Regulating Ammonia in Clean Air Act State Implementation Plans Do We Know Enough to Proceed?

The San Joaquin Valley has a serious particulate pollution problem that is caused in large part by ammonium nitrate and ammonium sulfate during winter stagnation events. Ammonium nitrate alone measured between 54 and 110 micrograms per cubic meter at monitoring sites throughout the air basin during episodes captured by the California Regional Particulate Air Quality Study (CRPAQS) field study in 2000-2001. The San Joaquin Valley has a $15 billion diversified farm economy and is California’s leading dairy region. Agriculture is estimated to contribute over 90 percent of the 397 ton per day ammonia emission inventory.

The San Joaquin Valley Air Pollution Control District (District) submitted a Particulate Matter less than 10 micrometers in diameter (PM10) State Implementation Plan to United States Environmental Protection Agency (EPA) in June 2003. The plan relies upon controls of oxides of nitrogen (NO_x) and oxides of sulfur (SO_x) to reduce nitrate and sulfate concentrations. This is based on field measurements that indicated that ammonia was nearly always in excess and NO_x/nitric acid was the limiting pollutant in the reaction. Modeling conducted by the California Air Resource Board using UAM-Aero was inconclusive regarding the benefit of control of each precursor. These inconclusive results led environmental groups following the process to comment that ammonia controls should be pursued immediately. EPA has so far allowed the District to forego ammonia controls pending the results of final CRPAQS reports due in 2005 and with a commitment to pursue controls should convincing evidence of their benefit be identified in the interim. The District also made the case that knowledge regarding the effectiveness of ammonia controls for agricultural sources was not sufficiently advanced to proceed with regulations. To resolve this situation satisfactorily, the District needs answers to three questions: 1. Are ammonia emission factors from all sources reliable enough to accurately identify the true causes of the problem? 2. Would ammonia emission reductions result in expedited attainment of PM10 and PM2.5 standards? 3. Are ammonia controls for agricultural sources effective and, if so, at what cost? The expenditure of millions of dollars by the agriculture industry and by government may hinge on the answer.

⁽¹⁾ E-mail: dave.mitchell@valleyair.org