Characterization of satellite-based proxies for estimating nucleation mode particles over South Africa
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Date
2015Author
Sundström, A.-M.
Laakso, L.
Beukes, J.P.
Van Zyl, P.G.
Josipovic, M.
Venter, A.D.
Jaars, K.
Pienaar, J.J.
Piketh, S.
Chiloane, E.K.
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Show full item recordAbstract
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 columnintegrated
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 (Nnuc) remained low.
At the time of the satellite overpass (13:00–14:00 LT) the
highest correlation is observed for SO2/CS (R2 D 0:2). However,
when the proxies are calculated using satellite data, only
NO2/AOD showed some correlation with Nnuc (R2 D 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 Nnuc. Hence, overall improvements in the formulation
of the proxies are needed
URI
http://hdl.handle.net/10394/16944https://www.atmos-chem-phys.net/15/4983/2015/
https://doi.org/10.5194/acp-15-4983-2015