Formation and Control of Atmospheric Aerosol Nitrate and Nitric Acid

This work focuses on the formation, transport and control of atmospheric nitric acid and nitrate aerosol using both theoretical modeling and experimental techniques. A mathematical model was developed that describes the formation and transport of photochemically produced atmospheric gases and nitrate aerosol, using fundamental thermodynamic data to determine the quantity and state of the aerosol nitrate produced. Model predictions compared favorably with the field data available. A sensitivity study of the model indicated that the predicted aerosol nitrate concentrations are highly dependent on temperature. The trajectory model was used to study the fate of nitrogen oxides emissions and the chemical reactions responsible for the formation of atmospheric nitric acid. A majority of the NOx emissions deposit out within 24 hours, primarily as HNO3. Previously it was believed that almost all of the atmospheric nitric acid was produced during daylight hours, however model results indicate that nighttime reactions can produce comparable quantities, especially in the upper portions of the boundary layer more than a hundred meters above ground. An experimental program was designed and executed to collect a set of data for use in studying nitrate formation, and for use in evaluating the accuracy of air quality models. A large quantity of aerosol nitrate was observed to accumulate overnight near the coast, presumably due to the reaction between HNO3 and sea salt aerosol or soil dust—like material. This aerosol is then transported inland the following afternoon, and can contribute to the high particulate nitrate levels found inland. Data from the experiment were used to test the hypothesis that atmospheric HNO3 and NH3 are in equilibrium with the aerosol phase. Most of the data are consistent with the assumption that an external mixture containing some pure NH4NO3 is present. Additional improvement is obtained if an internally mixed NH+4-NO-3-SO=4 aerosol is assumed to be present. Further evaluation of the air quality model against the data described above showed that the model accurately predicts the measured concentrations of O3, NO2, total nitrate, HNO3, and NH3. Representative emission control programs were tested using the model, and results indicated that NOx emission control will reduce HNO3(g), aerosol nitrate and PAN concentrations. For the particular trajectories studied, NOx control would also have reduced the peak O3 concentrations. Reducing NH3 emissions will reduce aerosol nitrate formation at the expense of increasing HNO3 concentrations. Controlling organic gas emissions will reduce O3 and PAN. Further research areas suggested by this work also are presented.