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Abstract In this study, a high-resolution simulation of fossil fuel CO 2 in Beijing and surrounding areas, China, during January 2014 is performed to investigate the characteristics and sources of Beijing fossil fuel CO 2 (FFCO 2) mixing ratios and the impact of regional transport. The model reasonably reproduces the observed meteorological fields in the study domain, including temperature, relative humidity, wind speed, and wind direction. The simulated CO 2 and CO mixing ratios in Beijing are in good agreement with the measurements. Elevated FFCO 2 levels are produced by the model in megacities, such as Beijing, Tianjin, Shijiazhuang, and Baoding. The model result shows that FFCO 2 mixing ratios are significantly correlated with observed total CO 2 , CO, and PM 2.5 concentrations in Beijing. Sensitivity experiments show that Beijing FFCO 2 is mainly from industry and residential emissions (35% for each), followed by power plant (21%) and transportation (9%) emissions in January 2014. Spatially, the largest contributor for Beijing FFCO 2 is the local source, and regional transport also plays an important role in winter. The impact of regional transport is associated with wind direction, in which south/east (north/west) wind tends to accumulate (dilute) Beijing FFCO 2. In addition, the roles of regional transport during haze and clean episodes are significantly distinct in winter, and more contribution is found in haze episodes. Highlights • Beijing FFCO 2 are significantly correlated with observed CO 2 , CO, and PM 2.5. • Beijing FFCO 2 is mainly from industry and residential emissions (35% for each). • The south/east (north/west) wind tends to accumulate (dilute) Beijing FFCO 2. • Distinct roles of regional transport for haze and clean episodes. [ABSTRACT FROM AUTHOR]

• Beijing haze is strongly related to enhanced atmospheric oxidizing capacity. • Two regimes separated by [O 3 ] of 80 μg m−3: AOC-sufficient and -deficient regimes. • Increasing oxidation capacity considerably weakens the emission mitigation effort. Although strict mitigation measures have been implemented since 2013 in Beijing-Tianjin-Hebei (BTH), China, air pollution still frequently occurs. Observations reveal that during pollution episodes in autumn, fine particulate matter (PM 2.5) concentrations have not decreased, and particularly, ozone (O 3) concentrations have increased remarkably from 2013 to 2015 in Beijing. Additionally, a concurrence of O 3 and particulate pollution with high secondary aerosol contributions has been observed frequently, indicating high atmospheric oxidizing capacity (AOC) during particulate pollution. The WRF-Chem model simulations show elevated O 3 concentrations and high fractions of oxygenated secondary aerosols (OSA) in PM 2.5 (0.53–0.73) during the severe pollution period. During daytime there exhibits an AOC-sufficient regime with the persistently high OSA fraction and an AOC-deficient regime with varied OSA fractions, separated by the O 3 level of 80 μg m−3. Our results suggest that increasing AOC can considerably weaken the emission mitigation effort by enhancing the secondary aerosol formation [ABSTRACT FROM AUTHOR]

Implementation of strict emission mitigation measures since 2013 has significantly changed air pollutants in the Beijing-Tianjin-Hebei region (BTH), China. Observations show that ozone (O 3) concentrations have increased by 62.40% (27.84%) and SO 2 concentrations have decreased by 56.42% (35.07%) during particulate pollution episodes in Beijing (BTH) in the autumn from 2013 to 2015. The measured nitrate concentration in Beijing has increased markedly, which to a large degree offsets the sulfate decrease caused by SO 2 emission mitigation. Using the WRF-Chem model, we demonstrate that the enhanced nitrate formation is primarily attributed to increasing atmospheric oxidizing capacity (AOC) and decreasing sulfate competition for base ions. A 9.41–46.24% (7.58–40.97%) decrease in OH radical (O 3) concentrations in October 2015 reduces nitrate and fine particulate matters (PM 2.5) concentrations by 2.51–18.18% and 3.15–18.90% in Beijing, respectively. Based on the scenario in October 2015, if the SO 2 emission increases by 20.00–100.00%, the PM 2.5 concentration increases by 3.02–11.21%, but the nitrate level decreases by 2.48–21.87% simultaneously. Our results suggest that the nitrate aerosol has become a dominant contributor to particulate pollution in Beijing and that decreasing AOC is critical to mitigate nitrate and PM 2.5 concentrations. ● Increasing nitrate concentration largely offsets the sulfate decrease since 2013. ● Increasing nitrate is mainly due to enhanced AOC and weakened sulfate competition. ● Decreasing AOC is beneficial to mitigate nitrate and PM 2.5 concentrations. [ABSTRACT FROM AUTHOR]

Beijing, the capital of China, is a mega city with a population of > 20 million. In recent years, the city has experienced heavy air pollution, with particulate matter (PM) being one of its top pollutants. In the last decade, extensive efforts have been made to characterize the sources, properties, and processes of PM in Beijing. Despite progress made by previous studies, there are still some important questions to be answered and addressed. The focus of this research is to study the impact of the heterogeneous hydrolysis of N 2 O 5 on the formation of nitrate (NO 3 − ) and ammonium (NH 4 + ) in Beijing. The results show that during heavy pollution days (e.g., during 14–17 September 2015, with PM 2.5 concentration over 100 μg/m 3 ), the concentrations of NO 2 and O 3 were high, with maxima of 90 and 240 μg/m 3 , respectively, providing high precursors for the formation of N 2 O 5 . In addition, the aerosol and sulfate concentrations were also high, with maxima of 201 μg/m 3 and 23 μg/m 3 respectively, providing reacting surface for the heterogeneous reaction. As a result, the hydrolysis of N 2 O 5 led to 21.0% enhancement of nitrate (NO 3 − ) and 7.5% enhancement of ammonium (NH 4 + ). It is worth to note that this important effect only occurred in high pollution days (PM 2.5 concentration over 100 μg/m 3 ). During low-pollution periods (PM 2.5 concentration < 100 μg/m 3 ), the effect of hydrolysis of N 2 O 5 on the formation of nitrate and ammonium was insignificant (variation rate < 5%). This study suggests that during heavy pollution periods, the hydrolysis of N 2 O 5 enhances the level of aerosol pollution in Beijing, and needs to be further studied in order to perform efficient air pollution control and mitigation strategies. [ABSTRACT FROM AUTHOR]