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Identifying the sources of atmospheric Carbon dioxide (CO2) is an important prerequisite for developing effective mitigation strategies. Here we conducted regular observations of the atmospheric CO2 mixing ratio and its carbon isotope compositions (i.e., Δ14C and δ13C) in Xi'an and Beijing during winter, to estimate source contributions of CO2 emissions in Chinese megacities. The results showed that CO2 emissions in both Xi'an and Beijing originated mainly from fossil‐fuel sources, which contributed 65 ± 3% and 82 ± 2% of the total CO2 enhancement, respectively, during the sampling period; the results also revealed a substantial biogenic CO2 contribution during winter. We further separated the fossil‐fuel sources into contributions from coal, oil and natural gas combustions. We found that coal combustion was the dominant anthropogenic source in Xi'an, accounting for 54 ± 4% of the total fossil‐fuel emissions, and oil and natural gas contribute almost equally to the emissions. In contrast, emission from natural‐gas combustion was the main fossil‐fuel source in Beijing, accounting for more than half of the total fossil‐fuel emissions, whereas, coal combustion contributed only 17 ± 10%. These top‐down results are generally consistent with emission inventory when seasonal variations of emissions are considered; some differences between the two methods indicated that the inventory for Xi'an might be underestimating the emissions from oil consumption. This pilot study confirms the potential of direct verification between top‐down and bottom‐up methods from the perspective of source attribution. Plain Language Summary: Quantifying Carbon dioxide (CO2) sources from cities is crucial for formulating policies and evaluating targets because cities have become the key region and basic unit for regulating emissions. Here, we take Xi'an and Beijing, two Chinese megacities, as the study case to conduct source attribution of atmospheric CO2 based on the powerful tracer ability of dual carbon isotopes (14C and 13C). We estimated the emission contributions of fossil source CO2 (CO2ff) and the fractions of different fossil fuels in CO2ff. We also performed independent evaluation for emission inventory data by the direct comparisons between the fractions of different fossil fuels obtained from "top‐down" and "bottom‐up" methods. This study can provide a new perspective for the verification of urban carbon emission data, and is of great significance to further improve the reliability of carbon budgets for Chinese cities. Key Points: The Carbon dioxide (CO2) emission patterns regarding the source contributions are completely different between Beijing and Xi'anBiogenic CO2 is also an important source for megacities and need to be considered in urban carbon budgetsWe confirm the potential of direct verification between top‐down and bottom‐up methods from a new perspective [ABSTRACT FROM AUTHOR]

The atmospheric oxidizing capacity (AOC) plays a key role in atmospheric chemistry, determining the removal of most reduced gases in the atmosphere. In this study, we assess the summertime AOC from 7 to 22 June 2014 associated with a field campaign conducted at Wangdu in the Beijing‐Tianjin‐Hebei region (BTH), China using the WRF‐Chem model. The model reasonably well reproduces the incident solar radiation, ambient O3 and fine particulate matter (PM2.5) concentrations, the photolysis rates of O3 and NO2, and radical concentrations against measurements in the BTH. The model results show high daytime AOC levels in urban areas and relatively low AOC in remote areas of the BTH. The diurnal cycle of the AOC exhibits a unimodal pattern with peaks at noon. OH radical contributes about 68% of the daily average AOC in Beijing, followed by O3 (27%) and NO3 (5%). At Wangdu, the production rate of OH radical reaches around 13 ppb h−1 at noon, which is mainly resulted from the reaction of HO2 with NO. The OH radical loss is primarily contributed by reactions with volatile organic compounds, followed by CO and NO2. Statistically, the population hourly or afternoon average O3 and Ox (=O3 + NO2) concentrations are significantly correlated with the AOC in Beijing, constituting potential simplified tracers to indicate the AOC level. Key Points: The summertime atmospheric oxidizing capacity (AOC) over the Beijing‐Tianjin‐Hebei area is assessed combining a field campaign and WRF‐Chem simulationOH radical contributes about 68% of the daily average AOC in Beijing, followed by O3 (27%) and NO3 (5%)Population hourly or afternoon average O3 and Ox (=O3 + NO2) concentrations constitute potential simplified tracers for the AOC level [ABSTRACT FROM AUTHOR]

Atmospheric aerosols scatter or absorb a fraction of the incoming solar radiation to cool or warm the atmosphere, decreasing surface temperature and altering atmospheric stability to further affect the dispersion of air pollutants in the planetary boundary layer (PBL). In the present study, simulations during a persistent and heavy haze pollution episode from 5 December 2015 to 4 January 2016 in the North China Plain (NCP) were performed using the Weather Research and Forecasting model with Chemistry (WRF-Chem) to comprehensively quantify contributions of aerosol shortwave radiative feedback (ARF) to near-surface (around 15 m above the ground surface) PM 2.5 mass concentrations. The WRF-Chem model generally performs well in simulating the temporal variations and spatial distributions of air pollutants concentrations compared to observations at ambient monitoring sites in the NCP, and the simulated diurnal variations of aerosol species are also consistent with the measurements in Beijing. Additionally, the model simulates the aerosol radiative properties, the downward shortwave flux, and the PBL height against observations in the NCP well. During the episode, ARF deteriorates the haze pollution, increasing the near-surface PM 2.5 concentrations in the NCP by 10.2 µ g m -3 or with a contribution of 7.8 % on average. Sensitivity studies have revealed that high loadings of PM 2.5 attenuate the incoming solar radiation reaching the surface to cool the low-level atmosphere, suppressing the development of the PBL, decreasing the surface wind speed, further hindering the PM 2.5 dispersion, and consequently exacerbating the haze pollution in the NCP. Furthermore, when the near-surface PM 2.5 mass concentration increases from around 50 to several hundred µ g m -3 , ARF contributes to the near-surface PM 2.5 by more than 20 % during daytime in the NCP, substantially aggravating the heavy haze formation. However, when the near-surface PM 2.5 concentration is less than around 50 µ g m -3 , ARF generally reduces the near-surface PM 2.5 concentration due to the consequent perturbation of atmospheric dynamic fields. [ABSTRACT FROM AUTHOR]

Although stringent emission mitigation strategies have been carried out since 2013 in Beijing–Tianjin–Hebei (BTH), China, heavy haze with high levels of fine particulate matter (PM 2.5) still frequently engulfs the region during wintertime and the nitrate contribution to PM 2.5 mass has progressively increased. N2O5 heterogeneous hydrolysis is the most important pathway of nitrate formation at nighttime. In the present study, the WRF-Chem model is applied to simulate a heavy haze episode from 10 to 27 February 2014 in BTH to evaluate contributions of N2O5 heterogeneous hydrolysis to nitrate formation and effects of organic coating. The model generally performs reasonably well in simulating meteorological parameters, air pollutants, and aerosol species against observations in BTH. N2O5 heterogeneous hydrolysis with all the secondary organic aerosol assumed to be involved in coating considerably improves the nitrate simulations compared to the measurements in Beijing. On average, organic coating decreases nitrate concentrations by 8.4 % in BTH during an episode, and N2O5 heterogeneous hydrolysis with organic coating contributes about 30.1 % of nitrate concentrations. Additionally, the reaction also plays a considerable role in the heavy haze formation, with a PM 2.5 contribution of about 11.6 % in BTH. Sensitivity studies also reveal that future studies need to be conducted to predict the organic aerosol hygroscopicity for accurately representing the organic coating effect on N2O5 heterogeneous hydrolysis. [ABSTRACT FROM AUTHOR]

The implementation of the Air Pollution Prevention and Control Action Plan in China since 2013 has profoundly altered the ambient pollutants in the Beijing–Tianjin–Hebei (BTH) region. Here we show observations of substantially increased O3 concentrations (about 30 %) and a remarkable increase in the ratio of organic carbon (OC) to elemental carbon (EC) in BTH during the autumn from 2013 to 2015, revealing an enhancement in atmospheric oxidizing capacity (AOC) and secondary organic aerosol (SOA) formation. To explore the impacts of increasing AOC on the SOA formation, a severe air pollution episode from 3 to 8 October 2015 with high O3 and PM 2.5 concentrations is simulated using the WRF-Chem model. The model performs reasonably well in simulating the spatial distributions of PM 2.5 and O3 concentrations over BTH and the temporal variations in PM 2.5 , O3 , NO2 , OC, and EC concentrations in Beijing compared to measurements. Sensitivity studies show that the change in AOC substantially influences the SOA formation in BTH. A sensitivity case characterized by a 31 % O3 decrease (or 36 % OH decrease) reduces the SOA level by about 30 % and the SOA fraction in total organic aerosol by 17 % (from 0.52 to 0.43, dimensionless). Spatially, the SOA decrease caused by reduced AOC is ubiquitous in BTH, but the spatial relationship between SOA concentrations and the AOC is dependent on the SOA precursor distribution. Studies on SOA formation pathways further show that when the AOC is reduced, the SOA from oxidation and partitioning of semivolatile primary organic aerosol (POA) and co-emitted intermediate volatile organic compounds (IVOCs) decreases remarkably, followed by those from anthropogenic and biogenic volatile organic compounds (VOCs). Meanwhile, the SOA decrease in the irreversible uptake of glyoxal and methylglyoxal on the aerosol surfaces is negligible. [ABSTRACT FROM AUTHOR]

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]

In the study, observational data analyses and the WRF-CHEM model simulations are used to investigate the role of sea-land and mountain-valley breeze circulations in a severe air pollution event occurred in Beijing-Tianjin-Hebei (BTH) during August 9–10, 2013. Both the wind observations and the model simulations have clearly indicated the evolution of the sea-land and mountain-valley breeze circulations during the event. The WRF-CHEM model generally reproduces the local meteorological circulations and also performs well in simulating temporal variations and spatial distributions of fine particulate matters (PM 2.5 ) and ozone (O 3 ) concentrations compared to observations in BTH. The model results have shown that the offshore land breeze transports the pollutants formed in Shandong province to the Bohai Gulf in the morning, causing the formation of high O 3 and PM 2.5 concentrations over the gulf. The onshore sea breeze not only causes the formation of a convergence zone to induce upward movement, mitigating the surface pollution to some degree, also recirculates the pollutants over the gulf to deteriorate the air quality in the coastal area. The upward valley breeze brings the pollutants in the urban area of Beijing to the mountain area in the afternoon, and the downward mountain breeze transports the pollutants back during nighttime. The intensity of the mountain-valley breeze circulation is weak compared to the land-sea breeze circulation in BTH. It is worth noting that the local circulations play an important role when the large-scale meteorological conditions are relatively weak. [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]