ACP Atmospheric Chemistry and Physics ACP Atmos. Chem. Phys. 1680-7324 Copernicus Publications Göttingen, Germany 10.5194/acp-15-4983-2015 Characterization of satellite-based proxies for estimating nucleation mode particles over South Africa Sundström A.-M. 1 Nikandrova A. 1 Atlaskina K. https://orcid.org/0000-0001-6068-9450 1 Nieminen T. https://orcid.org/0000-0002-2713-715X 1 Vakkari V. 1 2 Laakso L. 2 3 Beukes J. P. 3 Arola A. https://orcid.org/0000-0002-9220-0194 4 van Zyl P. G. https://orcid.org/0000-0003-1470-3359 3 Josipovic M. 3 Venter A. D. 3 Jaars K. 3 Pienaar J. J. 3 Piketh S. 3 Wiedensohler A. https://orcid.org/0000-0001-8298-491X 5 Chiloane E. K. 3 6 de Leeuw G. https://orcid.org/0000-0002-1649-6333 2 Kulmala M. https://orcid.org/0000-0003-3464-7825 1 Department of Physics, University of Helsinki, Helsinki, Finland Finnish Meteorological Institute, Helsinki, Finland Unit for Environmental Science and Management, North-West University, Potchefstroom, South Africa Finnish Meteorological Institute, Kuopio, Finland Leibniz Institute for Tropospheric Research, Leipzig, Germany Eskom Holdigns SOC Ltd, Sustainability Division, South Africa 04 05 2015 15 9 4983 4996 Copyright: © 2015 A.-M. Sundström et al. 2015 This work is licensed under the Creative Commons Attribution 3.0 Unported License. To view a copy of this licence, visit https://creativecommons.org/licenses/by/3.0/ This article is available from https://acp.copernicus.org/articles/15/4983/2015/acp-15-4983-2015.html The full text article is available as a PDF file from https://acp.copernicus.org/articles/15/4983/2015/acp-15-4983-2015.pdf

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 (NO<sub>2</sub> and SO<sub>2</sub>) 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 (<I>N</I><sub>nuc</sub>) remained low. At the time of the satellite overpass (13:00–14:00 LT) the highest correlation is observed for SO<sub>2</sub>/CS (<I>R</I><sup>2</sup> = 0.2). However, when the proxies are calculated using satellite data, only NO<sub>2</sub>/AOD showed some correlation with <I>N</I><sub>nuc</sub> (<I>R</I><sup>2</sup> = 0.2). This can be explained by the relatively high uncertainties related especially to the satellite SO<sub>2</sub> columns and by the positive correlation that is observed between the ground-based SO<sub>2</sub> and NO<sub>2</sub> concentrations. In fact, results show that the satellite NO<sub>2</sub> columns compare better with in situ SO<sub>2</sub> concentration than the satellite SO<sub>2</sub> 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 <I>N</I><sub>nuc</sub>. Hence, overall improvements in the formulation of the proxies are needed.

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