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An Investigation of Ammonia and Inorganic Particular Matter in California during the CalNex Campaign

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August 7, 2012
Luke Schiferl
Hosted by Colette Heald (advisor), Jeff Collett, Jay Ham (Soild and Crop Sciences)

Abstract

Over the last century, the rise of industrial agriculture has greatly increased the emission of ammonia (NH3) from livestock waste and synthetic crop fertilizers to the atmosphere. Ammonium (NH4) aerosol, which can be formed through the neutralization of atmospheric acids by NH3, is a key component of particulate matter (PM) in the atmosphere. PM causes negative human health effects and reduces visibility, and transport and deposition of excess NH3 can cause environmental degradation.

Airborne observations of gas precursors and inorganic aerosol taken during the CalNex campaign in May and June 2010 are used in this study to investigate the role of NH3 in PM formation in California and test the representation of the key processes relevant to this chemical system in the GEOS-Chem chemical transport model.

Evaluation of the 0.5°ree; x 0.667°ree; horizontal resolution nested model with observations shows a large underestimation (5.4 ppb median bias in the lowet 1km) of NH3 in the Central Valley. This NH3 underestimation is lower in the area surrounding Los Angeles (LA), only 1.4 ppb. Sulfur dioxide (SO2) is also underestimated in both regions, while nitric acid (HNO3) shows little bias. Near-surface simulated inorganic PM is under-predicted by 1.28 µg sm-3 in the LA region and over-predicted in the Valley by 0.27 µg sm-3.

Investigation of model sensitivity to the processes of gas-particle partitioning, wet deposition, dry deposition and emissions reveal that emissions have the largest potential for correction of model deficiencies. Increases to anthropogenic livestock NH3 emissions by a factor of 5 and anthropogenic SO2 emissions in the Valley by factors from 3 - 10 eliminates the bias in the simulation of gases in both regions and PM near LA, where under-prediction of nitrate (NO3-) is reduced from 0.64 µg sm-3 to 0.12 µg sm-3 in the lowest 1 km. An increase in NH3 emissions in the LA region is critical to capturing ammonium nitrate (NH4NO3) formation down-wind of the city core.

Using modified emissions simulation, seasonal PM differences in the two regions and the export of excess NH3 out of the Valley are explored. Mean June simulated PM concentration in the lowest 1 km is 3.48 µg sm-3 near LA (38% NO3 and 39% SO42-, by mass) and 1.98 µg sm-3 in the Valley (44% NO3 and 32% SO42-, by mass). These simulated PM concentrations are 2 times greater in the Valley in December than in June, when NH4NO3 formation is favored by colder temperatures. However LA simulated PM concentration falls by 53% in December compared to June, likely due to lower winter NH3 emissions. Both the model and IASI satellite observations indicate that large amounts of excess NH3 are transported from the Valley to southeastern California in the summertime which may negatively affect ecosystems in the area.