Characterization of satellite-based proxies for estimating nucleation mode particles over South Africa

Abstract. Proxies for estimating nucleation mode number concentrations and further simplification for their use with satellite data have been presented in Kulmala et al. (2011). In this paper we discuss the underlying assumptions for these simplifications and evaluate the resulting proxies over an area in South Africa based on a comparison with a suite of ground-based measurements available from four different stations. The proxies are formulated in terms of sources (concentrations of precursor gases (NO₂ and SO₂) and UVB radiation intensity near the surface) and a sink term related to removal of the precursor gases due to condensation on pre-existing aerosols. A-Train satellite data are used as input to compute proxies. Both the input data and the resulting proxies are compared with those obtained from ground-based measurements. In particular, a detailed study is presented on the substitution of the local condensation sink (CS) with satellite aerosol optical depth (AOD), which is a column-integrated parameter. One of the main factors affecting the disagreement between CS and AOD is the presence of elevated aerosol layers. Overall, the correlation between proxies calculated from the in situ data and observed nucleation mode particle number concentrations (N_nuc) remained low. At the time of the satellite overpass (13:00–14:00 LT) the highest correlation is observed for SO₂/CS (R² = 0.2). However, when the proxies are calculated using satellite data, only NO₂/AOD showed some correlation with N_nuc (R² = 0.2). This can be explained by the relatively high uncertainties related especially to the satellite SO₂ columns and by the positive correlation that is observed between the ground-based SO₂ and NO₂ concentrations. In fact, results show that the satellite NO₂ columns compare better with in situ SO₂ concentration than the satellite SO₂ column. Despite the high uncertainties related to the proxies calculated using satellite data, the proxies calculated from the in situ data did not better predict N_nuc. Hence, overall improvements in the formulation of the proxies are needed.