Abstract. Atmospheric aerosols have significant effects on human health and the climate. A large fraction of these aerosols originates from secondary new particle formation (NPF), where atmospheric vapors form small particles that subsequently grow into larger sizes. In this study, we characterize NPF events observed at a rural background site of Hada Al Sham (21.802° N, 39.729° E), located in western Saudi-Arabia, during the years 2013–2015. Our analysis shows that NPF events occur very frequently at the site, as 73 % of all the 454 classified days were NPF days. The high NPF frequency is likely 20 explained by the typically prevailing conditions of clear skies and high solar radiation, in combination with sufficient amounts of precursor vapors for particle formation and growth. In Hada Al Sham, the precursor vapors seem to be related to the transport of anthropogenic emissions from the coastal urban and industrial areas, since no NPF events are observed in air masses coming from the sparsely inhabited inland. The median particle formation and growth rates for the NPF days were 8.7 cm−3 s−1 (J7nm) and 7.4 nm h−1 (GR7–12nm), respectively, both showing highest values during late summer. In addition, the 25 formation and growth rates increase as a function of the condensation sink, likely reflecting the common anthropogenic sources of large primary particles and NPF precursor vapors. 76 % of the NPF days showed an unusual progression, where the observed diameter of the newly formed particle mode started to decrease after the growth phase. In comparison to most long-term measurements, the NPF events in Hada Al Sham are exceptionally frequent and strong. In addition, the frequency of the decreasing mode diameter events is higher than anywhere else in the world.
Review of “New particle formation, growth and shrinkage at a rural background site in western Saudi Arabia” by Hakala et al.