Wintertime Arctic Ocean sea water properties and primary marine aerosol concentrations

Sea spray aerosols are an important part of the climate system through their direct and indirect effects. Due to the diminishing sea ice, the Arctic Ocean is one of the most rapidly changing sea spray aerosol source areas. However, the influence of these changes on primary particle production is not known. <br><br> In laboratory experiments we examined the influence of Arctic Ocean water temperature, salinity, and oxygen saturation on primary particle concentration characteristics. Sea water temperature was identified as the most important of these parameters. A strong decrease in sea spray aerosol production with increasing water temperature was observed for water temperatures between −1°C and 9°C. Aerosol number concentrations decreased from at least 1400 cm<sup>−3</sup> to 350 cm<sup>−3</sup>. In general, the aerosol number size distribution exhibited a robust shape with one mode close to dry diameter <i>D</i><sub>p</sub> 0.2 μm with approximately 45% of particles at smaller sizes. Changes in sea water temperature did not result in pronounced change of the shape of the aerosol size distribution, only in the magnitude of the concentrations. Our experiments indicate that changes in aerosol emissions are most likely linked to changes of the physical properties of sea water at low temperatures. The observed strong dependence of sea spray aerosol concentrations on sea water temperature, with a large fraction of the emitted particles in the typical cloud condensation nuclei size range, provide strong arguments for a more careful consideration of this effect in climate models

Size-resolved cloud condensation nuclei concentration measurements in the Arctic: two case studies from the summer of 2008

The Arctic is one of the most vulnerable regions affected by climate change. Extensive measurement data are needed to understand the atmospheric processes governing this vulnerability. Among these, data describing cloud formation potential are of particular interest, since the indirect effect of aerosols on the climate system is still poorly understood. In this paper we present, for the first time, size-resolved cloud condensation nuclei (CCN) data obtained in the Arctic. The measurements were conducted during two periods in the summer of 2008: one in June and one in August, at the Zeppelin research station (78°54´ N, 11°53´ E) in Svalbard. Trajectory analysis indicates that during the measurement period in June 2008, air masses predominantly originated from the Arctic, whereas the measurements from August 2008 were influenced by mid-latitude air masses. CCN supersaturation (SS) spectra obtained on the 27 June, before size-resolved measurements were begun, and spectra from the 21 and 24 August, conducted before and after the measurement period, revealed similarities between the 2 months. From the ratio between CCN concentration and the total particle number concentration (CN) as a function of dry particle diameter (<i>D</i>_p) at a SS of 0.4 %, the activation diameter (<i>D</i>₅₀), corresponding to CCN / CN = 0.50, was estimated. <i>D</i>₅₀ was found to be 60 and 67 nm for the examined periods in June and August 2008, respectively. Corresponding <i>D</i>₅₀ hygroscopicity parameter (&kappa;) values were estimated to be 0.4 and 0.3 for June and August 2008, respectively. These values can be compared to hygroscopicity values estimated from bulk chemical composition, where &kappa; was calculated to be 0.5 for both June and August 2008. While the agreement between the 2 months is reasonable, the difference in &kappa; between the different methods indicates a size dependence in the particle composition, which is likely explained by a higher fraction of inorganics in the bulk aerosol samples

Comparison between summertime and wintertime Arctic Ocean primary marine aerosol properties

Primary marine aerosols (PMAs) are an important source of cloud condensation nuclei, and one of the key elements of the remote marine radiative budget. Changes occurring in the rapidly warming Arctic, most importantly the decreasing sea ice extent, will alter PMA production and hence the Arctic climate through a set of feedback processes. In light of this, laboratory experiments with Arctic Ocean water during both Arctic winter and summer were conducted and focused on PMA emissions as a function of season and water properties. Total particle number concentrations and particle number size distributions were used to characterize the PMA population. A comprehensive data set from the Arctic summer and winter showed a decrease in PMA concentrations for the covered water temperature (<i>T</i>_w) range between &minus;1&deg;C and 15&deg;C. A sharp decrease in PMA emissions for a <i>T</i>_w increase from &minus;1&deg;C to 4&deg;C was followed by a lower rate of change in PMA emissions for <i>T</i>_w up to about 6&deg;C. Near constant number concentrations for water temperatures between 6&deg;C to 10&deg;C and higher were recorded. Even though the total particle number concentration changes for overlapping <i>T</i>_w ranges were consistent between the summer and winter measurements, the distribution of particle number concentrations among the different sizes varied between the seasons. Median particle number concentrations for a dry diameter (<i>D</i>_<i>p</i>< 0.125&mu;m measured during winter conditions were similar (deviation of up to 3%), or lower (up to 70%) than the ones measured during summer conditions (for the same water temperature range). For <i>D</i>_<i>p</i> > 0.125&mu;m, the particle number concentrations during winter were mostly higher than in summer (up to 50%). The normalized particle number size distribution as a function of water temperature was examined for both winter and summer measurements. An increase in <i>T</i>_w from &minus;1&deg;C to 10&deg;C during winter measurements showed a decrease in the peak of relative particle number concentration at about a <i>D</i>_<i>p</i> of 0.180&mu;m, while an increase was observed for particles with <i>D</i>_<i>p</i> > 1&mu;m. Summer measurements exhibited a relative shift to smaller particle sizes for an increase of <i>T</i>_w in the range 7–11&deg;C. The differences in the shape of the number size distributions between winter and summer may be caused by different production of organic material in water, different local processes modifying the water masses within the fjord (for example sea ice production in winter and increased glacial meltwater inflow during summer) and different origin of the dominant sea water mass. Further research is needed regarding the contribution of these factors to the PMA production

Size-dependent hygroscopicity parameter ( κ ) and chemical composition of secondary organic cloud condensation nuclei

Secondary organic aerosol components (SOA) contribute significantly to the activation of cloud condensation nuclei (CCN) in the atmosphere. The CCN activity of internally mixed submicron SOA particles is often parameterized assuming a size-independent single-hygroscopicity parameter κ. In the experiments done in a large atmospheric reactor (SAPHIR, Simulation of Atmospheric PHotochemistry In a large Reaction chamber, Jülich), we consistently observed size-dependent κ and particle composition for SOA from different precursors in the size range of 50 nm–200 nm. Smaller particles had higher κ and a higher degree of oxidation, although all particles were formed from the same reaction mixture. Since decreasing volatility and increasing hygroscopicity often covary with the degree of oxidation, the size dependence of composition and hence of CCN activity can be understood by enrichment of higher oxygenated, low-volatility hygroscopic compounds in smaller particles. Neglecting the size dependence of κ can lead to significant bias in the prediction of the activated fraction of particles during cloud formation

A wave roughness Reynolds number parameterization of the sea spray source flux

Parameterizations of the sea spray aerosol source flux are derived as functions of wave roughness Reynolds numbers, R and R, for particles with radii between 0.176 and 6.61μm at 80% relative humidity. These source functions account for up to twice the variance in the observations than does wind speed alone. This is the first such direct demonstration of the impact of wave state on the variability of sea spray aerosol production. Global European Centre for Medium-Range Weather Forecasts operational mode fields are used to drive the parameterizations. The source flux from the R parameterizations varies from approximately 0.1 to 3 (R) and 5 (R) times that from a wind speed parameterization, derived from the same measurements, where the wave state is substantially underdeveloped or overdeveloped, respectively, compared to the equilibrium wave state at the local wind speed. Key Points: Sea spray aerosol source function is derived in terms of a wave Reynolds number The Reynolds number explains more flux variability than wind speed alone Wave state modifies the wind-driven flux by a factor between 0.1 and 3