Analysis of urban gas phase ammonia measurements from the 2002 Atlanta Aerosol Nucleation and Real‐Time Characterization Experiment (ANARChE)

Abstract

Gas phase ammonia (NH 3 ) measurements were made in July and August 2002 during the Atlanta Aerosol Nucleation and Real‐Time Characterization Experiment with two different chemical ionization mass spectrometry techniques. Correlations between the 1 min data from both instruments yielded a slope of 1.17 and an intercept of −0.295 ppbv, with a linear correlation coefficient ( r 2 ) of 0.71. Ambient NH 3 mixing ratios ranged from 0.4 to 13 ppbv. NH 3 observations were compared to the Community Multiscale Air Quality (CMAQ) modeling system as well as a thermodynamic equilibrium model, ISORROPIA, used by CMAQ to predict NH 3 partitioning. A morning rise in both observed and modeled NH 3 mixing ratios strongly suggests a regional influence due to automobile emissions. However, at midday the predicted NH 3 decreased to less than 0.5 ppbv, while the observations remained around 3 ppbv. Both observed and modeled ammonium nitrate levels were too low to support the observed midday NH 3 mixing ratios. ISORROPIA calculations of NH 3 constrained by the total measured ammonia mass (NH 3 + ammonium (NH 4 + )) agreed well with the observations (slope of 1.25 and r 2 of 0.75). For times when the net aerosol charge was near zero the agreement was excellent (slope of 1.22 and r 2 of 0.88). These results indicate that for most of the observed conditions, ISORROPIA could accurately predict NH 3 partitioning. The observations suggest that local sunlight‐ or temperature‐driven NH 3 sources, such as soil emissions, may be responsible for the discrepancy between the model results and measured values.