A new method for nitric acid and nitrate aerosol measurement using the dichotomous sampler

Abstract

A new sampling method suitable for the routine monitoring of nitric acid and particulate nitrate was tested at the Nitrogen Species Methods Comparison Study in Claremont, CA during September 1985. The method is based on prior observations that the internal surfaces of the Dichotomous sampler can efficiently denude nitric acid. A Dichotomous sampler was operated with both fine and coarse filter holders modified to accommodate a Teflon membrane prefilter and nylon membrane after filter. A Teflon-lined AIHL cyclone followed by Teflon and nylon filters in series sampled total fine nitrate. Particulate nitrate was determined with the Dichotomous sampler and nitric acid from the cyclone-Dichotomous sampler difference. Over 16 sampling periods, the fine nitrate and nitric acid showed excellent agreement with that obtained with two conventional MgO denuder difference sampling trains. Airborne concentrations of the major ionic species were determined with the Dichotomous sampler and good ion balances were obtained. Coarse nitrate was associated with marine air, consistent with formation by the reaction of nitric acid with sea salt. The resulting hydrochloric acid was measured by applying the cyclone-Dichotomous sampler difference method to chloride. Thus, the new sampling method is useful for the monitoring of the chemical species important for acid deposition.