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

by A. Hirsikko, V. Vakkari, P. Tiitta, J. Hatakka, V. M. Kerminen, A. M. Sundström, J. P. Beukes, H. E. Manninen, M. Kulmala, L. Laakso show all authors
Atmospheric Chemistry and Physics ()
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Recent studies have shown very high frequencies of atmospheric new\nparticle formation in different environments in South Africa. Our aim\nhere was to investigate the causes for two or three consecutive daytime\nnucleation events, followed by subsequent particle growth during the\nsame day. We analysed 108 and 31 such days observed in a polluted\nindustrial and moderately polluted rural environments, respectively, in\nSouth Africa. The analysis was based on two years of measurements at\neach site. After rejecting the days having notable changes in the air\nmass origin or local wind direction, i.e. two major reasons for observed\nmultiple nucleation events, we were able to investigate other factors\ncausing this phenomenon. Clouds were present during, or in between most\nof the analysed multiple particle formation events. Therefore, some of\nthese events may have been single events, interrupted somehow by the\npresence of clouds. From further analysis, we propose that the first\nnucleation and growth event of the day was often associated with the\nmixing of a residual air layer rich in SO2 (oxidized to sulphuric acid)\ninto the shallow surface-coupled layer. The second nucleation and growth\nevent of the day usually started before midday and was sometimes\nassociated with renewed SO2 emissions from industrial origin. However,\nit was also evident that vapours other than sulphuric acid were required\nfor the particle growth during both events. This was especially the case\nwhen two simultaneously growing particle modes were observed. Based on\nour analysis, we conclude that the relative contributions of estimated\nH2SO4 and other vapours on the first and second nucleation and growth\nevents of the day varied from day to day, depending on anthropogenic and\nnatural emissions, as well as atmospheric conditions.

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