Aerosol number to volume ratios in Southwest Portugal during ACE-2

Abstract

Past studies have indicated that long-term averages of the aerosol number to volume ratios (defined as the number ofparticles larger than a certain diameter divided by the particle volume over some range less than 1 μm) show littlevariability over the Atlantic. This work presents number to volume ratios (R) measured during the ACE-2 experiment onthe land-based Sagres field site located in Southwest Portugal. The values of R measured in Sagres compare reasonablywell with previous measurements over the Atlantic. The main emphasis of this work is therefore to investigate moreclosely possible reasons for the observed stability of the number to volume ratio. Aerosol number size distributionsmeasured in Sagres are parametrized by the sum of two log-normal distributions fitted to the accumulation and to theAitken mode. The main factor that limits the variability of R is that the parameters of these log-normal distributionsare not always independent but show some covariance. In polluted air mass types correlations between parameters ofthe Aitken and accumulation mode are mostly responsible for stabilizing R. In marine air mass types the variabilityof R is reduced by an inverse relationship between the accumulation-mode mean diameter and standard deviation, consistent with condensational processes and cloud processing working on the aerosol. However, despite this reduction, the variability of R in marine air mass types is still considerable and R is linearly dependent on the number concentrationof particles larger than 90 nm. This partly due to a tail of Aitken-mode particles extending to sizes larger than90 nm.