Spring time sources and sinks of Peroxyacetyl Nitrate in the UK and its contribution to acidification and nitrification of cloud water

During spring the atmospheric concentration of Peroxyacetyl Nitrate, PAN, in the UK varies over three orders of magnitude due to long range transport and local photochemical production. An experiment at Great Dun Fell was used to determine which of these sources of PAN controlled spring time concentrations and to quantify the contribution of PAN to oxidised nitrogen in cloud water. The gas phase mixing ratio of a wide range of trace gas pollutants, including PAN, along with cloud water chemical composition and aerosol/cloud droplet distributions were measured. A chemical model was developed to determine the contribution of PAN hydrolysis to cloud water nitrification and to quantify the sink strengths of PAN. Chemical reaction following thermal decomposition was found to be the dominant sink mechanism in the model but transport was the most important factor controlling PAN concentration. During cloud episodes PAN concentrations varied from 100 ppt to over 1 ppb but the calculated inorganic oxidised nitrogen loading in the cloud due to PAN dissolution and hydrolysis was an insignificant fraction of the observed total oxidised nitrogen loading of the cloud. Extrapolation of the model to persistent cloud in marine conditions showed that PAN has a negligible effect on cloud nitrification. It was concluded that PAN concentrations were heavily influenced by regional concentrations and that the hydrolysis of PAN in cloud water was an insignificant source of oxidised nitrogen in cloud water.