Formation and control of nitrogen-containing air pollutants
Creators: name: Russell, Armistead G.
name: Cass, Glen R.
Publisher: name: California Institute of Technology
Year: 1987

description: 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.
description: © 1987 Environmental Quality Laboratory. California Institute of Technology.

This work was supported by the California Air Resources Board under Agreement A2-150-32 and by gifts to the Environmental Quality Laboratory. 

This research project represents a sustained effort over a period of years by a large number of investigators at Caltech. Dr. Gregory J. McRae, now at Carnegie-Mellon University, was a major contributor to the early phases of the air quality modeling work reported in this study. Many of the modeling techniques employed here are an outgrowth of his Ph.D. thesis research. Lynn Hildemann co-authored the report section dealing with comparison of ambient NH_3 and HNO_3 levels to the predictions of models based on thermodynamic equilibrium between the aerosol and gas phases. Dr. Kenneth McCue participated in the execution of the many CPU-months of computer-based modeling calculations reported in Chapters 7 and 8. The ammonia emissions inventory procedures used in parts of this report were developed by Shohreh Gharib, Mary (Peterson) Ligocki, and Dr. James Tilden. Dr. John Seinfeld contributed to the air quality modeling study reproduced as Appendix B to this report. The participation of each of these co-authors is acknowledged at the start of the chapters to which they contributed. 

This work was conducted at the Environmental Quality Laboratory (EQL) of Caltech, and the support of its director, Dr. Norman H. Brooks is appreciated greatly. The manuscript was typed by Pat Houseworth, Christina Conti and Dixie Termin, Theresa Fall, Phil Dube, and Nancy Tomer helped in preparing many of the illustrations that are to follow. 

The list of people crucial to the project does not end there. Over twenty Caltech students and staff gave up two or more days of their time to help conduct the experiments described in this study. The South Coast Air Quality Management District (SCAQMD) provided much of the required air quality, meteorological and emissions data, and allowed the use of SCAQMD air monitoring sites for our experiments. We are grateful to the SCAQMD's staff members who helped in these matters, especially Bill Bope, Julian Foon, John Grissinger, Eric Lemke, and Chung Liu. 

The California Air Resources Board provided not only emissions data, and use of one of their monitoring sites, but, under Agreement A2-150-32. also provided most of the monetary support required to conduct this project. Paul Allen, Andrew Ranzieri, Charles Unger, and Kit Wagner helped provide emissions data and had helpful comments on the development of the modeling work. Doug Lawson, Jack Suder and Chuck Unger were the contract monitors at different times during this project.