Seasonal NH 3 emissions for the continental united states: Inverse model estimation and evaluation

Abstract

Significant uncertainty exists in the seasonal distribution of NH 3 emissions since the predominant sources are animal husbandry and fertilizer application. Previous studies that estimated bottom–up and top–down NH 3 emissions have provided the most comprehensive information available about the seasonality of NH 3 emissions. In this study, this bottom–up and top–down emission information is combined with the most recent 2001 USEPA National Emission Inventory (NEI) to construct a best prior estimate of seasonal NH 3 emissions. These emission estimates are then used in an annual 2001 USEPA Community Multiscale Air Quality (CMAQ) model simulation for the continental United States. A key objective of this study is to evaluate these prior NH 3 emission estimates and test the top–down inverse modeling method for a different year and a larger modeling domain than used previously. Based on the final posterior NH 3 emission estimates, the inverse modeling results suggest that the annual total NEI NH 3 emissions are reasonable and that a previous high bias in older USEPA emission inventories has been addressed in the updated inventory. Inverse modeling results suggest that the prior NH 3 emission estimates should be increased in the summer and decreased in the winter, while results for the spring and fall are questionable due to precipitation prediction biases. A final conclusion from this study is that total NH x (NH 3 and aerosol NH 4 +) air concentration data are essential for quantitative top–down analyses of NH 3 emissions that can extend beyond what is possible using precipitation chemistry data.