Abstract
Formation of Ultrafine particles (UFPs) in the urban atmosphere is expected to be less favored than in the rural atmosphere due to the high existing particle surface area acting as a sink for newly-formed particles Despite the large condensation sink (CS) values, previous comparative studies between rural and urban site reported higher frequency of new particle formation (NPF) events over urban sites in comparison to background sites as well as higher particle formation and growth rates attributed to the higher concentration of condensable species. The present study aims to better understand the environmental factors favoring, or disfavoring, atmospheric NPF over Lille, a large city North of France and to analyze their impact on particle number concentration using a long-term dataset (4 years : 1st July 2017 to 31st December 2020). The results highlight a strong seasonal variation of the NPF occurrences with a maximum observed during spring (27 events) and summer (53 events). It was found that high temperature (T > 295 K), low RH (RH < 45 %) and high solar radiation are ideal to observe NPF events over Lille. Relatively high values of condensation sink (CS ~ 2.10-2 s-1) are reported during event days suggesting that high CS does not inhibit the occurrence of NPFt over our site. Moreover, the particle Growth Rate (GR15.7-30 nm) was positively correlated with the temperature most probably linked to the higher emissions of precursors. Finally, the nucleation strength factor (NSF) was calculated to highlight the impact of those NPF events on particle number concentrations. NSF15.7-100 reaches a maximum of 4 in summer, indicating an enormous contribution of NPF events to particle number concentration at this time of the year.
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