Aerosol acidity plays an important role in atmospheric chemistry. China emits large amounts of SO₂, NO_x, and NH₃ into the atmosphere, but aerosol acidity is poorly characterized. In this study, simultaneous 1 h measurements of particulate and gaseous compositions along with the ISORROPIA‐II thermodynamic equilibrium model were used to study aerosol acidity during severe haze episodes in northern China. The summed concentration of sulfate, nitrate, and ammonium was 135 ± 51 μg/m³ with a maximum of 250 μg/m³, and the gas‐phase NH₃ mixing ratio was 22 ± 9 ppb. Fine particles were moderately acidic, with a pH range of 3.0–4.9 and an average of 4.2, which was higher than those in the United States and Europe. Excess NH₃ and high aerosol water content are responsible for the relatively lower aerosol acidity. These results suggest that the new pathways for sulfate production in China proposed by recent studies should be revisited.

The North China Plain has been identified as a significant hotspot of ammonia (NH₃) due to extensive agricultural activities. Satellite observations suggest a significant increase of about 30 % in tropospheric gas-phase NH₃ concentrations in this area during 2008–2016. However, the estimated NH₃ emissions decreased slightly by 7 % because of changes in Chinese agricultural practices, i.e., the transition in fertilizer types from ammonium carbonate fertilizer to urea, and in the livestock rearing system from free-range to intensive farming. We note that the emissions of sulfur dioxide (SO₂) have rapidly declined by about 60 % over the recent few years. By integrating measurements from ground and satellite, a long-term anthropogenic NH₃ emission inventory, and chemical transport model simulations, we find that this large SO₂ emission reduction is responsible for the NH₃ increase over the North China Plain. The simulations for the period 2008–2016 demonstrate that the annual average sulfate concentrations decreased by about 50 %, which significantly weakens the formation of ammonium sulfate and increases the average proportions of gas-phase NH₃ within the total NH₃ column concentrations from 26 % (2008) to 37 % (2016). By fixing SO₂ emissions of 2008 in those multi-year simulations, the increasing trend of the tropospheric NH₃ concentrations is not observed. Both the decreases in sulfate and increases in NH₃ concentrations show highest values in summer, possibly because the formation of sulfate aerosols is more sensitive to SO₂ emission reductions in summer than in other seasons. Besides, the changes in NO_x emissions and meteorological conditions both decreased the NH₃ column concentrations by about 3 % in the study period. Our simulations suggest that the moderate reduction in NO_x emissions (16 %) favors the formation of particulate nitrate by elevating ozone concentrations in the lower troposphere.

Biomass burning plays a significant role in air pollution and climate change. In this study, we used a method based on fire radiative energy (FRE) to develop a biomass burning emission inventory for China from 2003 to 2017. Daily fire radiative power (FRP) data derived from 1 km MODIS Thermal Anomalies/Fire products (MOD14/MYD14) were used to calculate FRE and combusted biomass. Available emission factors were assigned to four biomass burning types: forest, cropland, grassland, and shrubland fires. The farming system and crop types in different temperate zones were taken into account in this research. Compared with traditional methods, the FRE method was found to provide a more reasonable estimate of emissions from small fires. The estimated average annual emission ranges, with a 90 % confidence interval, were 91.4 (72.7–108.8) Tg CO₂ yr⁻¹, 5.0 (2.3–7.8) Tg CO yr⁻¹, 0.24 (0.05–0.48) Tg CH₄ yr⁻¹, 1.43 (0.53–2.35) Tg NMHC yr⁻¹, 0.23 (0.05–0.45) Tg NO_x yr⁻¹, 0.09 (0.02–0.17) Tg NH₃ yr⁻¹, 0.03 (0.01–0.05) Tg SO₂ yr⁻¹, 0.04 (0.01–0.08) Tg BC yr⁻¹, 0.27 (0.07–0.49) Tg OC yr⁻¹, 0.51 (0.19–0.84) Tg PM_2.5 yr⁻¹, 0.57 (0.15–1.05) Tg PM₁₀ yr⁻¹, where NMHC, BC, and OC are nonmethane hydrocarbons, black carbon, and organic carbon, respectively. Forest fires are determined to be the primary contributor to open fire emissions, accounting for 45 % of the total CO₂ emissions (average 40.8 Tg yr⁻¹). Crop residue burning ranked second place with a large portion of 39 % (average 35.3 Tg yr⁻¹). During the study period, emissions from forest and grassland fires showed a significant downward trend. Crop residue emissions continued to rise during 2003–2015 but dropped by 42 % in 2015–2016. Emissions from shrubland were negligible and little changed. Forest and grassland fires are concentrated in northeastern China and southern China, especially in the dry season (from October to March of the following year). Plain areas with high crop yields, such as the North China Plain, experienced high agricultural fire emissions in harvest seasons. Most shrubland fires were located in Yunnan and Guangdong provinces. The resolution of our inventory (daily, 1 km) is much higher than previous inventories, such as GFED4s and GFASv1.0. It could be used in global and regional air quality modeling.

Terrestrial ecosystems in China receive the world’s largest amount of reactive nitrogen (N) deposition. Recent controls on nitrogen oxides (NO_x = NO + NO₂) emissions in China to tackle air pollution are expected to decrease N deposition, yet the observed N deposition fluxes remain almost stagnant. Here we show that the effectiveness of NO_xemission controls for reducing oxidized N (NO_y = NO_x + its oxidation products) deposition is unforeseen in Eastern China, with one-unit reduction in NO_xemission leading to only 55‒76% reductions in NO_y-N deposition, as opposed to the high effectiveness (around 100%) in both Southern China and the United States. Using an atmospheric chemical transport model, we demonstrate that this unexpected weakened response of N deposition is attributable to the enhanced atmospheric oxidizing capacity by NO_xemissions reductions. The decline in N deposition could bear a penalty on terrestrial carbon sinks and should be taken into account when developing pathways for China’s carbon neutrality.

The atmospheric deposition of soluble (bioaccessible) iron enhances ocean primary productivity and subsequent atmospheric CO₂sequestration in iron-limited ocean basins, especially the Southern Ocean. While anthropogenic sources have been recently suggested to be important in some northern hemisphere oceans, the role in the Southern Ocean remains ambiguous. By comparing multiple model simulations with the new aircraft observations for anthropogenic iron, we show that anthropogenic soluble iron deposition flux to the Southern Ocean could be underestimated by more than a factor of ten in previous modeling estimates. Our improved estimate for the anthropogenic iron budget enhances its contribution on the soluble iron deposition in the Southern Ocean from about 10% to 60%, implying a dominant role of anthropogenic sources. We predict that anthropogenic soluble iron deposition in the Southern Ocean is reduced substantially (30‒90%) by the year 2100, and plays a major role in the future evolution of atmospheric soluble iron inputs to the Southern Ocean.

China is one of the largest agricultural countries in the world. Thus, NH₃ emission from agricultural activities in China considerably affects the country's regional air quality and visibility. In this study, a high-resolution agricultural NH₃ emission inventory compiled on 1 km × 1 km horizontal resolution was applied to calculate the NH₃ mass burden in China and reliably estimate the influence of NH₃ on agriculture. The key parameter emission factors of this inventory were enhanced by considering many experiment results, and the dynamic data of spatial and temporal information were updated using statistical data of 2015. In addition to fertilizers and husbandry, farmland ecosystems, livestock waste, crop residue burning, wood-based fuel combustion, and other NH₃ emission sources were included in this inventory. Furthermore, a source apportionment tool, namely, the Integrated Source Apportionment Method (ISAM) coupled with the air quality modeling system Regional Atmospheric Modeling System and Community Multiscale Air Quality, was applied to capture the contribution of NH₃ emitted from total agriculture (T_agr) in China. The aerosol mass concentration in 2015 was simulated, and results showed that the high mass concentration of NH₃ exceeded 10 µg m⁻³ and mainly appeared in the North China Plain, Central China, the Yangtze River Delta, and the Sichuan Basin. Moreover, the annual average contribution of T_agr NH₃ to PM_2.5 mass burden was 14 %–22 % in China. Specific to the PM_2.5 components, T_agr NH₃ contributed dominantly to ammonium formation (87.6 %) but trivially to sulfate formation (2.2 %). In addition, several brute-force sensitivity tests were conducted to estimate the impact of T_agr NH₃ emission reduction on PM_2.5 mass burden. In contrast to the result of ISAM, even though the T_agr NH₃ only provided 10.1 % contribution to nitrate under the current emission scenario, the reduction of nitrate could reach 95.8 % upon removal of the T_agr NH₃ emission. This deviation occurred because the contribution of NH₃ to nitrate should be small under a “rich NH₃”environment and large under a “poor NH₃” environment. Thus, the influence of NH₃ on nitrate formation would be enhanced with the decrease in ambient NH₃ mass concentration.