Regional Modeling of Ammonia Emissions from Native Soil Sources in California

Abstract

The development of a new emissions inventory of ammonia volatilization from native soil sources (excluding direct emissions from fertilizer application sources) for the state of California is discussed. Because a comprehensive measurement dataset is currently lacking upon which to build a reliable emissions inventory for NH3 from native soil sources in California, an ecosystem modeling approach that is based on satellite remote sensing and other geographic datasets was used to develop a new estimate of statewide biogenic emissions rates of N-NH3. The NASA?Ames version of the Carnegie?Ames?Stanford Approach (CASA) ecosystem model is applied for soil nitrogen gas emissions at 8-km spatial resolution. The NASA?CASA model estimates seasonal patterns in carbon fixation, nutrient allocation, litterfall, soil nitrogen mineralization, and soil NH3 emissions. The general conditions and spatial patterns favoring soil NH3 volatilization from soils (high pH, low moisture) have been integrated in the NASA?CASA formulation. Based on the modeling inventory estimate discussed here, statewide emissions of NH3 from native soil N sources could range from 12 to 57 ? 106 kg N-NH3 annually, depending on the importance of soil pH on emissions rates. The most important land cover types in terms of contributions to this statewide emissions inventory are the croplands and semiagricultural ecosystems that cover about 20% of the total area of the state, but which make up one-third to one-half of the total soil N sources for NH3 emissions annually. Other native soil source areas that contribute substantially to the statewide emissions inventory for emissions of NH3 are soils of evergreen needleleaf forests, woodland, and wooded grassland ecosystems, mainly on the basis of their large area coverage of the state's natural areas. The model predicts that October is the peak month overall for NH3 emissions from native soils in California. When totaled for the entire region, native soil sources of NH3 predicted for Central Valley counties are highest from July through January. This seasonal pattern in predicted native soil NH3 emissions is fairly consistent with observed seasonality in PM2.5 levels in the San Joaquin Valley Air Basin. It is hypothesized that the combination of productive vegetation communities growing on (even slightly) alkaline soils results in the largest annual emissions of NH3 from native soil N sources. However, a comparison is presented for these native soil sources of NH3 to a regional model budget for potential foliar absorption fluxes of ammonia, which implies that vegetation cover on a statewide basis could actually make ecosystems a strong net sink for locally emitted NH3 sources.