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Journal article

Multiple daytime nucleation events in semi-clean savannah and industrial environments in South Africa: Analysis based on observations

Hirsikko A, Vakkari V, Tiitta P, Hatakka J, Kerminen V, Sundström A, Beukes J, Manninen H, Kulmala M, Laakso L...(+10 more)

Atmospheric Chemistry and Physics, vol. 13, issue 11 (2013) pp. 5523-5532

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Abstract

Recent studies have shown very high frequencies of atmospheric new
particle formation in different environments in South Africa. Our aim
here was to investigate the causes for two or three consecutive daytime
nucleation events, followed by subsequent particle growth during the
same day. We analysed 108 and 31 such days observed in a polluted
industrial and moderately polluted rural environments, respectively, in
South Africa. The analysis was based on two years of measurements at
each site. After rejecting the days having notable changes in the air
mass origin or local wind direction, i.e. two major reasons for observed
multiple nucleation events, we were able to investigate other factors
causing this phenomenon. Clouds were present during, or in between most
of the analysed multiple particle formation events. Therefore, some of
these events may have been single events, interrupted somehow by the
presence of clouds. From further analysis, we propose that the first
nucleation and growth event of the day was often associated with the
mixing of a residual air layer rich in SO2 (oxidized to sulphuric acid)
into the shallow surface-coupled layer. The second nucleation and growth
event of the day usually started before midday and was sometimes
associated with renewed SO2 emissions from industrial origin. However,
it was also evident that vapours other than sulphuric acid were required
for the particle growth during both events. This was especially the case
when two simultaneously growing particle modes were observed. Based on
our analysis, we conclude that the relative contributions of estimated
H2SO4 and other vapours on the first and second nucleation and growth
events of the day varied from day to day, depending on anthropogenic and
natural emissions, as well as atmospheric conditions.

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