Aerosol size distribution in precipitation events in León, Spain

Precipitation and continuous particle number size distributions have been measured using a passive cavity aerosol spectrometer probe PCASP-X, in an urban environment: the city of León, Spain. Five rain events have been analyzed in detail, none of which registered very intense precipitation, ranging from 2.5 mm to 14.7 mm, with a maximum rain intensity of 5.6 mm/h. The study focuses on the influence of precipitation in aerosol size distributions before, during and after a rain event. Washout effects have been observed during rain events with intensities of over 3.2 ± 1.5 mm/h, resulting in a decrease of 20% in the number of particles detected, the decrease affecting large and small particles alike. However, if the rain intensity is about 0.6 mm/h or lower, the result is a considerable increase in the number of aerosols measured — up to 89% more — with an increase in the number of particles smaller than 1.3 µm and a decrease in the number of particles larger than 1.3 µm. It may be the case that during very weak rain intensity the probe does not discriminate adequately between aerosols and small precipitating droplets or the extremely small droplets that remain in suspension in the atmosphere. In consequence, it will be argued that measurements taken during very weak rain events (with an intensity of less than 0.6 mm/h), such as drizzle or dense fog, should be treated separately or not be considered. The performance of the accumulation and coarse modes has also been studied, as well as the count median diameters (CMDs) before, during and after the rain event.