Hydrocarbons are emitted into the Earth's atmosphere in very large quantities by human and biogenic activities. Their atmospheric oxidation processes almost exclusively yield RO radicals as reactive intermediates whose atmospheric fate is not yet fully unraveled. Herein, we show that gas-phase reactions of two RO radicals produce accretion products composed of the carbon backbone of both reactants. The rates for accretion product formation are very high for RO radicals bearing functional groups, competing with those of the corresponding reactions with NO and HO . This pathway, which has not yet been considered in the modelling of atmospheric processes, can be important, or even dominant, for the fate of RO radicals in all areas of the atmosphere. Moreover, the vapor pressure of the formed accretion products can be remarkably low, characterizing them as an effective source for the secondary organic aerosol.

Iodic acid (HIO3) is known to form aerosol particles in coastal marine regions, but predicted nucleation and growth rates are lacking. Using the CERN CLOUD (Cosmics Leaving Outdoor Droplets) chamber, we find that the nucleation rates of HIO3 particles are rapid, even exceeding sulfuric acid–ammonia rates under similar conditions. We also find that ion-induced nucleation involves IO3− and the sequential addition of HIO3 and that it proceeds at the kinetic limit below +10°C. In contrast, neutral nucleation involves the repeated sequential addition of iodous acid (HIO2) followed by HIO3, showing that HIO2 plays a key stabilizing role. Freshly formed particles are composed almost entirely of HIO3, which drives rapid particle growth at the kinetic limit. Our measurements indicate that iodine oxoacid particle formation can compete with sulfuric acid in pristine regions of the atmosphere.

α-Pinene (CH) represents one of the most important biogenic emissions in the atmosphere. Its oxidation products can significantly contribute to the secondary organic aerosol (SOA) formation. Here, we report on the formation mechanism of C and C accretion products from α-pinene oxidation, which are believed to be efficient SOA precursors. Measurements have been performed in a free-jet flow system. Detection of RO radicals and accretion products was carried out by recent mass spectrometric techniques using different ionization schemes. Observed C-RO radicals from α-pinene ozonolysis were O,O-CH(O) O with x = 0, 1, 2, 3 and from the OH radical reaction HO-CH(O)O with α = 0, 1, 2. All detected C accretion products can be explained via the accretion reaction RO + R'O → ROOR' + O starting from the measured C-RO radicals. We speculate that C accretion products are formed in an analogous way assuming CHO elimination. Addition of isoprene (CH), producing C-RO radicals, leads to C accretion products formed via cross-reactions with C-RO radicals. This process is competing with the formation of C/C products from the pure α-pinene oxidation. A similar behavior has been observed for ethylene additives that form C accretion products. In the atmosphere, a complex accretion product spectrum from self- and cross-reactions of available RO radicals can be expected. Modeling atmospheric conditions revealed that C/C product formation is only reduced by a factor of 1.2 or 3.6 in isoprene-dominated environments assuming a 2- or 15-fold isoprene concentration over α-pinene, respectively, as present in different forested areas.

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.