Determination of gas-phase nitric acid using a tracer technique

A tracer technique has been developed in this laboratory which allows the measurement of aqueous-phase oxidation of SO2 in clouds. We are now extending this technique to the chemistry of nitrogen species in clouds. Using a conservative tracer such as selenium we can determine the fraction of cloudwater nitrate that is the result of aerosol scavenging. At remote sites this allows the calculation of the nitric acid mixing ratio in the precloud air from measurements made during a cloud event. We have used this technique to determine the pre-cloud nitric acid partial pressure at Whiteface Mountain (WFM) for three cloud events. The calculated concentrations for an event on 2 August 1997 agree with measurements made at a second site below cloud level using filterpacks. Nitric acid concentrations for cloud events during July 1995, yielded values of <0.1–10.0 ppbv. At highly polluted sites closer to NOx sources the technique may provide a method of determining if there is any aqueous-phase oxidation of N(III) occurring.