Sources of sampling error for field measurement of nitric acid gas by a denuder system

Field measurements of atmospheric HNO ₃ gas by a denuder system equipped with four annular denuders were conducted in Taiwan. The positive errors from interfering N-compounds as well as the negative errors from incompleteness of the apparent absorption of the HNO ₃ gas, as sampled by one NaCl-denuder, were quantitatively determined and the error sources were identified.It is found that the concentrations of positive error and negative error might cancel each other resulting in less difference between the measured and the true HNO ₃ concentrations. The atmospheric HNO ₃ concentration and the ambient temperature were the major factors that influence the degree of negative errors. The apparent absorption efficiency might decrease to less than 80% if the HNO ₃ gas concentration were lower than 0.3μgm ^-3 or the ambient temperature were over 28°C. There are also evidences showed that the positive errors were from both interfering N-containing gases and nitrate-containing particles.