Aerosol Neutralization by Atmospheric Ammonia

Summary

The daily aerosol compositions at two sites in N. England have been measured using dichotomous samplers for a period of 18 months. A detailed analysis of the aerosol for one month established that within the fine fraction the ammonium ion accounts for ≥ 80% of the cations which neutralise the major anions SO ²⁻₄ , NO ⁻₃ and Cl⁻. The whole year’s data was analysed for seasonal trends in the ammonium levels and the degree of neutralisation of the aerosol. Although there is little Variation of the average monthly ammonium levels there is a clear deficiency of ammonium relative to the sum of the anions during winter. It is shown that this deficiency occurs mainly with fairly clean west to north westerly air masses of Atlantic origin having short overland traverses before reaching the sampling site. Conversely, air masses originating from southern U.K. and from Europe which generally have the highest aerosol concentrations also have high ammonium levels throughout the year.