Issues in ammonia and nitric acid measurements: Experiences in the midwest

In October 2003, the Midwest Regional Planning Organization (MRPO) and Central Regional Air Planning Association (CenRAP) initiated a 1-year field measurement study to assess ambient concentrations of ammonia, nitric acid, sulfur dioxide, and fine particulate nitrate, sulfate, and ammonium. Nine rural sites and one urban site were selected to make integrated 24-hour measurements every sixth day. The sampling platform consists of a basic denuder and an acidic denuder in series, followed by a filter pack containing Teflon™ and nylon filters; two different configurations of this basic sampling train are deployed. Because ammonia and nitric acid in particular are known to be difficult species to quantify, a number of quality assurance measures were undertaken to assess the precision and accuracy of the measurements. This paper presents the results of the quality assurance tests. The following tests are discussed and implications for data interpretation are noted: 1) comparison of collocated identical samplers; 2) comparison of collocated samplers with different denuder and inlet configurations; 3) losses to cyclones and impactors used as size selectors before the denuders; 4) denuder breakthrough; and 5) losses of ammonium and nitrate from Teflon™ filters.