Atmospheric Ammonia Emissions from Nitrogen Fertilizer Application: Impacts on Air Quality at Daily Scales

Abstract:

Ammonia (NH₃) is a key precursor to fine particulate matter (PM_2.5). Agricultural activities are the major sources of NH₃ emissions and have adverse impacts on human health due to the formation of ammonium aerosols. Ammonia has a short lifetime (~day), but the NH₃ ground monitoring network in the US only provides biweekly data, hindering the ability to capture daily scale NH₃ pollution episodes. Here we used daily satellite NH₃ observations from Infrared Atmospheric Sounding Interferometer (IASI), daily in-situ PM_2.5 observations from the US Environmental Protection Agency (US EPA) AirNow network, and land use data from US Department of Agriculture (USDA) to study the spatial distribution and magnitude of simultaneous NH₃ and PM_2.5 pollution episodes during the planting seasons, particularly around fertilization application. Increased NH₃ concentrations were observed over the Midwest, with the daily column abundances higher than 5×10¹⁶ molec/cm² in some regions on selected days, which is 7 times higher than the 95^th percentile of the annual averaged NH₃ column abundance in the contiguous US (0.66×10¹⁶ molec/cm²). Increased NH₃ columns are observed first in the southern US in early April and then progress to the northern US in early May. The temporal and spatial shifting of NH₃ blooms is consistent with the evolution of planting dates from the south to the north. Satellite NH₃ observations are capable of capturing the short-term high NH₃ emissions after fertilizer application. Simultaneous enhancement of both NH₃ and PM_2.5 concentrations were observed in the cropland dominated regions, suggesting that NH₃ emissions contribute to PM_2.5 formation in these regions. During these coincident episodes, PM_2.5 concentrations at times increased by more than 200% within 24 hours, leading to possible short-term exposures to high PM_2.5 during the planting season. Compared to other anthropogenic emissions, such as SO₂ and NO_x, NH₃ emissions are less well constrained, particularly episodic events such as around times of fertilization application. Our results highlight the potential health impact of agricultural NH₃ emissions and show that synchronizing multiple datasets, including satellite and ground monitoring networks, can provide critical insights into the role of agricultural NH₃ emissions in PM_2.5 formation.