VERTICAL-DISTRIBUTION OF AEROSOL-PARTICLES AND CCN IN CLEAR-AIR AROUND THE BRITISH-ISLES

We present vertical profiles of aerosol particles and cloud condensation nuclei (CCN) obtained during four flights around the British Isles under different synoptic situations and in different seasons. The calculated net downward radiative flux was reduced significantly by the presence of an aerosol layer close to the surface, especially for the two very polluted days. The heating rates in the aerosol layer increased by between a factor of 2 and more than an order of magnitude. Particle and CCN concentrations after air mass passage over the U.K. increased by about a factor of 2, the changes in the spectra being due to increases in the concentration of particles smaller than 0.2 mum in radius and to decreases in the concentration of large particles (5-10 mum). The changes in aerosol size distributions after air mass passage over land resulted in changes in microphysical and radiative characteristics of the clouds that developed. There is evidence that non-hygroscopic aerosol particles account for up to 50% of the aerosol particle number concentration during two heavily polluted flights.