[1] A major source of the primary marine aerosol is the bursting of air bubbles produced by breaking waves. Several source parameterizations are available from the literature, usually limited to particles with a dry diameter D p > 1 mm. The objective of this work is to extend the current knowledge to submicrometer particles. Bubbles were generated in synthetic seawater using a sintered glass filter, with a size spectra that are only partly the same spectra as measured in the field. Bubble spectra, and size distributions of the resulting aerosol (0.020-20.0 mm D p ) of the resulting aerosol, were measured for different salinity, water temperature (T w ), and bubble flux. The spectra show a minimum at $1 mm D p , which separates two modes, one at $0.1 mm, with the largest number of particles, and one at 2.5 mm D p . The modes show different behavior with the variation of salinity and water temperature. When the water temperature increases, the number concentration N p decreases for D p < 0.07 mm, whereas for D p > 0.35 mm, N p increases. The salinity effect suggests different droplet formation processes for droplets smaller and larger than 0.2 mm D p . The number of particles produced per size increment, time unit, and whitecap surface (È) is described as a linear function of T w and a polynomial function of D p . Combining È with the whitecap coverage fraction W (in percent), an expression results for the primary marine aerosol source flux dF 0 /dlogD p = W È (m À2 s À1 ). The results are compared with other commonly used formulations as well as with recent field observations. Implications for aerosol-induced effects on climate are discussed.

Abstract. Understanding Arctic climate change requires knowledge of both the external and the local drivers of Arctic climate as well as local feedbacks within the system. An Arctic feedback mechanism relating changes in sea ice extent to an alteration of the emission of sea salt aerosol and the consequent change in radiative balance is examined. A set of idealized climate model simulations were performed to quantify the radiative effects of changes in sea salt aerosol emissions induced by prescribed changes in sea ice extent. The model was forced using sea ice concentrations consistent with present day conditions and projections of sea ice extent for 2100. Sea salt aerosol emissions increase in response to a decrease in sea ice, the model results showing an annual average increase in number emission over the polar cap (70-90 • N) of 86 × 10 6 m −2 s −1 (mass emission increase of 23 µg m −2 s −1 ). This in turn leads to an increase in the natural aerosol optical depth of approximately 23%. In response to changes in aerosol optical depth, the natural component of the aerosol direct forcing over the Arctic polar cap is estimated to be between −0.2 and −0.4 W m −2 for the summer months, which results in a negative feedback on the system. The model predicts that the change in first indirect aerosol effect (cloud albedo effect) is approximately a factor of ten greater than the change in direct aerosol forcing although this result is highly uncertain due to the crude representation of Arctic clouds and aerosol-cloud interactions in the model. This study shows that both the natural aerosol direct and first indirect effects are strongly dependent on the Correspondence to: H. Struthers (hamish.struthers@itm.su.se) surface albedo, highlighting the strong coupling between sea ice, aerosols, Arctic clouds and their radiative effects.

[1] Bubbles bursting from whitecaps are considered to be the most effective mechanism for particulate matter to be ejected into the atmosphere from the Earth's oceans. To realistically predict the climate effect of marine aerosols, global climate models require process-based understanding of particle formation from bubble bursting. During a cruise on the highly biologically active waters of the northeastern Atlantic Ocean in the summer of 2006, the submicrometer primary marine aerosol produced by a jet of seawater impinging on a seawater surface was investigated. The produced aerosol size spectra were centered on 200 nm in dry diameter and were conservative in shape throughout the cruise. The aerosol number production was negatively correlated with dissolved oxygen (DO) in the water (r < À0.6 for particles of dry diameter D p > 200 nm). An increased surfactant concentration as a result of biological activity affecting the oxygen saturation is thought to diminish the particle production. The lack of influence of chlorophyll on aerosol production indicates that hydrocarbons produced directly by the photosynthesis are not essential for sea spray production. The upward mixing of deeper ocean water as a result of higher wind speed appears to affect the aerosol particle production, making wind speed influence aerosol production in more ways than by increasing the amount of whitecaps. The bubble spectra produced by the jet of seawater was representative of breaking waves at open sea, and the particle number production was positively correlated with increasing bubble number concentration with a peak production of 40-50 particles per bubble.

Abstract. Urban aerosol sources are important due to the health effects of particles and their potential impact on climate. Our aim has been to quantify and parameterise the urban aerosol source number flux F (particles m −2 s −1 ), in order to help improve how this source is represented in air quality and climate models. We applied an aerosol eddy covariance flux system 118.0 m above the city of Stockholm. This allowed us to measure the aerosol number flux for particles with diameters >11 nm. Upward source fluxes dominated completely over deposition fluxes in the collected dataset. Therefore, the measured fluxes were regarded as a good approximation of the aerosol surface sources. Upward fluxes were parameterised using a traffic activity (TA) database, which is based on traffic intensity measurements.The footprint (area on the surface from which sources and sinks affect flux measurements, located at one point in space) of the eddy system covered road and building construction areas, forests and residential areas, as well as roads with high traffic density and smaller streets. We found pronounced diurnal cycles in the particle flux data, which were well correlated with the diurnal cycles in traffic activities, strongly supporting the conclusion that the major part of the aerosol fluxes was due to traffic emissions.The emission factor for the fleet mix in the measurement area EF f m =1.4±0.1×10 14 veh −1 km −1 was deduced. This agrees fairly well with other studies, although this study has an advantage of representing the actual effective emission from a mixed vehicle fleet. Emission from other sources, not traffic related, account for a F 0 =15±18×10 6 m −2 s −1 . The urban aerosol source flux can then be written as F=EF f m TA+F 0 . In a second

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