From Megacities to Wildfire Smoke: Observational Constraints on Atmospheric Ammonia

March 25, 2026

Emily Lill

Committee: Emily Fischer (Advisor); Jeffrey Collett, Jr.; Jeffrey Pierce; Shantanu Jathar (Mechanical Engineering)

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Abstract

Urban ammonia (NH3) emissions are an increasingly important component of atmospheric nitrogen cycling and fine particulate matter (PM2.5) formation, yet they remain poorly constrained in both observations and models. While agricultural sources have historically dominated NH3 budgets, emerging evidence suggests that mobile sources and urban activities may play a larger role than currently represented in emission inventories. This dissertation quantifies the distribution, partitioning, and sources of reduced nitrogen (NHx = NH3 + NH4+) in major U.S. cities using aircraft observations from the 2023 Atmospheric Emissions and Reactions Observed from Megacities to Marine Areas (AEROMMA) campaign, and evaluates the ability of chemical transport models and satellite products to represent urban NH3.

In Chapter 3, we explore the relationships between NH3 and other urban pollutants using measurements collected from the NASA DC-8 aircraft during the Atmospheric Emissions and Reactions Observed from Megacities to Marine Areas (AEROMMA) field campaign in summer 2023. We report on NH3 abundance, NHx phase partitioning, enhancement ratios of NH3 to carbon monoxide (CO), ethyne (C2H3), and nitrogen oxides (NOx/y), and comparisons of enhancement ratios to previous studies. NH3 abundance and NHx phase partitioning varied based on city. Between Los Angeles (LA), Chicago, and New York City (NYC), LA had the highest average NH3 concentration (4.3 ± 4.0 ppb) whereas NYC had the lowest average NH3 concentration (0.8 ± 0.4 ppb). ΔNH3:ΔCO ratios ranged from 0.014 ± 0.002 ppb ppb−1 in NYC to 0.037 ± 0.004 in LA. The strong correlations with CO, C2H2, and NOx/y suggest that vehicles are the dominant source of NH3 during the summer months in these locations. The ΔNH3:ΔCO ratio over NYC observed during the AEROMMA campaign is consistent with National Emissions Inventory (NEI) emissions for on-road vehicles, whereas the ΔNH3:ΔCO ratios observed over LA and Chicago during AEROMMA are significantly higher than expected from the NEI on-road emissions.

In Chapter 4, we evaluate GEOS-Chem simulations against aircraft observations to assess biases in NH3, NH4+, and NHx. The model systematically underestimates urban NH3 enhancements, particularly in traffic-influenced plumes, suggesting that mobile-source emissions are underestimated in current inventories. Biases in nitric acid (HNO3) further propagate into errors in simulated ammonium nitrate formation (NH4NO3), indicating that improvements in both emissions and chemical mechanisms are needed. We also examine wildfire-influenced air masses and find that transported smoke exhibits higher observed NH4+ than simulated, pointing to incomplete representation of biomass burning emissions and aging pathways.

In Chapter 5, we evaluate the effects of the record-breaking 2023 Canadian wildfire season on NH3 concentration and NH4+ deposition across the Upper Midwest. This study integrates satellite observations, ground-based data, and in situ aircraft measurements. In May - June 2023, NH3 concentrations increased at 83% of ground sites, and NH₄⁺ deposition flux rose at 100% of ground sites in the Upper Midwest. Satellite data showed significantly higher column-averaged NH3 in 47% of grid cells in the Upper Midwest. On 1 August, a smoke plume over the Midwest corresponded with an AEROMMA flight observing enhanced NH3, NH4+, carbon monoxide, and acetonitrile. These findings highlight the substantial impact of wildfire smoke on NH3 and NH4+ at regional scales, with implications for nitrogen cycling, air quality, and atmospheric modeling.