Read this Nature paper about airborne particles if you care about your every breath

NEWS AND VIEWS  13 MAY 2020

Airborne particles might grow fast in cities

By Hugh Coe

Nanoscale particles have been observed to form and grow in the atmospheres of many cities, contradicting our understanding of particle-formation processes. Experiments now reveal a possible explanation for this mystery.

(发展中国家必经之路吗？）

Article

Rapid growth of new atmospheric particles by nitric acid and ammonia condensation

Wang, M. et al. Nature 581, 184–189 (2020).

New-particle formation is a major contributor to urban smog1,2, but how it occurs in
cities is often puzzling3. If the growth rates of urban particles are similar to those found in
cleaner environments (1–10 nanometres per hour), then existing understanding
suggests that new urban particles should be rapidly scavenged by the high concentration
of pre-existing particles. Here we show, through experiments performed under
atmospheric conditions in the CLOUD chamber at CERN, that below about +5 degrees
Celsius, nitric acid and ammonia vapours can condense onto freshly nucleated particles
as small as a few nanometres in diameter. Moreover, when it is cold enough (below −15
degrees Celsius), nitric acid and ammonia can nucleate directly through an acid–base
stabilization mechanism to form ammonium nitrate particles. Given that these vapours
are often one thousand times more abundant than sulfuric acid, the resulting particle
growth rates can be extremely high, reaching well above 100 nanometres per hour.
However, these high growth rates require the gas-particle ammonium nitrate system to
be out of equilibrium in order to sustain gas-phase supersaturations. In view of the strong
temperature dependence that we measure for the gas-phase supersaturations, we
expect such transient conditions to occur in inhomogeneous urban settings, especially
in wintertime, driven by vertical mixing and by strong local sources such as traffic. Even
though rapid growth from nitric acid and ammonia condensation may last for only a few
minutes, it is nonetheless fast enough to shepherd freshly nucleated particles through
the smallest size range where they are most vulnerable to scavenging loss, thus greatly
increasing their survival probability. We also expect nitric acid and ammonia nucleation
and rapid growth to be important in the relatively clean and cold upper free troposphere,
where ammonia can be convected from the continental boundary layer and nitric acid is
abundant from electrical storms4,5.

（不是应该由中国科学家领衔吗？；），还好有中国单位参与 ：））