Formation and control of nitrogen-containing air pollutants

This work focuses on the formation, transport and control of nitrogen-containing air pollutants. Particular attention is paid to the problem of understanding how to control atmospheric aerosol nitrate and nitric acid concentrations. In the course of this study, additional insights are gained into the effects of emission controls on other co-pollutants, including NO_2, O_3, PAN and ammonia. Computer-based theoretical models are employed that relate emissions of reactive organic gases, oxides of nitrogen and ammonia to downwind pollutant concentrations. Both trajectory models that follow the path of a single air parcel and grid models that examine an entire air basin are used. The trajectory version of the atmospheric model is used extensively to test the new features of the chemical mechanism that is built into these models. Nighttime atmospheric chemical reactions that can lead to nitric acid formation are examined, and are found to produce significant amounts of HNO_3. The hypothesis that atmospheric HNO_3 and NH_3 are in equilibrium with the aerosol phase is tested, and found to be a useful basis for predicting the ambient HNO_3 and NH_3 levels. Ambient measurements on O_3 and NO_2 concentrations are routinely available from governmental air monitoring stations, but short-term average data on the concentrations of HNO_3, NH_3, PAN and aerosol nitrate needed to test the performance of our models are lacking. Therefore, a major field experiment is conducted as part of this study during August, 1982, to acquire such a model verification data set in the South Coast Air Basin that surrounds Los Angeles. The product of the measured nitric acid and ammonia concentrations ranges from less than 1 ppbv^2 to greater than 300 ppbv^2 during the experiment, providing a wide range of conditions over which comparisons can be drawn between chemical equilibrium calculations and experimental results. The ionic material in the aerosol phase is found to be chemically more complex than is assumed by present theoretical models for the equilibrium between NH_3, HNO_3 and the aerosol phase, and includes significant amounts of Na^+, Ca^(2+), Mg^(2+), K^+ and Cl^- in addition to NH_4^+, SO_4^(-2) and NO_3. Results of the experiment show that aerosol nitrate levels in excess of 20 μgm^(-3) accumulate in near-coastal locations on the morning of 31 August, followed by subsequent transport across the air basin. Trajectory analysis shows that the afternoon aerosol nitrate peak observed inland at Rubidoux near Riverside on August 31 is associated with the same air mass that contains the high morning nitrate levels near the coast, indicating that description of both transport and atmospheric chemical reactions is important in understanding regional nitrate dynamics. The performance of both the trajectory- and grid-based versions of the photochemical models used here is evaluated by comparison to the August 1982 field experiments. The trajectory model produces excellent agreement between observations and predictions, especially along the transport path from Long Beach to Riverside where all of the model's input data requirements can be satisfied by actual measured values. The predictions of the grid-based model for O_3 and PAN are in good agreement with observations. The absolute value of the total inorganic nitrate. NH_3 and HNO_3 predictions on average are within a few ppb of the observations. Lacking an inventory of ionic and alkaline aerosol emissions, accurate apportionment of total inorganic nitrate between the aerosol and gas phases is not possible at coastal locations. At mid-basin sites like Anaheim, where NH_4NO_3 is the dominant nitrate aerosol species present, the aerosol nitrate levels predicted by the model are in good agreement with observed values. The completed grid-based airshed model then is used to study the effect of specific emission control measures on ambient NO_2, total inorganic nitrate (TN = HNO_3 + aerosol nitrate). HNO_3, aerosol nitrate, PAN, NH_3 and ozone concentrations in the Los Angeles area, NO_x and reactive hydrocarbon (RHC) emission reductions of up to 61 % and 37%, respectively, are examined. NO_2 and TN concentration reductions in excess of 50% averaged over 20 monitoring sites are achieved at the highest level of emission control studied. The distribution of TN air quality improvements between HNO_3 and aerosol nitrate is affected by the NH_3 emission rate of the NO_x control technologies employed. Peak 1-hr O_3 concentrations at many sites in the eastern portion of the air basin studied decline by more than 25% at the highest NO_x and RHC control levels studied, with the final increment of NO_x control alone capable of producing O_3 concentration improvements at locations with the highest O_3 concentrations