Abstract. Accurate ammonia (NH3) emission inventories are critical for PM2.5 mitigation, yet bottom-up estimates remain uncertain, particularly for sector-resolved estimates in humid subtropical regions such as Guangdong, where urban-industrial emissions in the Pearl River Delta (PRD) coexist with dispersed agricultural sources in surrounding non-PRD areas. Here we integrate a ground-based NH3 network, Fengyun-4B (FY-4B) geostationary NH3 retrievals, a localized 3 km × 3 km prior inventory, and stretched-grid GEOS-Chem High Performance (GCHP) simulations at 0.2° × 0.2° resolution to inversely constrain monthly agricultural and non-agricultural NH3 emissions in the PRD and non-PRD Guangdong in 2023. The posterior simulation improved agreement with observations, reducing NRMSE from 55.3 % to 48.4 % and changing NMB from −9.0 % to 4.6 %, with further support from independent NH3, NH4+, and deposition measurements. Provincial anthropogenic NH3 emissions decreased from 477.6 to 441.5 kt yr−1: agricultural emissions declined from 437.2 to 362.5 kt yr−1, mainly through warm-season reductions in non-PRD areas, whereas non-agricultural emissions increased from 40.4 to 79.0 kt yr−1, especially during the cool season. Hypothetically removing agricultural (non-agricultural) NH3 emissions in Guangdong provided provincial PM2.5 reductions of 3.7 ± 1.5 (0.7 ± 0.4) μg m−3 and avoided premature deaths of 3251 (1064), highlighting the need to combine dispersed agricultural source controls and targeted non-agricultural source controls over populated PRD.

Purpose of Review As the most abundant alkaline trace gas in the atmosphere, NH 3 plays a critical role in the formation of atmospheric particulate matter and nitrogen cycling in ecosystems. NH 3 emissions have been increasing globally over the past few decades. To provide a clearer understanding of atmospheric NH 3 , this paper presents a systematic review of the literature on the sources and variability of atmospheric NH 3 and describes the contribution of atmospheric NH 3 to PM 2.5 . Recent Findings (1) The primary source of atmospheric NH 3 emissions is agriculture; other sources include combustion-related emissions and volatilization from soil and oceans. However, recent studies have revealed the major role of nonagricultural sources in urban areas. (2) The spatiotemporal variability of atmospheric NH 3 is complex, and its mechanisms are not entirely clear. (3) Atmospheric NH 3 can participate in multiple atmospheric chemical processes and to the formation of fine particulate matter. Summary This review summarizes the latest knowledge on the sources and variability of atmospheric NH 3 and highlights the necessity of controlling atmospheric NH 3 emissions. However, significant knowledge gaps still exist in understanding the sources, trends, and effects of atmospheric NH 3 . Therefore, further research is essential to investigate the influencing factors and environmental effects of atmospheric NH 3 concentrations, providing a scientific basis for the development of effective NH 3 control strategies.

Ammonia (NH3) has major effects on the environment and climate. In situ measurements of NH3 concentrations taken between June 2009 and July 2020 at an urban site in Beijing were analyzed to study its long-term behavior, responses to meteorological conditions, and influences on the formation of secondary inorganic aerosols (SIAs). The 11-year average NH3 mixing ratio was 26.9±19.3 ppb (median 23.5 ppb). The annual average NH3 mixing ratio increased from 2009 to 2017 by 50 % and then decreased by 49 % from 2017 to 2020. Notably, the long-term trend for NH3 at the ground level did not align with the trends derived from satellite observations and emission estimates. The NH3 concentration exhibited a stronger correlation with the daily variation in water vapor (H2O) concentration than with air temperature. Thermodynamic modeling revealed the nonlinear response of SIAs to NH3, with increased sensitivity when its concentration was reduced to 40 % of the initial level. Although reducing NH3 concentrations can improve air quality during winter, controlling acid gas concentrations has a greater effect than controlling NH3 concentrations on reducing SIA concentrations, until NH3 and acidic gas concentrations are reduced below 80 % of their current levels. Nevertheless, the increased mass proportion of ammonium salts in SIAs during the observation period indicates that future control measures for NH3 concentrations may need to be prioritized in Beijing.

Ammonia (NH3) plays an important role in particulate matter formation; hence, its atmospheric level is relevant to human health and climate change. Due to different relative distributions of NH3 sources, concentrations of atmospheric NH3 may behave differently in urban and rural areas. However, few parallel long-term observations of NH3 exist to reveal the different behaviors of NH3 concentrations at urban and rural sites in a same region. In this study, online ammonia analyzers were used to continuously observe atmospheric NH3 concentrations at an urban site and a suburban site in Beijing from 13 January 2018 to 13 January 2019. The observed mixing ratio of NH3 averaged 21±14 ppb (range of 1.6–133 ppb) at the urban site and 22±15 ppb (range of 0.8–199 ppb) at the suburban site. The NH3 mixing ratios at the urban and suburban sites exhibited similar seasonal variations, with high values in summer and spring and low values in autumn and winter. The hourly mean NH3 mixing ratios at the urban site were highly correlated (R=0.849, P<0.01) with those at the suburban site; however, the average diurnal variations in the NH3 mixing ratios at the urban and suburban sites differed significantly, which implies different contributions from NH3 sources and sinks at the urban and suburban sites. In addition to the emission sources, meteorological factors were closely related to the changes in the NH3 concentrations. For the same temperature (relative humidity) at the urban and suburban sites, the NH3 mixing ratios increased with relative humidity (temperature). Relative humidity was the factor with the strongest influence on the NH3 mixing ratio in different seasons at the two sites. The relationships between the NH3 concentrations and temperature (relative humidity) varied from season to season and showed differences between the urban and suburban sites. The reasons for the different relationships need to be investigated in future studies. Higher wind speed mainly from the northwest sector lowered the NH3 mixing ratios at both sites. Similarly to other primary pollutants in Beijing, the NH3 mixing ratios were high when impacted by air masses from the southern sector.

General comments: This manuscript reports a year long parallel measurements of NH3 concentration at urban of suburban sites of Beijing. Statistical analysis, seasonal variation, diel profile and the relationship with temperature, RH, H2O, wind speed and wind

Abstract. Ammonia (NH3) plays an important role in particulate matter formation; however, few long-term observations with a high temporal resolution have been conducted on the NH3 concentrations in Beijing. In this study, online ammonia analyzers were used to observe continuously the atmospheric NH3 concentrations at an urban site and a suburban site in Beijing from January 13, 2018, to January 13, 2019. The average mixing ratio of NH3 at the urban site was 21 ± 14 ppb (range: 1.6–133 ppb) and that at the suburban site was 22 ± 15 ppb (range: 0.8–199 ppb). The NH3 mixing ratios at the urban and suburban sites exhibited similar seasonal variations, with high values being observed in the summer and spring and low values being observed in the autumn and winter. The hourly mean NH3 mixing ratios at the urban site were highly correlated (R = 0.849, P

Abstract. Ammonia (NH3) plays an important role in particulate matter formation; however, few long-term observations with a high temporal resolution have been conducted on the NH3 concentrations in Beijing. In this study, online ammonia analyzers were used to observe continuously the atmospheric NH3 concentrations at an urban site and a suburban site in Beijing from January 13, 2018, to January 13, 2019. The average mixing ratio of NH3 at the urban site was 21 ± 14 ppb (range: 1.6–133 ppb) and that at the suburban site was 22 ± 15 ppb (range: 0.8–199 ppb). The NH3 mixing ratios at the urban and suburban sites exhibited similar seasonal variations, with high values being observed in the summer and spring and low values being observed in the autumn and winter. The hourly mean NH3 mixing ratios at the urban site were highly correlated (R = 0.849, P