Effect of ambient NH3 levels on PM2.5 composition in the Great Smoky Mountains National Park

Continuous measurements were made of gaseous NH3, gaseous HNO3, PM2.5 sulfate, aerosol mass, air temperature, and relative humidity (RH), and 24-h filter samples of PM2.5 aerosols were analyzed to determine the effects of ambient NH3 levels on PM2.5 aerosol composition. These measurements were conducted from 2 to 15 August 2002, at Look Rock, TN, at an air monitoring station located on a mountain ridge (elevation ∼800 m MSL) on the southwestern edge of the Great Smoky Mountains National Park. A strong diurnal pattern was observed for gaseous NH3 levels at this site with higher daytime and much lower NH3 during the night hours. The diurnal variability of both gaseous NH3 concentrations and RH suggests that conditions controlling aerosol nitrate formation and acidic aerosol neutralization are complex. The observations during this 2-week long sampling period showed that measurable NH4NO3 aerosol was not present under the ambient conditions encountered. However, modest perturbations from observed ambient conditions could have enabled NH4NO3 aerosol production. The extent of neutralization of sulfate aerosol collected on filters indicates that there was insufficient regional NH3 to neutralize the acidic sulfate aerosols to the degree predicted from local NH3 and RH conditions. Incomplete neutralization of acidic sulfate could also result if neutralization proceeds more slowly under field conditions compared to laboratory conditions or model predictions. Continuous measurements of aerosol ammonium and/or acidity are needed to determine if ammonia availability or kinetic limitations (or both) restrict neutralization of acidic sulfate aerosols, a key factor for modeling and exposure studies.