Bridging the spatial gaps of the Ammonia Monitoring Network using satellite ammonia measurements

Ammonia (NH₃) is a key precursor to fine particulate matter (PM_2.5) and a primary form of reactive nitrogen. The limited number of NH₃ observations hinders further understanding of its impacts on air quality, climate, and biodiversity. Currently, NH₃ ground monitoring networks are few and sparse across most of the globe, and even in the most established networks, large spatial gaps exist between sites and only a few sites have records that span longer than a decade. Satellite NH₃ observations can be used to discern trends and fill spatial gaps in networks, but many factors influence the syntheses of the vastly different spatiotemporal scales between surface network and satellite measurements. To this end, we intercompared surface NH₃ data from the Ammonia Monitoring Network (AMoN) and satellite NH₃ total columns from the Infrared Atmospheric Sounding Interferometer (IASI) in the contiguous United States (CONUS) and then performed trend analyses using both datasets. We explored the sensitivity of correlations between the two datasets to factors such as satellite data availability and distribution over the surface measurement period as well as agreement within selected spatial and temporal windows. Given the short lifetime of atmospheric ammonia and consequently sharp gradients, smaller spatial windows show better agreement than larger ones except in areas of relatively uniform, low concentrations where large windows and more satellite measurements improve the signal-to-noise ratio. A critical factor in the comparison is having satellite measurements across most of the measurement period of the monitoring site. When IASI data are available for at least 80% days of AMoN's 2-week sampling period within a 25 km spatial window of a given site, IASI NH₃ column concentrations and the AMoN NH₃ surface concentrations have a correlation of 0.74, demonstrating the feasibility of using satellite NH₃ columns to bridge the spatial gaps existing in the surface network NH₃ concentrations. Both IASI and AMoN show increasing NH₃ concentrations across CONUS (median: 6.8%·yr^-1 vs. 6.7%·yr^-1) in the last decade (2008 - 2018), suggesting theNH₃ will become a greater contributor to nitrogen deposition. NH₃ trends at AMoN sites are correlated with IASI NH₃ trends (r = 0.66), and show similar spatial patterns, with the highest increases in the Midwest and eastern U.S. In spring and summer, increases of NH₃ were larger than 10%·yr^-1 in the eastern U.S. and Midwest (cropland dominated) and the western U.S. (pastureland dominated), respectively. NH₃ hotpots (defined as regions where the IASI NH₃ column is larger than the 95^th percentile of 11-year CONUS map, 6.7 × 10¹⁵ molec/cm²), also experiencing increasing concentrations over time, with a median of NH₃ trend of 4.7% · yr¹. IASI data show large NH₃ increases in urban areas (8.1%·yr^-1), including 8 of the top 10 most populous regions in the CONUS, where AMoN sites are sparse. A comparison between IASI NH₃ concentration trends and state-level NH₃ emission trends is then performed to reveal that positive correlations exist in states with strong agricultural NH₃ emissions while negative correlations in states with low NH₃ emissions and large NO_x emissions, suggesting the different roles of emission and partitioning in NH₃ increases. The increases in NH₃ could have detrimental effects on nearby eco-sensitive regions through nitrogen deposition and on aerosol chemistry in the densely populated urban areas, and therefore should be carefully monitored and studied.