Development and Evaluation of an Ammonia Bi-Directional Flux Model for Air Quality Models

Abstract

Ammonia is an important contributor to particulate matter in the atmosphere and can significantly impact terrestrial and aquatic ecosystems. Surface exchange between the atmosphere and biosphere is a key part of the ammonia cycle. Agriculture, in particular, is a large source of ammonia emitted to the atmosphere, mostly from animal operations and fertilized crops, while dry and wet deposition are the primary sinks of atmospheric ammonia. Although, current air quality models consider all of these source and sink processes, algorithms for emissions from fertilized crops and dry deposition are too simplistic to provide accurate accounting of the net surface fluxes. New modeling techniques are being developed that replace current ammonia emission from fertilized crops and ammonia dry deposition with a bi-directional surface flux model. Comparisons of the ammonia bi-direction flux algorithm to field experiments involving both lightly fertilized soybeans and heavily fertilized corn are presented and discussed. Initial tests and evaluation of CMAQ modeling results for a full year (2002) at 12 km grid resolution including implementation of a soil nitrification model and the ammonia bi-directional flux algorithm result in improved NH_xwet deposition.