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. Atmospheric aerosols have significant effects on humanhealth and the climate. A large fraction of these aerosols originates fromsecondary new particle formation (NPF), where atmospheric vapors form smallparticles that subsequently grow into larger sizes. In this study, wecharacterize 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 veryfrequently at the site, as 73 % of all the 454 classified days were NPFdays. The high NPF frequency is likely explained by the typically prevailingconditions of clear skies and high solar radiation, in combination withsufficient amounts of precursor vapors for particle formation and growth.Several factors suggest that in Hada Al Sham these precursor vapors arerelated to the transport of anthropogenic emissions from the coastal urbanand industrial areas. The median particle formation and growth rates for theNPF days were 8.7 cm −3  s −1 ( J7 nm ) and 7.4 nm h −1 (GR 7−12 nm ), respectively, both showing highest values during late summer.Interestingly, the formation and growth rates increase as a function of thecondensation sink, likely reflecting the common anthropogenic sources of NPFprecursor vapors and primary particles affecting the condensation sink. A total of 76 % of the NPF days showed an unusual progression, where the observeddiameter of the newly formed particle mode started to decrease after thegrowth phase. In comparison to most long-term measurements, the NPF eventsin Hada Al Sham are exceptionally frequent and strong both in terms offormation and growth rates. In addition, the frequency of the decreasingmode diameter events is higher than anywhere else in the world.

New particle formation, growth and apparent shrinkage at a rural background site in western Saudi Arabia