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

Abstract. Ammonia (NH₃) is a key precursor to fine particulate matter (PM_2.5) and a primary form of reactive nitrogen. The limited observations of NH₃ hinders further understanding of its impacts on air quality, climate, and biodiversity. Currently, NH₃ ground monitoring networks are limited in number across 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 networks 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), stressing the rising importance of NH₃ in terms of nitrogen deposition. NH₃ trends for AMoN sites correlates with IASI NH₃ trend IASI and AMoN NH₃ trend (r = 0.66) and show a similar spatial pattern, with the highest increases in the Midwest and eastern U.S., and NH₃ trend for AMoN sites correlates with IASI NH₃ trend (r = 0.66). In spring and summer, increases of NH₃ were larger than 10 % · yr⁻¹ in the eastern U.S. and Midwest (cropland dominated) and western U.S. (pastureland dominated), respectively. In terms of trend in NH₃ hotpots (defined as regions where the IASI NH₃ column is larger than the 95th percentile of 11-year CONUS map, 6.7 × 1015 molec/cm²), these largest emissions sources are also experiencing increasing concentrations over time with the median of NH₃ trend is 4.7 % · yr⁻¹. 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. The increasing NH₃ could have detrimental effects on nearby eco-sensitive regions through nitrogen deposition and on aerosol chemistry in the densely populated urban areas, hence needs immediate attention.