supersaturation (0.55 % and 0.73 %). Particle growth and number concentrations were reasonably reproduced, though the simulated PSD was broader and flatter than measurement. Simulated CCN concentrations agreed well with measurements at moderate to high supersaturation (0.37 %-0.73 %) but were overestimated at low supersaturation (0.19 %). Sensitivity analysis highlights the importance of accurately representing both PSD and kappa for reliable CCN prediction, especially at supersaturation < 0.4 %. This study also highlights that HOM formation, finer PSD resolution and improved kappa parameterizations are warranted in future chemical transport models, and evaluates the ability and limitations of this benchmark model.

An important property of the compounds comprising organic aerosol is volatility, typically described in terms of saturation vapor pressure or saturation concentration (measured in µg/m3). The volatility of aerosol constituents can be estimated based on their molecular formula using different parametrizations or measured experimentally by using a chemical ionization mass spectrometer (CIMS) coupled to a filter inlet for gases and aerosols (FIGAERO). In this technique, aerosol sample is collected semi-online and evaporated via gradually heated nitrogen flow desorbing organic constituents to be measured by CIMS. From the temperature at which detected chemical species reach their maximum signal, it is possible to determine the respective compositions’ volatility.In 2024, the FIGAERO-CIMS was deployed at the CHANEL (household chemicals amplifying urban aerosol pollution) measurement campaign at the SAPHIR chamber at Jülich Research Centre, Germany. During the campaign, complex reactive mixtures representing urban air scenarios were injected into the chamber, and exposed to both day- and night-time oxidation via opening or closing the roof to natural sunlight. We developed a multi-peak fitting algorithm to fully fit each composition’s thermogram (signal vs. desorption temperature), resulting in multiple nominal saturation concentrations per detected composition. We interpret these as combinations of simple volatility-driven desorption and decomposition (typically at higher temperatures) of larger compounds, such as accretion products.We tracked the chemical composition and volatility of secondary organic aerosol throughout its formation and subsequent aging in the chamber over several hours. The chemical composition measured by FIGAERO-CIMS was compared with other co-located online mass spectrometric techniques, e.g., CIMS following online aerosol evaporation by a heated sheath flow (WALL-E). Our initial results show how aerosol volatilities typically decreased with age, as more oxygen was incorporated. Further, night-time conditions resulted in both increased organonitrate formation and lower product volatility relative to day-time conditions.

Particle linear depolarization ratio is a widely used parameter in lidar research to distinguish different aerosol types and the thermodynamic phase of water. It is most frequently measured at ultraviolet and visible wavelengths (355 and 532 nm), yet multi-wavelength observations suggest that this parameter can vary substantially with wavelength. In this work, we assessed particle linear depolarization ratios at 1565 nm using Halo Photonics StreamLine Doppler lidars. We examined the depolarization ratio through three case studies featuring extremely fresh and aged smoke, and volcanic ash aerosol particles in the troposphere. Both fresh and aged smoke aerosol particles induced low values. Specifically, aerosol layers dominated by extremely fresh smoke showed a depolarization ratio of 0.017 ± 0.004, whereas aged long-range transported smoke particles exhibited marginally higher values. Volcanic aerosol layers induced high depolarization ratios with layer mean values of 0.45 ± 0.01. For the extremely fresh smoke case, we further estimated the smoke mass concentration using the lidar observations at 1565 nm and found good agreement with the in situ observations. These results demonstrate that Halo Doppler lidars operating at 1565 nm wavelength are capable of distinguishing several key aerosol types, enabling a comprehensive characterization of atmospheric conditions by simultaneously observing aerosol properties and wind dynamics.

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.

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.

Secondary organic aerosol (SOA), formed by oxidation of volatile organic compounds, significantly influence air quality and climate. Biogenic highly oxygenated organic molecules (HOM), particularly those formed from monoterpenes, play a key role in SOA formation and growth. As the most important daytime oxidant, hydroxyl radical (OH•) initiated HOM from monoterpenes is believed to be mainly formed via OH addition channel. However, for α-pinene and limonene, we found that the contribution of hydrogen abstraction channel by OH contribute a significantly to HOM formation. We will present our observations and theoretical calculations, showing the role of hydrogen-abstraction and alkoxy radicals for fast autoxidation leading to HOM formation. We also provide formation mechanisms of and yields of HOM, suggesting the non-negligible contribution of the hydrogen abstraction channel to ambient SOA, particularly in OH-rich areas.

Highly oxygenated organic compounds (HOMs) from α-pinene oxidation are of great interest because of their importance in secondary organic aerosol (SOA) formation. Despite intensive investigations, the mechanisms of HOM formation from first-generation peroxy radicals to HOM-peroxy radicals (HOM-RO2⚫) and to HOM-closed shell products are not well understood. One reason is that HOM-alkoxy radicals (HOM-RO⚫) are likely to contribute to the propagation of oxidative radical chains (alkoxy-peroxy pathway) because isomerization of functionalized alkoxy radicals can compete with their fragmentation (and reaction with O2), as shown by theoretical kinetics. However, HOM-RO⚫ reaction steps are difficult to verify in mechanisms. In this work, we have investigated HOM formation by varying the significance of the alkoxy-peroxy pathway as a function of NOx, OH⚫, and CO. HOM-RO⚫ are likely formed with high branching ratios in reactions of HOM-RO2⚫ with peroxy radicals and NO in analogy to simpler alkoxy radicals. We provide experimental evidence that for HOM-RO⚫ the branching into isomerization is about 50 % (±14 %). Thus, HOM-RO⚫ can play a central role in HOM formation, since alkoxy-peroxy pathways can compete with direct autoxidation. We observed significant concentrations of HOM-RO2⚫, despite fast termination by NO, and shifts to higher O/C for HOM-RO2⚫ and termination products with increasing NO. At NO concentrations > 1.5 ppb, the alkoxy-peroxy pathway may even prevail in propagating the oxidative radical chain leading to HOM formation. The increasing sink of HOM-RO2⚫ with increasing concentration of peroxy radicals and NO is compensated by an increasing source via the alkoxy-peroxy pathway. This manuscript is dedicated to Astrid Kiendler-Scharr (deceased, 6 February 2023).

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.

Fire is an integral part of savanna and grassland biomes and globally approximately half of landscape fire emissions originate from savannas and grasslands. Emissions of trace gases and aerosol particles from landscape fires are characterised by emission factors (EFs), which denote the amount of emitted substance per mass of combusted biomass. EFs vary depending on both the biomass that is consumed in the fire and the combustion characteristics of the fire, i.e. the ratio of flaming to smouldering combustion. However, emission inventories tend to use only one average EF for each biome. Here, we use a set of 27 laboratory experiments to characterise the effect of combustion characteristics on submicron aerosol EFs from savanna and grassland biomass acquired from South Africa as well as boreal forest floor samples from Finland. Combustion experiments were carried out at the ILMARI facility in Kuopio, Finland from May to June 2022 under an open stack mimicking natural burning and dilution. Sample was injected into a 29 m3 environmental chamber for ageing studies. Chemical and physical properties of both fresh and aged smoke were observed with a host of instruments including e.g. AMS, FIGAERO-CIMS, VOCUS, SP2 and SMPS. The ratio of flaming to smouldering combustion was characterised by modified combustion efficiency (MCE), i.e. CO2/(CO2+CO). The increase of organic aerosol EF with increasing smouldering fraction (i.e. decreasing MCE) was very similar for both the grassland and savanna combustion experiments. Surprisingly, also the boreal forest floor EFs closely follow the same trend, where smouldering-dominated combustion EFs are more than 10 times higher than EFs for flaming combustion. We observed also that the submicron aerosol particle size distribution shifts towards larger sized particles with increasing smouldering fraction. Furthermore, both the number and the mass of the size distribution cannot be fully characterised with a single log-normal size distribution, which needs to be considered when converting mass emissions into number size distribution in simulations.

Biogenic secondary organic aerosols (SOAs) can be formed from the oxidation of plant volatiles in the atmosphere. Herbivore-induced plant volatiles (HIPVs) can elicit plant defenses, but whether such ecological functions persist after they form SOAs was previously unknown. Here we show that Scots pine seedlings damaged by large pine weevils feeding on their roots release HIPVs that trigger defenses in neighboring conspecific plants. The biological activity persisted after HIPVs had been oxidized to form SOAs, which was indicated by receivers displaying enhanced photosynthesis, primed volatile defenses, and reduced weevil damage. The elemental composition and quantity of SOAs likely determines their biological functions. This work demonstrates that plant-derived SOAs can mediate interactions between plants, highlighting their ecological significance in ecosystems.

Particulate matter represents one of the most severe air pollutants globally. Organic aerosol (OA) comprises 30-70 % of submicron particle mass in urban areas. An effective way to mitigate OA particulate pollutants is to reduce the formation of secondary organic aerosol (SOA). Here, we studied the effect of titanium dioxide (TiO2) photocatalytic seeds on the formation and mitigation of SOA particles from alpha-pinene or toluene oxidation in chamber. For the first time, we discovered that under ultraviolet (UV) irradiation, the presence of TiO2 directly removed internally mixed alpha-pinene SOA mass by 53.7 % within 200 mins, and also directly removed SOA matter in an externally mixed state that is not in direct contact with TiO2 surface: the mass of externally mixed alpha-pinene SOA was reduced by 21.9 % within 81 mins, and the toluene SOA mass was reduced by 46.6 % in 145mins. In addition, the presence of TiO2 effectively inhibited the formation of SOA particles with a SOA mass yield of zero. This study brings up an innovative concept for air pollution control - the direct photocatalytic degradation of OA with aid of TiO2-based photocatalysts. Our novel findings will potentially bring practical applications in air pollution abatement and regional, even global aerosol-climate interactions.

AbstractActive vegetation fires in south‐eastern (SE) Europe resulted in a notable increase in the number concentration of aerosols and cloud condensation nuclei (CCN) particles at two high latitude locations—the SMEAR IV station in Kuopio, Finland, and the Zeppelin Observatory in Svalbard, high Arctic. During the fire episode aerosol hygroscopicity κ slightly increased at SMEAR IV and at the Zeppelin Observatory κ decreased. Despite increased κ in high CCN conditions at SMEAR IV, the aerosol activation diameter increased due to the decreased supersaturation with an increase in aerosol loading. In addition, at SMEAR IV during the fire episode, in situ measured cloud droplet number concentration (CDNC) increased by a factor of ∼7 as compared to non‐fire periods which was in good agreement with the satellite observations (MODIS, Terra). Results from this study show the importance of SE European fires for cloud properties and radiative forcing in high latitudes.

Abstract Anthropogenic nitrogen oxides may influence the cloud condensation nuclei (CCN) activity of biogenic secondary organic aerosols (SOA) in both daytime photooxidation and nighttime NO3 oxidation, which has significant implications for the climatic impact of SOA. We investigated the influence of NOx on the CCN activity of monoterpene‐derived SOA in OH oxidation and in NO3 oxidation. In OH oxidation, NOx had little influence on the hygroscopic parameter κ of organic aerosol (κOrg), which was attributed to the minor fraction of organic nitrates (ON) in SOA (<24%), resulted from the low branching ratio of RO2 + NO to form ON. In contrast, in NO3 oxidation κOrg was much reduced compared to OH/O3 oxidation due to a dominant fraction of ON. We report κ of MT‐derived ON formed in photo‐oxidation and NO3 oxidation (0.029–0.052) for the first time to our knowledge, which may be used to improve model simulations of CCN concentrations.

Filter Inlet for Gases and Aerosols (FIGAERO) inlet coupled with Time-of-Flight Chemical Ionization Mass Spectrometers (ToF-CIMS) have been successfully employed in numerous studies during past years (Thornton et. al., 2020). The instrument can be used to study both gas-phase and particle-phase chemical compounds over a broad range of chemical functionalities. The inbuilt controlled thermal desorption mechanism also allows for the investigation of volatility of the compounds present in the particle phase. Such a particle volatility measurement using FIGAERO-ToF-CIMS relies on accurate identification of the Tmax for individual compounds, i.e. the temperatures at which highest respective signals are observed. These Tmax values can be converted to saturation vapor pressure Psat and saturation mass concentration C* values with a reliable calibration (Ylisirniö et. al., 2021). However, due to a lack of calibration compounds with known Psat at the low end of the volatility range (Psat <10-9 (Pa) / C* < -10-4 (μg/m3)), the current calibration procedure can only cover the desorption temperature range up to ~80-100 ˚C, while desorption temperatures of FIGAERO-ToF-CIMS can reach 200 ˚C and Tmax values are routinely identified up to 160 ˚C. In this study we aim to extend the FIGAERO-ToF-CIMS calibration values to cover lower Psatvalues by utilizing a range of different approaches to increase the accuracy of the volatility measurement.MethodsTmax values of Polyethylene Glycols (PEG’s) from PEG 5 to 15 were measured with FIGAERO-CIMS and corresponding C* values were either measured (for PEG 5-9) with isothermal evaporation experiments or estimated (PEG 5-15) with several different methods. Used estimation methods were quantum chemical modelling (COSMOtherm), desorption modelling (Schobesberger et. al., 2018), and parametrizations including Modified Grain Model, EVAPORATION, SIMPOL, and by Li et. al., 2016, Stolzenburg et. al., 2018 and Mohr et. al., 2019. Additionally, a parametrization equation according to Li et. al. 2016 was fitted to PEG 4-9 literature data and C* values of PEG 10-15 were estimated with this fit. ResultsFigure 1 shows the measured and estimated C*(298 K) values as function of the measured Tmaxvalues. The spread of results with different models is increasingly larger especially at the higher order PEG’s, with over 10 orders of magnitude difference between models. Two independent measurement methods agree with each other within one order of magnitude. Best agreement with measurements is with desorption model, Modified Grain Model and COSMOTherm.Figure 1. Tmax vs. C*. Results from different measurements or estimates are shown in the figure legend. Note that some markers are very close to each other. This work was supported by Academy of Finland Flagship funding (grant no. 337550) Li et. al., 2016, Atmos. Chem. Phys., 16, 3327-3344.Krieger et. al., 2018, Atmos. Meas. Tech., 11(1), 49–63.Mohr et. al., 2019, Nat. Comm. 10, Article number: 4442.Schobesberger et. al., 2018, Atmos. Chem. Phys., 18(20), 14757–14785.Stolzenburg et. al., 2018, PNAS, 115 (37) 9122-9127.Thornton et. al., 2020, Acc. Chem. Res., 53(8), 1415–1426.Ylisirniö et. al., 2021, Atmos. Meas. Tech., 14(1), 355–367.

Highly oxygenated organic molecules (HOMs) play a pivotal role in the formation of secondary organic aerosol (SOA). Therefore, the distribution and yields of HOMs are fundamental to understand their fate and chemical evolution in the atmosphere, and it is conducive to ultimately assess the impact of SOA on air quality and climate change. In this study, gas-phase HOMs formed from the reaction of limonene with OH radicals in photooxidation were investigated with SAPHIR (Simulation of Atmospheric PHotochemistry In a large Reaction chamber), using a time-of-flight chemical ionization mass spectrometer with nitrate reagent ion (NO3--CIMS). A large number of HOMs, including monomers (C9–10) and dimers (C17–20), were detected and classified into various families. Both closed-shell products and open-shell peroxy radicals (RO2) were identified under low NO (0.06–0.1 ppb) and high NO conditions (17 ppb). C10 monomers are the most abundant HOM products and account for over 80 % total HOMs. Closed-shell C10 monomers were formed from a two peroxy radical family, C10H15Ox⚫ (x=6–15) and C10H17Ox⚫ (x=6–15), and their respective termination reactions with NO, RO2, and HO2. While C10H17Ox⚫ is likely formed by OH addition to C10H16, the dominant initial step of limonene plus OH, C10H15Ox⚫, is likely formed via H abstraction by OH. C10H15Ox⚫ and related products contributed 41 % and 42 % of C10 HOMs at low and high NO, demonstrating that the H-abstraction pathways play a significant role in HOM formation in the reaction of limonene plus OH. Combining theoretical kinetic calculations, structure–activity relationships (SARs), data from the literature, and the observed RO2 intensities, we proposed tentative mechanisms of HOM formation from both pathways. We further estimated the molar yields of HOMs to be 1.97-1.06+2.52 % and 0.29-0.16+0.38 % at low and high NO, respectively. Our study highlights the importance of H abstraction by OH and provides the yield and tentative pathways in the OH oxidation of limonene to simulate the HOM formation and assess the role of HOMs in SOA formation.

Many recent secondary organic aerosol (SOA) studies mainly focus on biogenic SOA particles formed under low NOx conditions and thus are applicable to pristine environments with minor anthropogenic influence. Although interactions between biogenic volatile organic compounds and NOx are important in, for instance, suburban areas, there is still a lack of knowledge about the volatility and processes controlling the evaporation of biogenic SOA particles formed in the presence of high concentrations of NOx. Here we provide detailed insights into the isothermal evaporation of α-pinene SOA particles that were formed under low NOx and high NOx conditions to investigate the evaporation process and the evolution of particle composition during the evaporation in more detail. We coupled Filter Inlet for Gases and AEROsols-Chemical Ionization Mass Spectrometer (FIGAERO-CIMS) measurements of the molecular composition and volatility of the particle phase with isothermal evaporation experiments conducted under a range of relative humidity (RH) conditions from low RH (<7 % RH) to high RH (80 % RH). Very similar changes were observed in particle volatility at any set RH during isothermal evaporation for the α-pinene SOA particles formed under low NOx and high NOx conditions. However, there were distinct differences in the initial composition of the two SOA types, possibly due to the influence of NOx on the RO2 chemistry during SOA formation. Such compositional differences consequently impacted the primary type of aqueous-phase processes in each type of SOA particle in the presence of particulate water.

This document contains additional information about the modelling closure study of aerosol-interactions using UCLALES-SALSA and in-cloud observations from the Puijo 2020 campaign.

We carried out a closure study of aerosol–cloud interactions during stratocumulus formation using a large eddy simulation model UCLALES–SALSA (University of California Los Angeles large eddy simulation model–sectional aerosol module for large applications) and observations from the 2020 cloud sampling campaign at Puijo SMEAR IV (Station for Measuring Ecosystem–Atmosphere Relations) in Kuopio, Finland. The unique observational setup combining in situ and cloud remote sensing measurements allowed a closer look into the aerosol size–composition dependence of droplet activation and droplet growth in turbulent boundary layer driven by surface forcing and radiative cooling. UCLALES–SALSA uses spectral bin microphysics for aerosols and hydrometeors, and incorporates a full description of their interactions into the turbulent-convective radiation-dynamical model of stratocumulus. Based on our results, the model successfully described the probability distribution of updraught velocities and consequently the size dependency of aerosol activation into cloud droplets, and further recreated the size distributions for both interstitial aerosol and cloud droplets. This is the first time such a detailed closure is achieved not only accounting for activation of cloud droplets in different updraughts, but also accounting for processes evaporating droplets and drizzle production through coagulation–coalescence. We studied two cases of cloud formation, one diurnal (24 September 2020) and one nocturnal (31 October 2020), with high and low aerosol loadings, respectively. Aerosol number concentrations differ more than 1 order of magnitude between cases and therefore, lead to cloud droplet number concentration (CDNC) values which range from less than 100 cm−3 up to 1000 cm−3. Different aerosol loadings affected supersaturation at the cloud base, and thus the size of aerosol particles activating to cloud droplets. Due to higher CDNC, the mean size of cloud droplets in the diurnal high aerosol case was lower. Thus, droplet evaporation in downdraughts affected more the observed CDNC at Puijo altitude compared to the low aerosol case. In addition, in the low aerosol case, the presence of large aerosol particles in the accumulation mode played a significant role in the droplet spectrum evolution as it promoted the drizzle formation through collision and coalescence processes. Also, during the event, the formation of ice particles was observed due to subzero temperature at the cloud top. Although the modelled number concentration of ice hydrometeors was too low to be directly measured, the retrieval of hydrometeor sedimentation velocities with cloud radar allowed us to assess the realism of modelled ice particles. The studied cases are presented in detail and can be further used by the cloud modellers to test and validate their models in a well-characterized modelling setup. We also provide recommendations on how increasing amount of information on aerosol properties could improve the understanding of processes affecting cloud droplet number and liquid water content in stratiform clouds.