1Department of Mechanical Engineering, University of Colorado, Boulder, Colorado, USA
2US Environmental Protection Agency, Research Triangle Park, North Carolina, USA
3Atmospheric and Environmental Research, Inc., Lexington, Massachusetts, USA
4Environment Canada, Toronto, Ontario, Canada
5Jet Propulsion Laboratory, California Institute of Technology Pasadena, CA, USA
6Program in Atmospheric and Oceanic Sciences, Princeton University, Princeton, New Jersey, USA
7Geophysical Fluid Dynamics Laboratory/National Oceanic and Atmospheric Administration, Princeton, New Jersey, USA
Received: 07 Dec 2014 – Discussion started: 20 Feb 2015
Abstract. Bidirectional air–surface exchange of ammonia (NH3) has been neglected in many air quality models. In this study, we implement the bidirectional exchange of NH3 in the GEOS-Chem global chemical transport model. We also introduce an updated diurnal variability scheme for NH3 livestock emissions and evaluate the recently developed MASAGE_NH3 bottom-up inventory. While updated diurnal variability improves comparison of modeled-to-hourly in situ measurements in the southeastern USA, NH3 concentrations decrease throughout the globe, up to 17 ppb in India and southeastern China, with corresponding decreases in aerosol nitrate by up to 7 μg m−3. The ammonium (NH4+) soil pool in the bidirectional exchange model largely extends the NH3 lifetime in the atmosphere. Including bidirectional exchange generally increases NH3 gross emissions (7.1 %) and surface concentrations (up to 3.9 ppb) throughout the globe in July, except in India and southeastern China. In April and October, it decreases NH3 gross emissions in the Northern Hemisphere (e.g., 43.6 % in April in China) and increases NH3 gross emissions in the Southern Hemisphere. Bidirectional exchange does not largely impact NH4+ wet deposition overall. While bidirectional exchange is fundamentally a better representation of NH3 emissions from fertilizers, emissions from primary sources are still underestimated and thus significant model biases remain when compared to in situ measurements in the USA. The adjoint of bidirectional exchange has also been developed for the GEOS-Chem model and is used to investigate the sensitivity of NH3 concentrations with respect to soil pH and fertilizer application rate. This study thus lays the groundwork for future inverse modeling studies to more directly constrain these physical processes rather than tuning bulk unidirectional NH3 emissions.
Revised: 10 Oct 2015 – Accepted: 09 Nov 2015 – Published: 19 Nov 2015
Zhu, L., Henze, D., Bash, J., Jeong, G.-R., Cady-Pereira, K., Shephard, M., Luo, M., Paulot, F., and Capps, S.: Global evaluation of ammonia bidirectional exchange and livestock diurnal variation schemes, Atmos. Chem. Phys., 15, 12823-12843, doi:10.5194/acp-15-12823-2015, 2015.