Sea spray is one of the largest natural aerosol sources and plays an important role in the Earth’s radiative budget. These particles are inherently hygroscopic, that is, they take-up moisture from the air, which affects the extent to which they interact with solar radiation. We demonstrate that the hygroscopic growth of inorganic sea salt is 8–15% lower than pure sodium chloride, most likely due to the presence of hydrates. We observe an increase in hygroscopic growth with decreasing particle size (for particle diameters <150 nm) that is independent of the particle generation method. We vary the hygroscopic growth of the inorganic sea salt within a general circulation model and show that a reduced hygroscopicity leads to a reduction in aerosol-radiation interactions, manifested by a latitudinal-dependent reduction of the aerosol optical depth by up to 15%, while cloud-related parameters are unaffected. We propose that a value of κs=1.1 (at RH=90%) is used to represent the hygroscopicity of inorganic sea salt particles in numerical models.

Abstract.We have developed an inorganic sea spray source function that is based upon state-of-the-art measurements of sea spray aerosol production using a temperature-controlled plunging jet sea spray aerosol chamber. The size-resolved particle production was measured between 0.01 and 10 µm dry diameter. Particle production decreased non-linearly with increasing seawater temperature (between −1 and 30 • C) similar to previous findings. In addition, we observed that the particle effective radius, as well as the particle surface, particle volume and particle mass, increased with increasing seawater temperature due to increased production of particles with dry diameters greater than 1 µm. By combining these measurements with the volume of air entrained by the plunging jet we have determined the size-resolved particle flux as a function of air entrainment. Through the use of existing parameterisations of air entrainment as a function of wind speed, we were subsequently able to scale our laboratory measurements of particle production to wind speed. By scaling in this way we avoid some of the difficulties associated with defining the "white area" of the laboratory whitecap -a contentious issue when relating laboratory measurements of particle production to oceanic whitecaps using the more frequently applied whitecap method.The here-derived inorganic sea spray source function was implemented in a Lagrangian particle dispersion model (FLEXPART -FLEXible PARTicle dispersion model). An estimated annual global flux of inorganic sea spray aerosol of 5.9 ± 0.2 Pg yr −1 was derived that is close to the median of estimates from the same model using a wide range of existing sea spray source functions. When using the source function derived here, the model also showed good skill in predicting measurements of Na + concentration at a number of field sites further underlining the validity of our source function.In a final step, the sensitivity of a large-scale model (NorESM -the Norwegian Earth System Model) to our new source function was tested. Compared to the previously implemented parameterisation, a clear decrease of sea spray aerosol number flux and increase in aerosol residence time was observed, especially over the Southern Ocean. At the same time an increase in aerosol optical depth due to an increase in the number of particles with optically relevant sizes was found. That there were noticeable regional differences may have important implications for aerosol optical properties and number concentrations, subsequently also affecting the indirect radiative forcing by non-sea spray anthropogenic aerosols.

The two monoterpenes, Δ3-carene and α-pinene, have very similar chemical structures, which leads to the assumption that the formation of secondary organic aerosol (SOA) is also similar. This study aims to investigate if the formation of SOA from ozonolysis of Δ3-carene and α-pinene is actually analogous. This is addressed by conducting smog chamber studies of Δ3-carene ozonolysis and comparing the results to similar studies of α-pinene. Detailed offline analysis using ultra-high-performance liquid chromatography coupled to mass spectrometry seeks to elucidate differences in the product distribution between Δ3-carene and α-pinene. The experimental findings are supported by quantum chemical calculations of formation free energies of the first-generation oxidation products, cis-3-caric acid and cis-pinic acid. The smog chamber studies and detailed offline analysis show a considerable difference in the SOA formation from Δ3-carene and α-pinene. Δ3-carene produces higher SOA mass and larger particles, whereas α-pinene produces a larger number of smaller particles. This indicates that Δ3-carene oxidation products tend to condense on already existing particles, whereas α-pinene oxidation products to a larger degree contribute to new particle formation. The detailed offline analysis shows a much less diverse product distribution in Δ3-carene SOA compared to α-pinene SOA. In addition, three dimers from Δ3-carene are identified for the first time. The quantum chemical calculations indicate that cis-3-caric acid is expected to be more efficient in condensing on already existing particles compared to cis-pinic acid, which is in good agreement with the experimental results.

Abstract. Little is known about the effects of subzero temperatures on the formation of secondary organic aerosol (SOA) from α-pinene. In the current work, ozone-initiated oxidation of α-pinene at initial concentrations of 10 and 50 ppb, respectively, is performed at temperatures of 20, 0, and −15 ∘C in the Aarhus University Research on Aerosol (AURA) smog chamber during the Aarhus Chamber Campaign on Highly Oxygenated Organic Molecules and Aerosols (ACCHA). Herein, we show how temperature influences the formation and chemical composition of α-pinene-derived SOA with a specific focus on the formation of organic acids and dimer esters. With respect to particle formation, the results show significant increase in particle-formation rates, particle number concentrations, and particle mass concentrations at low temperatures. In particular, the number concentrations of sub-10 nm particles were significantly increased at the lower 0 and −15 ∘C temperatures. Temperature also affects the chemical composition of formed SOA. Here, detailed offline chemical analyses show that organic acids contribute from 15 % to 30 % by mass, with highest contributions observed at the lowest temperatures, indicative of enhanced condensation of these semivolatile species. In comparison, a total of 30 identified dimer esters were seen to contribute between 4 % and 11 % to the total SOA mass. No significant differences in the chemical composition (i.e. organic acids and dimer esters) of the α-pinene-derived SOA particles are observed between experiments performed at 10 and 50 ppb initial α-pinene concentrations, thus suggesting a higher influence of reaction temperature compared to that of α-pinene loading on the SOA chemical composition. Interestingly, the effect of temperature on the formation of dimer esters differs between the individual species. The formation of less oxidized dimer esters – with oxygen-to-carbon ratio (O:C)<0.4 – is shown to increase at low temperatures, while the formation of the more oxidized species (O:C>0.4) is suppressed, consequently resulting in temperature-modulated composition of the α-pinene-derived SOA. Temperature ramping experiments exposing α-pinene-derived SOA to changing temperatures (heating and cooling) reveal that the chemical composition of the SOA with respect to dimer esters is governed almost solely by the temperature at which oxidization started and is insusceptible to subsequent changes in temperature. Similarly, the resulting SOA mass concentrations were found to be more influenced by the initial α-pinene oxidation temperatures, thus suggesting that the formation conditions to a large extent govern the type of SOA formed, rather than the conditions to which the SOA is later exposed. For the first time, we discuss the relation between the identified dimer ester and the highly oxygenated organic molecules (HOMs) measured by chemical ionization–atmospheric pressure interface–time-of-flight mass spectrometer (CI-APi-ToF) during the ACCHA experiments. We propose that, although very different in chemical structures and O:C ratios, many dimer esters and HOMs may be linked through similar RO2 reaction pathways and that dimer esters and HOMs merely represent two different fates of the RO2 radicals.