Aerosol climate effects are intimately tied to interactions with water. Here we combine hygroscopicity measurements with direct observations about the phase of secondary organic aerosol (SOA) particles to show that water uptake by slightly oxygenated SOA is an adsorption‐dominated process under subsaturated conditions, where low solubility inhibits water uptake until the humidity is high enough for dissolution to occur. This reconciles reported discrepancies in previous hygroscopicity closure studies. We demonstrate that the difference in SOA hygroscopic behavior in subsaturated and supersaturated conditions can lead to an effect up to about 30% in the direct aerosol forcing—highlighting the need to implement correct descriptions of these processes in atmospheric models. Obtaining closure across the water saturation point is therefore a critical issue for accurate climate modeling.

Secondary organic aerosols (SOA) forms a major fraction of organic aerosols in the atmosphere. Knowledge of SOA properties that affect their dynamics in the atmosphere is needed for improving climate models. By combining experimental and modeling techniques, we investigated the factors controlling SOA evaporation under different humidity conditions. Our experiments support the conclusion of particle phase diffusivity limiting the evaporation under dry conditions. Viscosity of particles at dry conditions was estimated to increase several orders of magnitude during evaporation, up to 109 Pa s. However, at atmospherically relevant relative humidity and time scales, our results show that diffusion limitations may have a minor effect on evaporation of the studied α‐pinene SOA particles. Based on previous studies and our model simulations, we suggest that, in warm environments dominated by biogenic emissions, the major uncertainty in models describing the SOA particle evaporation is related to the volatility of SOA constituents.

Aerosol and molecular processing in the atmosphere occurs in a complex and variable environment consisting of multiple phases and interfacial regions. To explore the effects of such conditions on the reactivity of chemical systems, we employ an environmental simulation chamber to investigate the multiphase photolysis of pyruvic acid, which photoreacts in the troposphere in aqueous particles and in the gas phase. Upon irradiation of nebulized pyruvic acid, acetic acid and carbon dioxide are rapidly generated, which is consistent with previous literature on the bulk phase photolysis reactions. Additionally, we identify a new C product, zymonic acid, a species that has not previously been reported from pyruvic acid photolysis under any conditions. Its observation here, and corresponding spectroscopic signatures, indicates it could be formed by heterogeneous reactions at the droplet surface. Prior studies of the aqueous photolysis of pyruvic acid have shown that high-molecular-weight compounds are formed via radical reactions; however, they are inhibited by the presence of oxygen, leading to doubt as to whether the chemistry would occur in the atmosphere. Identification of dimethyltartaric acid from the photolysis of multiphase pyruvic acid in air confirms radical polymerization chemistry can compete with oxygen reactions to some extent under aerobic conditions. Evidence of additional polymerization within the particles during irradiation is suggested by the increasing viscosity and organic content of the particles. The implications of multiphase specific processes are then discussed within the broader scope of atmospheric science.

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.

Abstract. Four measurement campaigns were performed in two different environments -inside the harbour areas in the city centre of Helsinki, and along the narrow shipping channel near the city of Turku, Finland -using a mobile laboratory van during winter and summer conditions in 2010-2011. The characteristics of gaseous (CO, CO 2 , SO 2 , NO, NO 2 , NO x ) and particulate (number and volume size distributions as well as PM 2.5 ) emissions for 11 ships regularly operating on the Baltic Sea were studied to determine the emission parameters. The highest particle concentrations were 1.5 × 10 6 and 1.6 × 10 5 cm −3 in Helsinki and Turku, respectively, and the particle number size distributions had two modes. The dominating mode peaked at 20-30 nm, and the accumulation mode at 80-100 nm. The majority of the particle mass was volatile, since after heating the sample to 265 • C, the particle volume of the studied ship decreased by around 70 %. The emission factors for NO x varied in the range of 25-100 g (kg fuel) −1 , for SO 2 in the range of 2.5-17.0 g (kg fuel) −1 , for particle number in the range of (0.32-2.26) × 10 16 # (kg fuel) −1 , and for PM 2.5 between 1.0-4.9 g (kg fuel) −1 . The ships equipped with SCR (selective catalytic reduction) had the lowest NO x emissions, whereas the ships with DWI (direct water injection) and HAMs (humid air motors) had the lowest SO 2 emissions but the highest particulate emissions. For all ships, the averaged fuel sulphur contents (FSCs) were less than 1 % (by mass) but none of them was below 0.1 % which will be the new EU directive starting 1 January 2015 in the SO x emission control areas; this indicates that ships operating on the Baltic Sea will face large challenges.