Abstract. Gas-phase ethene ozonolysis experiments were conducted at room temperature to determine formic acid yields as a function of relative humidity (RH) using the integrated EXTreme RAnge chamber-Chemical Ionisation Mass Spectrometry technique, employing a CH 3 I ionisation scheme. RHs studied were <1, 11, 21, 27, 30 % and formic acid yields of (0.07±0.01) and (0.41±0.07) were determined at <1 % RH and 30 % RH respectively, showing a strong water dependence. It has been possible to estimate the ratio of the rate coefficient for the reaction of the Criegee biradical, CH 2 OO with water compared with decomposition. This analysis suggests that the rate of reaction with water ranges between 1×10 −12 -1×10 −15 cm 3 molecule −1 s −1 and will therefore dominate its loss with respect to bimolecular processes in the atmosphere. Global model integrations suggest that this reaction between CH 2 OO and water may dominate the production of HC(O)OH in the atmosphere.

Environmental context. With large reductions in anthropogenic emissions of many ozone-depleting gases in response to the Montreal Protocol, gases with biogenic sources have become relatively more important in recent years. The global budgets of the biogenic halocarbons are unbalanced with known sinks outweighing sources, suggesting that additional natural sources are required to balance the budgets. In the present study, an investigation has been carried out to determine the importance of leaf cutter ants as a missing source of the biogenic halocarbons, which will reduce the discrepancy of the global budget of the halocarbons. Abstract. Leaf cutter ant colonies are shown to be a potentially significant new source of biogenic halocarbons. Fungus cultivated by these ant species may emit CH3Br, CH3I, CH3Cl, CH2Cl2 and CHCl3 in significant quantities, contributing to their respective global atmospheric budgets. The study suggests that the mixing ratios of CH3Br, CH3I, CH3Cl, CH2Cl2 and CHCl3 in the ant colony under test were significantly higher than background levels, by on average a factor of 1.5–5.0. Sampling was carried out during three stages of ant colony development (new, moderately active and highly active) and it was found that levels of these halocarbons were elevated during the active phases of the ant colony life cycle. A very rough estimate of the possible emission of CH3Br, CH3I, CH3Cl, CH2Cl2 and CHCl3 from ant colonies globally are 0.50, 0.02, 0.80, 0.15 and 0.22 Gg year–1.

The first airborne measurements of formic acid mixing ratios over the United Kingdom were measured on the FAAM BAe-146 research aircraft on the 16 March 2010 with a chemical ionisation mass spectrometer using I^&minus; reagent ions. The I^&minus; ionisation scheme was able to measure formic acid mixing ratios at 1 Hz in the lower boundary layer. <br><br> In-flight standard addition calibrations from a formic acid source were used to determine the instrument sensitivity of 35&plusmn;6 ion counts pptv⁻¹ s⁻¹ and a limit of detection of 25 pptv. Routine measurements were made through a scrubbed inlet to determine the instrumental background. Three plumes of formic acid were observed over the UK, originating from London, Humberside and Tyneside. The London plume had the highest formic acid mixing ratio throughout the flight, peaking at 358 pptv. No significant correlations of formic acid with NO_x and Ozone were found. <br><br> A trajectory model was employed to determine the sources of the plumes and compare modelled mixing ratios with measured values. The model underestimated formic acid concentrations by up to a factor of 2. This is explained by missing sources in the model, considered to be primary emissions of formic acid of mainly anthropogenic origin and lack of precursor emissions, such as isoprene, from biogenic sources

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Abstract. A Lagrangian trajectory model used to simulate photochemistry has been extended to include a simple parameterisation of primary and secondary aerosol particles. The model uses emission inventories of primary particles for the UK from the NAEI (National Atmospheric Emissions Inventory for the UK), and for Europe from the TNO (Institute of Environmental Sciences, Energy Research and Process Innovation, the Netherlands) respectively, to transport tracers representing PM10. One biogenic and two anthropogenic organic compounds were chosen as surrogates to model the formation of condensable material suitable for the production of secondary organic aerosol (SOA). The SOA is added to the primary PM10 and compared to measured PM10 at one urban and two rural UK receptor sites. The results show an average under-prediction by factors of 4.5 and 8.9 in the urban and rural cases respectively. The model is also used to simulate production of two secondary inorganic species, H2SO4 and HNO3, which are assumed, as a limiting case, to be present in the particle phase. The relationships between modelled and measured total PM10 improved with the addition of secondary inorganic compounds, and the overall model under-prediction factors are reduced to 3.5 and 3.9 in the urban and rural cases respectively. Nevertheless, our conclusion is that current emissions and chemistry do not appear to provide sufficient information to model PM10 well (i.e. to within a factor of two). There is a need for further process studies to inform global climate modelling that includes climate forcing by aerosol.