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Airborne Flux Measurements of Ammonia over the Southern Great Plains
SIEGFRIED SCHOBESBERGER, Emma D'Ambro, Ben H. Lee, Qiaoyun Peng, Mikhail Pekour, Jerome Fast, Joel A. Thornton, University of Eastern Finland
Abstract Number: 310
Working Group: Instrumentation and Methods
Abstract
Ammonia (NH3) plays an important role in atmospheric and environmental chemistry, from the formation of inorganic and organic aerosol, to soil acidification and nutrient cycles. Its dominant source are anthropogenic emissions, primarily from agricultural activities, and it thereby contributes substantially to fine-particle pollution in many regions. However, there are high uncertainties in attributing atmospheric NH3 to specific sources, and current emission inventories substantially underestimate many major point sources. The quantification of NH3 is challenging, due to the wide range of ambient mixing ratios and its infamous propensity to interact with surfaces, causing losses and slow response times.
In this study, we present a new technique for detecting NH3 using a chemical ionization mass spectrometer (CIMS). The CIMS was deployed on a G-1 aircraft during the Holistic Interactions of Shallow Clouds, Aerosols, and Land Ecosystems (HI-SCALE) campaign over Oklahoma, specifically around the ARM Southern Great Plains field site, in 2016. The instrument was modified to enable quantifiable airborne measurements throughout tropospheric pressures, and to alternatingly use iodide anion and deuterated benzene cation ionization. In this mode, and aided by a high-flow core-sampling setup, we obtained a formidable device for measuring in-situ mixing ratios of NH3. The high sensitivity and response time of ~1 s allowed us to calculate vertical NH3 fluxes via eddy covariance. Besides measuring the free-troposphere and boundary-layer NH3 levels, we detected sharp plumes of highly elevated (10’s of parts per billion) NH3 downwind from a fertilizer plant. The fluxes let us also constrain the NH3 emission rates for ubiquitous agricultural area sources. The HI-SCALE flight profiles focused on studying shallow convective clouds, and additional instrumentation provided aerosol chemical speciation and size distributions, which allows us to put our measurement results in perspective of aerosol properties and cloud processing.