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 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.
Several factors suggest that in Hada Al Sham these precursor vapors are
related to the transport of anthropogenic emissions from the coastal urban
and industrial areas. The median particle formation and growth rates for the
NPF days were 8.7 cm−3 s−1 (J7 nm) and 7.4 nm h−1
(GR7−12 nm), respectively, both showing highest values during late summer.
Interestingly, the formation and growth rates increase as a function of the
condensation sink, likely reflecting the common anthropogenic sources of NPF
precursor vapors and primary particles affecting the condensation sink. A total of 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 both in terms of
formation and growth rates. In addition, the frequency of the decreasing
mode diameter events is higher than anywhere else in the world.
Reduction in anthropogenic emissions suppressed new particle formation and growth: Insights from the COVID‐19 lockdown
