Organic aerosols affect the climate by scattering or absorbing incoming solar radiation. Secondary organic material (SOM), which represents the major chemical constituent of atmospheric aerosol particles, is produced by the oxidation of atmospheric volatile organic compounds (VOCs). SOM in clouds, fogs, and aerosols undergoes concentration/dilution cycles due to the evaporation/condensation of water droplets. These physical processes could lead to the chemical processing of SOM and the formation of new, light-absorbing compounds. In this study, model SOM was generated through smog chamber photooxidation and flow tube ozonolysis of various atmospherically relevant anthropogenic and biogenic VOCs, including toluene (TOL), d-limonene (LIM), α-pinene (APIN), β-pinene (BPIN), and isoprene (ISO). Collected SOM was extracted in water, and the solutions were acidified with sulfuric acid to pH 2 and dried to simulate the evaporation of acidic particles containing SOM. Significant changes in mass absorption coefficients (MACs) were observed after the evaporation and redissolution of SOM in the presence of sulfuric acid. At visible wavelengths, the MAC values of most SOM increased after the evaporation, with the fractional increase being the largest for LIM/O3 SOM at 400 nm (fractional increase of 65.0). Exceptions to evaporation increasing MAC values in the presence of sulfuric acid were ISO/OH and TOL/OH/NO x . Light-absorbing species in LIM/O3 SOM were chromatographically separated and detected using a photodiode array detector and a high-resolution electrospray ionization mass spectrometer. The increase in MAC was accompanied by the appearance of more than 300 organosulfate peaks. Five potential brown carbon (BrC) chromophores in LIM/O3 SOM were separated and assigned chemical formulas, including C10H16SO6, C10H14SO6, C10H16SO5, C11H16SO7, and C11H18SO8. This study suggests that evaporation-driven processes may occur in the atmosphere, substantially modifying the molecular composition and optical properties of SOM. The evaporation of filter extracts from the field or laboratory could similarly produce organosulfates as artifacts if the extract is sufficiently acidic before the evaporation. We recommend that complete drying of particulate matter filter extracts should be avoided in future work.

Abstract. The project MarParCloud (Marine biological production, organic aerosol Particles and marine Clouds: a process chain) aims to improve our understanding of the genesis, modification and impact of marine organic matter (OM) from its biological production, to its export to marine aerosol particles and, finally, to its ability to act as ice-nucleating particles (INPs) and cloud condensation nuclei (CCN). A field campaign at the Cape Verde Atmospheric Observatory (CVAO) in the tropics in September–October 2017 formed the core of this project that was jointly performed with the project MARSU (MARine atmospheric Science Unravelled). A suite of chemical, physical, biological and meteorological techniques was applied, and comprehensive measurements of bulk water, the sea surface microlayer (SML), cloud water and ambient aerosol particles collected at a ground-based and a mountain station took place. Key variables comprised the chemical characterization of the atmospherically relevant OM components in the ocean and the atmosphere as well as measurements of INPs and CCN. Moreover, bacterial cell counts, mercury species and trace gases were analyzed. To interpret the results, the measurements were accompanied by various auxiliary parameters such as air mass back-trajectory analysis, vertical atmospheric profile analysis, cloud observations and pigment measurements in seawater. Additional modeling studies supported the experimental analysis. During the campaign, the CVAO exhibited marine air masses with low and partly moderate dust influences. The marine boundary layer was well mixed as indicated by an almost uniform particle number size distribution within the boundary layer. Lipid biomarkers were present in the aerosol particles in typical concentrations of marine background conditions. Accumulation- and coarse-mode particles served as CCN and were efficiently transferred to the cloud water. The ascent of ocean-derived compounds, such as sea salt and sugar-like compounds, to the cloud level, as derived from chemical analysis and atmospheric transfer modeling results, denotes an influence of marine emissions on cloud formation. Organic nitrogen compounds (free amino acids) were enriched by several orders of magnitude in submicron aerosol particles and in cloud water compared to seawater. However, INP measurements also indicated a significant contribution of other non-marine sources to the local INP concentration, as (biologically active) INPs were mainly present in supermicron aerosol particles that are not suggested to undergo strong enrichment during ocean–atmosphere transfer. In addition, the number of CCN at the supersaturation of 0.30 % was about 2.5 times higher during dust periods compared to marine periods. Lipids, sugar-like compounds, UV-absorbing (UV: ultraviolet) humic-like substances and low-molecular-weight neutral components were important organic compounds in the seawater, and highly surface-active lipids were enriched within the SML. The selective enrichment of specific organic compounds in the SML needs to be studied in further detail and implemented in an OM source function for emission modeling to better understand transfer patterns, the mechanisms of marine OM transformation in the atmosphere and the role of additional sources. In summary, when looking at particulate mass, we see oceanic compounds transferred to the atmospheric aerosol and to the cloud level, while from a perspective of particle number concentrations, sea spray aerosol (i.e., primary marine aerosol) contributions to both CCN and INPs are rather limited.