Quantifying the dry deposition of ammonia in ammonia-rich and ammonia-poor environments using a surrogate surface approach

In this study, dry deposition samples were collected with both an aerodynamically smooth water surface (WSS) and a knife-edge surrogate surface (KSS) in Potsdam, NY, Solomons, MD, and at a farm in upstate NY intermittently between March 2000 and August 2001. The WSS and the citric acid-coated paper filter on the KSS were used to measure total ammonium deposition (NH3 and p-NH4+), while a greased Mylar disk on the KSS was used to measure particulate NH4+ (p-NH4+) deposition. An annular denuder system (ADS) was used to measure NH3 gas phase and particulate NH4+ concentrations. In side-by-side experiments with two WSSs, no statistical difference was found between simultaneously measured total ammonium deposition at any of the sites. Average total NH3 fluxes measured with the WSS in Potsdam, Solomons, and at the farm were 3.1, 2.1, and 310 mg NH4+ m−2 day−1, respectively. Similarly gaseous NH3 fluxes measured with the citric acid-coated paper on the KSS were 3.6, 2.1, and 500 mg NH4+m−2 day−1, respectively. The p-NH4+ fluxes to the KSS-greased strip were negligible at all three locations. Fluxes measured by the citric acid paper filters were slightly greater than those measured with the WSS suggesting that the acidic dry surfaces had a higher affinity for ammonia than the water in the WSS. The average gas-phase ammonia concentrations in Potsdam, Solomons, MD and at the farm were 0.60×10−3, 2.1×10−3, and 0.25 mg NH4+ m−3, respectively. The average ammonium concentrations were 0.50×10−3, 1.2×10−3, and 14×10−3 mg NH4+ m−3, respectively.