Regional and global impacts of Criegee intermediates on atmospheric sulphuric acid concentrations and first steps of aerosol formation.

Carbonyl oxides ("Criegee intermediates"), formed in the ozonolysis of alkenes, are key species in tropospheric oxidation of organic molecules and their decomposition provides a non-photolytic source of OH in the atmosphere (Johnson and Marston, Chem. Soc. Rev., 2008, 37, 699, Harrison et al, Sci, Total Environ., 2006, 360, 5, Gäb et al., Nature, 1985, 316, 535, ref. 1-3). Recently it was shown that small Criegee intermediates, C.I.'s, react far more rapidly with SO2 than typically represented in tropospheric models, (Welz, Science, 2012, 335, 204, ref. 4) which suggested that carbonyl oxides could have a substantial influence on the atmospheric oxidation of SO2. Oxidation of 502 is the main atmospheric source of sulphuric acid (H2SO4), which is a critical contributor to aerosol formation, although questions remain about the fundamental nucleation mechanism (Sipilä et al., Science, 2010, 327, 1243, Metzger et al., Proc. Natl. Acad. Sci. U. S. A., 2010 107, 6646, Kirkby et al., Nature, 2011, 476, 429, ref. 5-7). Non-absorbing atmospheric aerosols, by scattering incoming solar radiation and acting as cloud condensation nuclei, have a cooling effect on climate (Intergovernmental Panel on Climate Change (IPCC), Climate Change 2007: The Physical Science Basis, Cambridge University Press, 2007, ref. 8). Here we explore the effect of the Criegees on atmospheric chemistry, and demonstrate that ozonolysis of alkenes via the reaction of Criegee intermediates potentially has a large impact on atmospheric sulphuric acid concentrations and consequently the first steps in aerosol production. Reactions of Criegee intermediates with SO2 will compete with and in places dominate over the reaction of OH with SO2 (the only other known gas-phase source of H2SO4) in many areas of the Earth's surface. In the case that the products of Criegee intermediate reactions predominantly result in H2SO4 formation, modelled particle nucleation rates can be substantially increased by the improved experimentally obtained estimates of the rate coefficients of Criegee intermediate reactions. Using both regional and global scale modelling, we show that this enhancement is likely to be highly variable spatially with local hot-spots in e.g. urban outflows. This conclusion is however contingent on a number of remaining uncertainties in Criegee intermediate chemistry.

Secondary organic aerosol formation from a large number of reactive man-made organic compounds.

A photochemical trajectory model has been used to examine the relative propensities of a wide variety of volatile organic compounds (VOCs) emitted by human activities to form secondary organic aerosol (SOA) under one set of highly idealised conditions representing northwest Europe. This study applied a detailed speciated VOC emission inventory and the Master Chemical Mechanism version 3.1 (MCM v3.1) gas phase chemistry, coupled with an optimised representation of gas-aerosol absorptive partitioning of 365 oxygenated chemical reaction product species. In all, SOA formation was estimated from the atmospheric oxidation of 113 emitted VOCs. A number of aromatic compounds, together with some alkanes and terpenes, showed significant propensities to form SOA. When these propensities were folded into a detailed speciated emission inventory, 15 organic compounds together accounted for 97% of the SOA formation potential of UK man made VOC emissions and 30 emission source categories accounted for 87% of this potential. After road transport and the chemical industry, SOA formation was dominated by the solvents sector which accounted for 28% of the SOA formation potential.

Modelling the ambient distribution of organic compounds during the august 2003 ozone episode in the southern UK.

A photochemical trajectory model containing speciated emissions of 124 non-methane volatile organic compounds (VOC), and a comprehensive description of the chemistry of VOC degradation, has been used to simulate the chemical evolution of boundary layer air masses arriving at a field campaign site in the southern UK during a widespread and prolonged photochemical pollution event in August 2003. The simulated concentrations and distributions of organic compounds at the arrival location are compared with observations of a series of hydrocarbons and carbonyl compounds, which were measured using GC-FID and multidimensional GC methods. The comparison of the simulated and observed distributions of 34 emitted hydrocarbons provides some support for the magnitude and applied emissions speciation of anthropogenic hydrocarbons, but is indicative of an under representation of the input of biogenic hydrocarbons, particularly at elevated temperatures. Simulations of the detailed distribution of ca. 1250 carbonyl compounds, formed primarily from the degradation of the 124 emitted VOC, focus on 61 aldehydes, ketones, dicarbonyls, hydroxycarbonyls and aromatic aldehydes which collectively account for ca. 90% of the simulated total molar concentration of carbonyls. The simulated distributions indicate that the photolysis of formaldehyde and alpha-dicarbonyls make major contributions to free radical production for the arrival conditions of five case study trajectories. The simulated concentrations of hydroxycarbonyls demonstrate preferential formation of the 1,4-substituted isomers (compared with 1,2- and 1,3-isomers of the same carbon number), which are formed during the initial oxidation sequence of longer chain alkanes.