Field intercomparison of filter pack and impactor sampling for aerosol nitrate, ammonium, and sulphate at coastal and inland sites

An intercomparison has been performed on the coarse (>2.5 μm) and fine fraction (<2.5 μm) mass concentrations of particulate nitrate, ammonium, and sulphate determined simultaneously by filter pack and MOUDI samplers. Five intensive field campaigns have been carried out in summer and winter seasons, both at coastal sites (Mace Head, Ireland, and Tenerife, Spain) and at an inland site (University of Birmingham, West Midlands, UK). Comparison between particle sulphate measurements shows that sulphate measurements are the same with both filter pack and MOUDI, independent of sampling site or season. For both nitrate and ammonium, the MOUDI results are observed to be usually less than those from the filter pack, especially in the case of polluted air masses. During periods when the measured concentration products [NH3][HNO3] are low (<0.1 ppbv2), the ammonium concentrations obtained with the two samplers are matched very well with each other, but for nitrate, the filter pack system provides 15% higher mass concentrations than the MOUDI which are attributable to different inlet efficiencies. During more polluted periods, however, when there are high levels of gaseous ammonia and nitric acid in the atmosphere (with the measured concentration products [NH3][HNO3]>0.1 ppbv2), significantly negative sampling artefacts are observed for both nitrate and ammonium concentrations obtained with the impactor relative to the filter pack. Nevertheless, it is shown that both filter pack and MOUDI are capable of collecting ammonium nitrate from polluted air masses although the absolute efficiency is unknown. From the measurements obtained with the MOUDI in summertime at both coastal and inland sites, it is observed that about 64% of collected particle nitrate is present in the coarse (>2.5 μm) mode; but in wintertime, only about 29% of particle nitrate is found to reside in the coarse particles collected at the inland site. The lower efficiency of the MOUDI for ammonium nitrate relative to the Teflon filter is in marked contrast to the reported higher efficiency of the Berner impactor than the Teflon filter for sampling nitrate from polluted air.