A chemical ionization mass spectrometer for ground-based measurements of nitric acid

A chemical ionization mass spectrometer (CIMS) instrument has been developed for high-precision measurements of gaseous nitric acid (HNO₃) specifically under high- and variable-humidity conditions in the boundary layer. The instrument's background signals (i.e., signals detected when HNO₃-free air in measured), which depend on the humidity and HNO₃ concentration of the sample air, are the most important factor affecting the limit of detection (LOD). A new system to provide HNO₃-free air without changing both the humidity and the pressure of the sampled air was developed to measure the background level accurately. The detection limit was about 23 parts per trillion by volume (pptv) for 50-s averages. Field tests, including an intercomparison with the diffusion scrubber technique, were carried out at a surface site in Tokyo, Japan, in October 2003 and June 2004. A comparison between the measured concentrations of HNO₃ and particulate nitrate indicated that the interference from particulate nitrate was not detectable (i.e., less than about 1 %). The intercomparison indicated that the two independent measurements of HNO₃ agreed to within the combined uncertainties of these measurements. This result demonstrates that the CIMS instrument developed in this study is capable of measuring HNO₃ mixing ratios with the precision, accuracy, and time resolution required for atmospheric science.