Abstract
Abstract. Physical and chemical properties of boundary layer clouds, together with relevant aerosol properties, were investigated during the first Pallas Cloud Experiment (First Pace) conducted in northern Finland between 20 October and 9 November 2004. Two stations located 6 km apart from each other at different altitudes were employed in measurements. The low-altitude station was always below the cloud layer, whereas the high-altitude station was inside clouds about 75% of the time during the campaign. Direct measurements of cloud droplet populations showed that our earlier approach of determining cloud droplet residual particle size distributions and corresponding activated fractions using continuous aerosol number size distribution measurements at the two stations is valid, as long as the cloud events are carefully screened to exclude precipitating clouds and to make sure the same air mass has been measured at both stations. We observed that a non-negligible fraction of cloud droplets originated from Aitken mode particles even at moderately-polluted air masses. We found clear evidence on first indirect aerosol effect on clouds but demonstrated also that no simple relation between the cloud droplet number concentration and aerosol particle number concentration exists for this type of clouds. The chemical composition of aerosol particles was dominated by particulate organic matter (POM) and sulphate in continental air masses and POM, sodium and chlorine in marine air masses. The inorganic composition of cloud water behaved similarly to that of the aerosol phase and was not influenced by inorganic trace gases.
Highlights
Clouds constitute perhaps the largest source of uncertainty in predicting the behavior of the Earth’s climate system (IPCC, 2007; Baker and Peter, 2008)
Our main goals in this paper are i) to demonstrate the performance of the method developed by Komppula et al (2005) for determining the cloud droplet activating properties of aerosols from continuous size distribution measurements, ii) to provide additional data on how particles of different size are activated into cloud droplets in different air mass types, and iii) to get further insight into the connections between cloud microphysics, aerosol number size distribution and associated chemistry
Low particle number concentrations were related to air masses coming almost directly from the Arctic Ocean, whereas high particle number concentrations were associated with continental air masses coming from west and south or with air influenced by the Kola Peninsula industrial area in Russia 350–400 km east of our measurement site
Summary
Clouds constitute perhaps the largest source of uncertainty in predicting the behavior of the Earth’s climate system (IPCC, 2007; Baker and Peter, 2008). Global climate model simulations and satellite measurements are the only means to determine the connection between atmospheric aerosols, clouds and Earth’s climate system. Both these approaches are subject to large uncertainties at the moment. Reported climate forcing estimates associated with aerosol-cloud interactions are usually substantially lower for satellite-based estimates as compared with climate model simulations (McComiskey and Feingold, 2008; Quaas et al, 2008). Aircraft measurements and observations at elevated stations provide a complementary way to investigate aerosolcloud interactions, especially what it comes to the dependence of various cloud properties on the size distribution and chemical composition of the underlying aerosol particle population. Our main goals in this paper are i) to demonstrate the performance of the method developed by Komppula et al (2005) for determining the cloud droplet activating properties of aerosols from continuous size distribution measurements, ii) to provide additional data on how particles of different size are activated into cloud droplets in different air mass types, and iii) to get further insight into the connections between cloud microphysics, aerosol number size distribution and associated chemistry
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