Description: Wind, chemical enhancement, phytoplankton activity, and surfactants are potential factors driving the air-sea gas exchange of carbon dioxide (CO2). We investigated their effects on the gas transfer velocity of CO2 in a large annular wind-wave tank filled with natural seawater from the North Atlantic Ocean. Experiments were run under 11 different wind speed conditions (ranging from 1.5 ms−1 to 22.8 ms−1), and we increased the water pCO2 concentration twice by more than 950 μatm for two of the seven experimental days. We develop a conceptual box model that incorporated the thermodynamics of the marine CO2 system. Surfactant concentrations in the sea surface microlayer (SML) ranged from 301 to 1015 μgL−1 (as Triton X-100 equivalents) with enrichments ranged from 1.0 to 5.7 in comparison to the samples from the underlying bulk water. With wind speeds up to 8.5 ms−1, surfactants in the SML can reduce the gas transfer velocity by 54%. Wind-wave tank experiments in combination with modeling are useful tools for obtaining a better understanding of the gas transfer velocities of CO2 across the air-sea boundary. The tank allowed for measuring the gas exchange velocity under extreme low and high wind speeds; in contrast, most previous parametrizations have fallen short because measurements of gas exchange velocities in the field are challenging, especially at low wind conditions. High variability in the CO2 transfer velocities suggests that gas exchange is a complex process not solely controlled by wind forces, especially in low wind conditions.

Description: Size-resolved and vertical profile measurements of single particle chemical composition (sampling altitude range 50–3000 m) were conducted in July 2014 in the Cana- dian high Arctic during an aircraft-based measurement cam- paign (NETCARE 2014). We deployed the single parti- cle laser ablation aerosol mass spectrometer ALABAMA (vacuum aerodynamic diameter range approximately 200– 1000 nm) to identify different particle types and their mix- ing states. On the basis of the single particle analysis, we found that a significant fraction (23 %) of all analyzed parti- cles (in total: 7412) contained trimethylamine (TMA). Two main pieces of evidence suggest that these TMA-containing particles originated from emissions within the Arctic bound- ary layer. First, the maximum fraction of particulate TMA occurred in the Arctic boundary layer. Second, compared to particles observed aloft, TMA particles were smaller and less oxidized. Further, air mass history analysis, associated wind data and comparison with measurements of methane- sulfonic acid give evidence of a marine-biogenic influence on particulate TMA. Moreover, the external mixture of TMA- containing particles and sodium and chloride (“Na/Cl-”) containing particles, together with low wind speeds, sug- gests particulate TMA results from secondary conversion of precursor gases released by the ocean. In contrast to TMA- containing particles originating from inner-Arctic sources, particles with biomass burning markers (such as levoglucosan and potassium) showed a higher fraction at higher al- titudes, indicating long-range transport as their source. Our measurements highlight the importance of natural, marine inner-Arctic sources for composition and growth of summer- time Arctic aerosol.

Description: Aerosol particles impact the Arctic climate system both directly and indirectly by modifying cloud properties, yet our understanding of their vertical distribution, chemical composition, mixing state, and sources in the summertime Arctic is incomplete. In situ vertical observations of particle properties in the high Arctic combined with modelling analy- sis on source attribution are in short supply, particularly dur- ing summer. We thus use airborne measurements of aerosol particle composition to demonstrate the strong contrast be- tween particle sources and composition within and above the summertime Arctic boundary layer. In situ measure- ments from two complementary aerosol mass spectrometers, the Aircraft-based Laser Ablation Aerosol Mass Spectrom- eter (ALABAMA) and an Aerodyne high-resolution time- of-flight aerosol mass spectrometer (HR-ToF-AMS), are pre- sented alongside black carbon measurements from an single particle soot photometer (SP2). Particle composition anal- ysis was complemented by trace gas measurements, satel- lite data, and air mass history modelling to attribute parti- cle properties to particle origin and air mass source regions. Particle composition above the summertime Arctic bound- ary layer was dominated by chemically aged particles, con- taining elemental carbon, nitrate, ammonium, sulfate, and organic matter. From our analysis, we conclude that the pres- ence of these particles was driven by transport of aerosol and precursor gases from mid-latitudes to Arctic regions. Specifically, elevated concentrations of nitrate, ammonium, and organic matter coincided with time spent over vegeta- tion fires in northern Canada. In parallel, those particles were largely present in high CO environments (〉 90 ppbv ). Ad- ditionally, we observed that the organic-to-sulfate ratio was enhanced with increasing influence from these fires. Besides vegetation fires, particle sources in mid-latitudes further in- clude anthropogenic emissions in Europe, North America, and East Asia. The presence of particles in the Arctic lower free troposphere, particularly sulfate, correlated with time spent over populated and industrial areas in these regions. Further, the size distribution of free tropospheric particles containing elemental carbon and nitrate was shifted to larger diameters compared to particles present within the boundary layer. Moreover, our analysis suggests that organic matter, when present in the Arctic free troposphere, can partly be identified as low molecular weight dicarboxylic acids (ox- alic, malonic, and succinic acid). Particles containing dicar- boxylic acids were largely present when the residence time of air masses outside Arctic regions was high. In contrast particle composition within the marine boundary layer was largely driven by Arctic regional processes. Air mass history modelling demonstrated that alongside primary sea spray particles, marine biogenic sources contributed to secondary aerosol formation via trimethylamine, methanesulfonic acid, sulfate, and other organic species. Our findings improve our knowledge of mid-latitude and Arctic regional sources that influence the vertical distribution of particle chemical com- position and mixing state in the Arctic summer.