Abstract. Limited availability of ammonia (NH 3 ) observations is currently a barrier for effective monitoring of the nitrogen cycle. It prevents a full understanding of the atmospheric processes in which this trace gas is involved and therefore impedes determining its related budgets. Since the end of 2007, the Infrared Atmospheric Sounding Interferometer (IASI) satellite has been observing NH 3 from space at a high spatio-temporal resolution. This valuable data set, already used by models, still needs validation. We present here a first attempt to validate IASI-NH 3 measurements using existing independent ground-based and airborne data sets. The yearly distributions reveal similar patterns between ground-based and space-borne observations and highlight the scarcity of local NH 3 measurements as well as their spatial heterogeneity and lack of representativity. By comparison with monthly resolved data sets in Europe, China and Africa, we show that IASI-NH 3 observations are in fair agreement, but they are characterized by a smaller variation in concentrations. The use of hourly and airborne data sets to compare with IASI individual observations allows investigations of the impact of averaging as well as the representativity of independent observations for the satellite footprint. The importance of considering the latter and the added value of densely located airborne measurements at various altitudes to validate IASI-NH 3 columns are discussed. Perspectives and guidelines for future validation work on NH 3 satellite observations are presented.

Abstract. Limited availability of ammonia (NH3) observations is currently a barrier for effective monitoring of the nitrogen cycle. It prevents a full understanding of the atmospheric processes in which this trace gas is involved and therefore impedes determining its related budgets. Since the end of 2007, the Infrared Atmospheric Sounding Interferometer (IASI) satellite has been observing NH3 from space at a high spatiotemporal resolution. This valuable data set, already used by models, still needs validation. We present here a first attempt to validate IASI-NH3 measurements using existing independent ground-based and airborne data sets. The yearly distributions reveal similar patterns between ground-based and space-borne observations and highlight the scarcity of local NH3 measurements as well as their spatial heterogeneity and lack of representativity. By comparison with monthly resolved data sets in Europe, China and Africa, we show that IASI-NH3 observations are in fair agreement but that they are characterized by a smaller variation in concentrations. The use of hourly and airborne data sets to compare with IASI individual observations allows to investigate the impact of averaging as well as the representativity of independent observations for the satellite footprint. The importance of considering the latter and the added value of densely located airborne measurements at various altitudes to validate IASI-NH3 columns are discussed. Perspectives and guidelines for future validation work on NH3 satellite observations are presented.

Ammonia (NH 3 ) plays a critical role in atmospheric chemistry and can exacerbate haze formation. Agricultural emissions have been known as a primary source of global atmospheric NH 3 . However, with accelerating urbanization and optimized agricultural production, the dominance of agricultural emissions has become less clear. We investigated the contributions of nonagricultural and agricultural sources to atmospheric NH 3 based on measurements of NH 3 isotopes at nine sites in Quzhou County, a typical agricultural county in the North China Plain. We found that Quzhou had extremely high NH 3 concentrations (annual average across all sites of 40.3 ± 3.3 μg m −3 ). We compared the sources of seasonal NH 3 contributions in rural and urban areas through 15 N-stable isotope analyses, which provides new insights into NH 3 sources compared with the traditional emission inventories. In rural areas, agricultural sources (fertilizer application and livestock production) make significant contributions (56 ± 3%) to NH 3 emissions in the winter, whereas there were larger contributions of nonagricultural sources [fossil fuel, waste, and biomass burning (56 ± 2%)] relative to agricultural sources in urban areas. More effective strategies are still needed for better manure management and vegetable/fruit production in the winter and for controlling nonagricultural sources, even in counties dominated by agriculture.

<p><strong>Abstract.</strong> China is experiencing intense air pollution caused in large part by anthropogenic emissions of reactive nitrogen (Nr). Atmospheric ammonia (NH<sub>3</sub>) and nitrogen dioxide (NO<sub>2</sub>) are the most important precursors for Nr compounds (including N<sub>2</sub>O<sub>5</sub>, HNO<sub>3</sub>, HONO and particulate NO<sub>3</sub><sup>−</sup> and NH<sub>4</sub><sup>+</sup>) in the atmosphere. Understanding the changes of NH<sub>3</sub> and NO<sub>2</sub> has important implications for the regulation of anthropogenic Nr emissions, and is a requirement for assessing the consequence of environmental impacts. We conducted the temporal trend analysis of atmospheric NH<sub>3</sub> and NO<sub>2</sub> on a national scale since 1980 based on emission data (during 1980–2010), satellite observations (for NH<sub>3</sub> since 2008 and for NO<sub>2</sub> since 2005) and atmospheric chemistry transport modeling (during 2008–2015). <br><br> Based on the emission data, during 1980–2010, both significant continuous increasing trend of NH<sub>3</sub> and NO<sub>x</sub> were observed from REAS (Regional Emission inventory in Asia, for NH<sub>3</sub> 0.17 kg N ha<sup>−1</sup> y<sup>−2</sup> and for NO<sub>x</sub> 0.16 kg N ha<sup>−1</sup> y<sup>−2</sup>) and EDGAR (Emissions Database for Global Atmospheric Research, for NH<sub>3</sub> 0.24 kg N ha<sup>−1</sup> y<sup>−2</sup> and for NO<sub>x</sub> 0.17 kg N ha<sup>−1</sup> y<sup>−2</sup>) over China. Based on the satellite data and atmospheric chemistry transport modeling named as the Model for Ozone and Related chemical Tracers, version 4 (MOZART-4), the NO<sub>2</sub> columns over China increased significantly (<i>p</i><0.01) from 2005 to 2011 and then decreased significantly from 2011 to 2015; the satellite-retrieved NH<sub>3</sub> columns from 2008 to 2014 had no big changes but increased in 2015 (large increase from satellite IASI, but slight increase from MOZART-4). The decrease in NO<sub>2</sub> columns since 2011 may result from more stringent strategies taken to control NO<sub>x</sub> emissions during the 12th Five-Year-Plan, while no control policy focused on NH<sub>3</sub> emissions. Our findings provided an overall insight on the temporal trends of both NO<sub>2</sub> and NH<sub>3</sub> since 1980 based on emission data, satellite observations and atmospheric transport modeling. These findings can provide a scientific background for policy-makers that are attempting to control atmospheric pollution in China. Moreover, the multivariate data used in this study have implications for estimating long-term Nr deposition datasets to assess its impact on soil, forest, water and greenhouse balance.</p>

<p><strong>Abstract.</strong> Ammonia (NH<sub>3</sub>), as an alkaline gas in the atmosphere, can cause direct or indirect effects on the air quality, soil acidification, climate change as well as human health. Estimating surface NH<sub>3</sub> concentrations is critically important for modelling the dry deposition of NH<sub>3</sub>, which has important impacts on the natural environment. However, sparse monitoring sites make it challenging and difficult to understand the global distribution of surface NH<sub>3</sub> concentrations both in time and space. We estimated the global surface NH<sub>3</sub> concentrations for the years of 2008–2016 using the satellite NH<sub>3</sub> observations combining its vertical profiles from the GEOS-Chem. The accuracy assessment indicates that the satellite-based approach has achieved a high predictive power for annual surface NH<sub>3</sub> concentrations (R<sup>2</sup> = 0.76 and RMSE = 1.50 μg N m<sup>-3</sup>). The satellite-derived surface NH<sub>3</sub> concentrations had higher consistency with the ground-based measurements in China (R<sup>2</sup> = 0.71 and RMSE = 2.6 μg N m<sup>-3</sup>) than the US (R<sup>2</sup> = 0.45 and RMSE = 0.76 μg N m<sup>-3</sup>) and Europe (R<sup>2</sup> = 0.45 and RMSE = 0.86 μg N m<sup>-3</sup>) at a yearly scale. Annual surface NH<sub>3</sub> concentrations higher than 6 μg N m<sup>-3</sup> are mainly concentrated in the North China Plain of China and Northern India, followed by 2–6 μg N m<sup>-3</sup> mainly in southern and northeastern China, India, western Europe and eastern United States (US). High surface NH<sub>3</sub> concentrations were found in the croplands in China, US and Europe, and surface NH<sub>3</sub> concentrations in the croplands in China were approximately double than those in the croplands in the US and Europe. The liner trend analysis shows that a significant increase rate of surface NH<sub>3</sub> concentrations (> 0.2 μg N m<sup>-3</sup> y<sup>-1</sup>) appeared in the eastern China during 2008–2016, and a middle increase rate (0.1–0.2 μg N m<sup>-3</sup> y<sup>-1</sup>) occurred in northern Xinjiang over China. NH<sub>3</sub> increase was also found in agricultural regions in middle and eastern US with an annual increase rate of lower than 0.10 μg N m<sup>-3</sup> y<sup>-1</sup>. The satellite-derived surface NH<sub>3</sub> concentrations help us to determine the NH<sub>3</sub> pollution status in the areas without monitoring sites and to estimate the dry deposition of NH<sub>3</sub> in the future.</p>