Airborne flux measurements of ammonia over the Southern Great Plains using chemical ionization mass spectrometry

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. Measured NH₃ mixing ratios spanned from <10 to 100s of parts per trillion in the free troposphere, to sharp plumes of highly elevated mixing ratios (10’s of parts per billion) downwind from a fertilizer plant. These plumes are of the order expected based on the U.S. Environmental Protection Agency’s National Emissions Inventory (NEI). The high sensitivity and response time of ~1 s allowed us to also calculate vertical NH₃ fluxes via eddy covariance. We used the continuous wavelet transform method to maximize the spatial resolution of the derived fluxes, remaining limited to ~1-2 km by the flight altitudes and related turbulence scales. Together with flux footprint considerations, the measurements let us constrain the NH₃ emission rates for ubiquitous agricultural area sources in rural Oklahoma. Typically, the derived area emission rates clearly exceeded the values provided by the NEI. In addition, our measurements captured large point sources that appeared to be missing in the NEI, at least one identified as a large cattle farm.