Abstract
A list of authors and their affiliations appears at the end of the paper 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.
Highlights
A list of authors and their affiliations appears at the end of the paper New-particle formation is a major contributor to urban smog[1,2], but how it occurs in cities is often puzzling[3]
If the growth rates of urban particles are similar to those found in cleaner environments (1–10 nanometres per hour), existing understanding suggests that new urban particles should be rapidly scavenged by the high concentration of pre-existing particles
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
Summary
A list of authors and their affiliations appears at the end of the paper New-particle formation is a major contributor to urban smog[1,2], but how it occurs in cities is often puzzling[3]. New-particle formation in highly polluted megacities is often perplexing, because the apparent particle growth rates are only modestly faster (by a factor of roughly three) than growth rates in remote areas, whereas the vapour condensation sink (to background particles) is up to two orders of magnitude larger (Extended Data Fig. 1) This implies a very low survival probability in the ‘valley of death’, where particles with diameters (dp) of 10 nm or less have high Brownian diffusivities and will be lost by coagulational scavenging unless they grow rapidly[7,15]. Even a small fractional supersaturation of nitric acid and ammonia vapours with respect to ammonium nitrate has the potential to drive very rapid particle growth, carrying very small, freshly nucleated particles through the valley of death in a few minutes These rapid growth events can exceed 100 nm h−1 under urban conditions—an order of magnitude higher than previous observations—and the growth will continue until the vapours are exhausted and conditions return to equilibrium. Such transients will be difficult to identify in inhomogeneous urban environments, yet have the potential to explain the puzzling observations of new-particle formation in highly a b polluted megacities
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