Observations of the Conversion of Nitric Acid to Ammonium Nitrate Over California During ITCT 2002

Photochemical processing of anthropogenic emissions from the west coast of the continental US was examined from the NOAA WP-3 aircraft on several flights during the Intercontinental Transport and Chemical Transformation (ITCT) experiment in May 2002. Airborne measurements of fine particle (< 1 micron diameter) composition, fine particle volume, trace gases, and gas-phase particle precursors are used to describe gas-to-particle conversion observed within the planetary boundary layer over Los Angeles, San Francisco, and the neighboring valleys. Correlations of fast response (1 s) measurements of nitric acid, carbon monoxide, and particle volume are used to identify the sources of the measured ammonium nitrate particles. Conversion of gas-phase nitric acid to ammonium nitrate particles was observed both in regions characterized by high ammonia emissions from livestock sources and in the Los Angeles urban plume upwind of agricultural emissions of ammonia. In an air mass that was transported to an altitude of 4 km over the mountains surrounding Los Angeles, all observed nitrates were partitioned to the particle phase. This partitioning was caused by the sharp decrease in the ammonium nitrate dissociation constant with decreasing ambient temperature when this air mass was lifted from the ground. This conversion of nitric acid to ammonium nitrate during transport is important for understanding fine particle concentration and the lifetime of NOx emissions in the troposphere.