The effect of clouds on aerosol growth in the rural atmosphere

Measurements of accumulation mode aerosol in the atmospheric boundary layer under cloudy and cloud-free conditions, and in the lower free troposphere under cloud-free conditions, were conducted over the rural northwest of England. Normalised size distributions in the cloud-free Ž . Ž . boundary layer CFBL and the cloud-free free troposphere CFFT exhibited almost identical spectral similarities with both size distributions possessing a concentration peak mode-radius of f0.05 mm or less. By comparison, aerosol distributions observed in cloudy air exhibited a distinctive log-normal distribution with mode-radii occurring at f0.1 mm concomitant with a local minimum at f0.05 mm. The consistent and noticeable difference in spectral features observed between cloudy and cloud-free conditions suggest that a greater amount of gas-to-particle conversion occurs on cloudy days, presumably through in-cloud aqueous phase oxidation processes, leading to larger sized accumulation mode particles. Apart from the distinct difference between cloudy and cloud-free aerosol spectra on cloudy days, aerosol concentration and mass were observed to be significantly enhanced above that of the ambient background in the vicinity of clouds. Volatility analysis during one case of cloud processing indicated an increase in the relative contribution of aerosol mass volatile at temperatures characteristic of sulphuric acid, along Ž . with a smaller fraction of more volatile material possibly nitric acid andror organic aerosol . Growth-law analysis of possible growth mechanisms point to aqueous phase oxidation of aerosol precursors in cloud droplets as being the only feasible mechanism capable of producing the Ž . observed growth. The effect of cloud processing is to alter the cloud condensation nuclei CCN ) Corresponding author. Tel.: q358-9-191-8365; fax: q358-9-191-8318. Ž . E-mail address: [email protected] C.D. O’Dowd . 0169-8095r00r$ see front matter q2000 Elsevier Science B.V. All rights reserved. Ž . PII: S0169-8095 00 00046-6 ( ) C.D. O’Dowd et al.rAtmospheric Research 54 2000 201–221 202 supersaturation spectrum in a manner which increases the availability of CCN at lower cloud supersaturations. q 2000 Elsevier Science B.V. All rights reserved.