The stability of particulate nitrate in the Los Angeles atmosphere

Simultaneous measurements of atmosphere particulate nitrate (NO₃^?) and its gas phase precursor nitric acid (HONO₂) were conducted in Los Angeles during severe smog episodes, and the results were compared to those predicted on the basis of thermodynamic data for the HONO₂ (g) + NH₃ (g) = NH₄ NO₃ (s) equilibrium. Over the wide range of conditions studied (four-hour averaged NO₃^? = 0.5?44.3 μg m^?3 HONO₂ = 1.4?36.0 μg m^?3, T = 10?33°C, humidity = 16?99%, ozone up to 460 ppb) the solid equilibrium model was found to be applicable to only about one-third of the total number of data sets. In this case good agreement was found between measured nitrate values and those expected from comparison of measured and equilibrium nitric acid concentrations. Samples collected at humidities above the deliquescent point of ammonium nitrate at the sampling temperature, for which the solid NH₄ NO₃ equilibrium no longer applies and solution chemistry must be considered, accounted for two-thirds of the data sets. For this subset, good agreement was also found between experimental results and theoretical considerations.