A comparison of three methods for measurement of atmospheric nitric acid and aerosol nitrate and ammonium

Three methods for measuring gaseous ambient nitric acid in the low concentration range 0–15 μ m⁻³ were compared under field conditions in Southwestern Ontario during 1–14 June 1982. The methods employed were (1) tunable diode laser absorption, (2) a tungstic acid denuder tube and (3) a filter pack containing a Teflon-nylon-W41 filter combination operated over 3-h sampling periods. In general, the three methods measured nitric acid with adequate sensitivity (sub ppb) and they correlated reasonably over the low ambient concentrations considered here. However, there were some notable differences (up to a factor of 2 at night) by the tungstic acid and filter pack methods. During the field comparison, particulate nitrate and ammonium were also measured (0–6 μg m⁻³ range) by the tungstic acid and filter pack methods. These correlated well but some differences (less than 20%) were observed in the particulate nitrate measurements. In view of the low concentrations measured and the resultant larger experimental error, specific loss or interfering mechanisms could not be unambiguously identified.