Examination of Atmospheric Gas-Particle Partitioning of Ammonium Nitrate over Major North American Cities

Fine particulate matter (PM_2.5) is an air pollutant composed of inhalable solid or liquid particles less than 2.5 microns in diameter. Long-term exposure to elevated concentrations of these particles increases the risk of cardiovascular and pulmonary disease in humans. They can be emitted directly or formed by atmospheric reactions of precursor emissions. Ammonium nitrate (NH₄NO₃) is a major contributor to PM_2.5 mass and forms in the atmosphere by the gas-to-particle partitioning of ammonia (NH₃) and nitric acid (HNO₃) gases. Understanding this process is important for being able to predict when and how much ammonium nitrate forms in the atmosphere and how to develop effective pollution control strategies. Here we examined a large dataset obtained from the NASA DC-8 aircraft during the NOAA 2023 Atmospheric Emissions and Reactions Observed from Megacities to Marine Areas (AEROMMA) field campaign for examples of where this gas-to-particle partitioning occurred using measurements of particulate ammonium (NH₄⁺) and nitrate (NO₃^-) and gas phase NH₃ and HNO₃. A primary focus of the field campaign was to measure atmospheric aerosol and gas phase species over the large North American cities of Chicago, Toronto, New York City, and Los Angeles. Here we found 20 out of 51 profiles from six flights demonstrated an increasing fraction of nitrate in the particulate phase with altitude. Out of these 20 profiles, four had a negative 1:1 correlation in the amounts of both gas phase species with the corresponding aerosol phase species. Ten profiles had a negative 1:1 correlation for either nitrate/HNO₃ or ammonium/NH₃, and six profiles did not have this negative 1:1 correlation for either species. Profiles where there was a negative 1:1 correlation for both species occurred over well-mixed atmospheres, where total (gas+particle) nitrate, total (gas+particle) ammonium, potential temperature, water vapor, and carbon monoxide did not vary with altitude. Profiles where only one species had this correlation in a well-mixed atmosphere could indicate the formation of something other than ammonium nitrate. Finally, comparisons of the measured gas phase concentrations with modeled thermodynamic values indicated that the air masses in the altitude profiles analyzed were generally in equilibrium for ammonium nitrate. This study demonstrated that gas-to-particle partitioning occurred in the atmosphere over several locations in North America and that higher ammonium nitrate mass concentrations were present above the surface due to the lower temperatures there.