A Review of Atmospheric Chemistry Research in China:Photochemical Smog, Haze Pollution,and Gas-Aerosol Interactions

In this paper we present a review of atmospheric chemistry research in China over the period 2006-2010, focusing on tropospheric ozone, aerosol chemistry, and the interactions between trace gases and aerosols in the polluted areas of China. Over the past decade, China has suffered severe photochemical smog and haze pollution, especially in North China, the Yangtze River Delta, and the Pearl River Delta. Much scientific work on atmospheric chemistry and physics has been done to address this large-scale, complex environmental problem. Intensive field experiments, satellite data analyses, and model simulations have shown that air pollution is significantly changing the chemical and physical characters of the natural atmosphere over these parts of China. In addition to strong emissions of primary pollutants, photochemical and heterogeneous reactions play key roles in the formation of complex pollution. More in-depth research is recommended to reveal the formation mechanism of photochemical smog and haze pollution and their climatic effects at the urban, regional, and global scales.

Monthly Variations of Atmospheric Circulations Associated with Haze Pollution in the Yangtze River Delta and North China

Haze pollution in early winter(December and January) in the Yangtze River Delta(YRD) and in North China(NC)are both severe;however, their monthly variations are significantly different. In this study, the dominant large-scale atmospheric circulations and local meteorological conditions were investigated and compared over the YRD and NC in each month. Results showed that the YRD(NC) is dominated by the so-called Scandinavia(East Atlantic/West Russia)pattern in December, and these circulations weaken in January. The East Asian December and January monsoons over the YRD and NC have negative correlations with the number of haze days. The local descending motion facilitates less removal of haze pollution over the YRD, while the local ascending motion facilitates less removal of haze pollution over NC in January, despite a weaker relationship in December. Additionally, the monthly variations of atmospheric circulations showed that adverse meteorological conditions restrict the vertical(horizontal) dispersion of haze pollution in December(January) over the YRD, while the associated local weather conditions are similar in these two months over NC.

Numerical simulation of an extreme haze pollution event over the North China Plain based on initial and boundary condition ensembles

The North China Plain often su ers heavy haze pollution events in the cold season due to the rapid industrial development and urbanization in recent decades.In the winter of 2015,the megacity cluster of Beijing Tianjin Hebei experienced a seven-day extreme haze pollution episode with peak PM2.5(particulate matter(PM)with an aerodynamic diameter≤2.5μm)concentration of 727μg m 3.Considering the in uence of meteorological conditions on pollu-tant evolution,the e ects of varying initial conditions and lateral boundary conditions(LBCs)of the WRF-Chem model on PM2.5 concentration variation were investigated through ensemble methods.A control run(CTRL)and three groups of ensemble experiments(INDE,BDDE,INBDDE)were carried out based on difierent initial conditions and LBCs derived from ERA5 reanalysis data and its 10 ensemble members.The CTRL run reproduced the meteorological conditions and the overall life cycle of the haze event reasonably well,but failed to capture the intense oscillation of the instantaneous PM2.5 concentration.However,the ensemble forecasting showed a considerable advantage to some extent.Compared with the CTRL run,the root-mean-square error(RMSE)of PM2.5 concentration decreased by 4.33%,6.91%,and 8.44%in INDE,BDDE and INBDDE,respectively,and the RMSE decreases of wind direction(5.19%,8.89%and 9.61%)were the dominant reason for the improvement of PM2.5 concentration in the three ensemble experiments.Based on this case,the ensemble scheme seems an e ective method to improve the prediction skill of wind direction and PM2.5 concentration by using the WRF-Chem model.

A new approach to spatial source apportionment of haze pollution in large scale and its application in China

Long-lasting expansion of haze pollution in China has already presented a stern challenge to regional joint prevention and control. There is an urgent need to enlarge and reconstruct the coverage of joint prevention and control of air pollution in key area. Air quality models can identify and quantify the regional contribution of haze pollution and its key components with the help of numerical simulation, but it is difficult to be applied to larger spatial scale due to the complexity of model parameters. The time series analysis can recognize the existence of spatial interaction of haze pollution between cities, but it has not yet been used to further identify the spatial sources of haze pollution in large scale. Using econometric framework of time series analysis, this paper developed a new approach to perform spatial source apportionment. We applied this approach to calculate the contribution from spatial sources of haze pollution in China, using the monitoring data of particulate matter(PM_(2.5)) across 161 Chinese cities. This approach overcame the limitation of numerical simulation that the model complexity increases at excess with the expansion of sample range, and could effectively deal with severe large-scale haze episodes.

Remote impacts from the tropical Indian Ocean on haze pollution in January over the Yangtze River Delta

Haze events in the Yangtze River Delta(YRD)region have recently been occurring more frequently and with dramatic damages inflicted on human and ecosystem health.In this study,observational analyses and numerical experiments are used to investigate the meteorological conditions associated with haze pollution,with the main emphasis on the impacts of the preceding sea surface temperature(SST)in the tropical Indian Ocean(TIO).The results show that the December SST in the TIO has a significant positive correlation with the number of haze days in January over the YRD,especially during 1999–2017.In December,the positive SST anomalies in the TIO heat the overlying air,and then in the following January provoke a Matsuno–Gill-like pattern and a series of Rossby wave–like trains in the upper troposphere,transmitting signals to the YRD and downstream through the Sea of Japan and Aleutian Islands.The cyclonic anomalies in the YRD seem to significantly weaken the East Asian jet stream by means of anomalous easterlies,and subsequently affect the climate in the region.Near the surface,the increased surface air temperature and southerly winds,along with the decreased surface wind speed,accompanied by influences from upstream areas,are conducive to the occurrence of haze.These observational results were also reproduced well in CESM-LE simulations.

A Haze Feature Extraction and Pollution Level Identification Pre-Warning Algorithm

The prediction of particles less than 2.5 micrometers in diameter(PM2.5)in fog and haze has been paid more and more attention,but the prediction accuracy of the results is not ideal.Haze prediction algorithms based on traditional numerical and statistical prediction have poor effects on nonlinear data prediction of haze.In order to improve the effects of prediction,this paper proposes a haze feature extraction and pollution level identification pre-warning algorithm based on feature selection and integrated learning.Minimum Redundancy Maximum Relevance method is used to extract low-level features of haze,and deep confidence network is utilized to extract high-level features.eXtreme Gradient Boosting algorithm is adopted to fuse low-level and high-level features,as well as predict haze.Establish PM2.5 concentration pollution grade classification index,and grade the forecast data.The expert experience knowledge is utilized to assist the optimization of the pre-warning results.The experiment results show the presented algorithm can get better prediction effects than the results of Support Vector Machine(SVM)and Back Propagation(BP)widely used at present,the accuracy has greatly improved compared with SVM and BP.

Countermeasures against Haze Pollution in Xi'an City

Major causes of haze pollution in Xi＇an City were discussed firstly, and current situation of air pollution caused by vehicle exhaust in Xi＇an City was analyzed. Finally, countermeasures against haze pollution in Xi＇an City were put forward, such as strictly implementing I/M （ Inspection/ Maintenance） system of in-use vehicles in Xran City, and paying more attention to in-use vehicles with high pollution to reduce haze.

The effect of hosting international mega-events on urban haze pollution in China:Based on a quasi-natural experiment

With the rapid economic and social development of China,more and more Chinese cities are hosting important international events.However,the long-term effects of hosting international events on urban haze pollution are rarely discussed in existing literature.Based on the quasi-natural experiment of China hosting the G20 Hangzhou Summit in 2016,this paper applies the synthetic control method (SCM) for the first time to evaluate the impact of the G20 Hangzhou Summit on urban haze pollution in China.Compared with the traditional method,SCM measures the long-term impact of hosting the G20 Summit on haze pollution more scientifically.The results show that hosting the G20 Summit did improve the haze pollution in Hangzhou.The effectiveness and robustness of the above conclusions are objectively confirmed by multi-dimensional tests such as placebo test,permutation test,iterative test and difference-in-differences model.Further mechanism analysis shows that foreign direct investment and technological innovation are important channels for the G20 Summit to curb haze pollution in Hangzhou.At the same time,this paper employs a series of methods to systematically evaluate the impact of the G20 Hangzhou Summit on haze pollution in both time and space dimensions.This paper provides a credible basis and empirical support for the impact of international mega-events on air pollution in China.

COATS:Comprehensive observation on the atmospheric boundary layer three-dimensional structure during haze pollution in the North China Plain

The North China Plain(NCP)is troubled by severe haze pollution and the evolution of haze pollution is closely related to the atmospheric boundary layer(ABL).However,experimental and theoretical studies on the physical-chemical processes of the ABL in the NCP are lacking,with many scientific problems to be addressed.To solve these problems,the Comprehensive Observation on the Atmospheric boundary layer Three-dimensional Structure(COATS)during haze pollution was carried out in the NCP from 2016 to 2020.The COATS experiment adopted a"point-line-surface"spatial layout,obtaining both spatial-temporal profiles of the meteorological and environmental elements in the ABL and the turbulent transport data of fine particulate matter(PM_(2.5))in winter and summer.The research achievements are as follows.The spatial-temporal distribution characteristics of the ABL structure and PM_(2.5)concentrations in NCP were determined.The typical thermal structure of persistent heavy haze events and the pollutant removal mechanism by low-level jets were revealed.It was determined that the spatial structure of the ABL adjusted by the Taihang Mountains is responsible for the heterogeneous distribution of haze pollution in the NCP,and that mountain-induced vertical circulations can promote the formation of elevated pollution layers.The restraints of the atmospheric internal boundaries on horizontal diffusion of pollutants were emphasized.The contribution of the ABL to haze pollution in winter and summer was qualitatively compared and quantitatively estimated.The turbulent transport nature behind the relationship between the atmospheric boundary layer height(ABLH)and surface PM_(2.5)concentrations was analyzed.The concept of"aerosol accumulation layer"was defined,and the applicability of the material method in determining ABLH was clarified.A measurement system for obtaining the turbulent flux of PM_(2.5)concentrations was developed,and the turbulence characteristics of PM_(2.5)concentrations were demonstrated.The COATS experiment is of great theoretical significance for thoroughly understanding the physical mechanisms of the ABL during haze pollution and filling the knowledge gap on the impacts of the ABL three-dimensional structure on haze pollution.The results of this study are conducive to the improvement and development of ABL parameterization schemes and serve as a scientific basis for formulating regional pollution prevention and control measures.

Control of fine particulate nitrate during severe winter haze in“2+26”cities

The“2+26”cities,suffering the most severe winter haze pollution,have been the key region for air quality improvement in China.Increasing prominent nitrate pollution is one of the most challenging environmental issues in this region,necessitating development of an effective control strategy.Herein,we use observations,and state-of-the-art model simulations with scenario analysis and process analysis to quantify the effectiveness of the future SO_(2)-NO_(X)-VOC-NH_(3) emission control on nitrate pollution mitigation in“2+26”cities.Focusing on a serious winter haze episode,we find that limited NO_(X)emission reduction alone in the short-term period is a less effective choice than VOC or NH_3emission reduction alone to decrease nitrate concentrations,due to the accelerated NO_(X)-HNO_(3)conversion by atmospheric oxidants and the enhanced HNO_(3)to NO_(3)^(-)partition by ammonia,although deep NO_(X)emission reduction is essential in the long-term period.The synergistic NH_(3) and VOC emission control is strongly recommended,which can counteract the adverse effects of nonlinear photochemistry and aerosol chemical feedback to decrease nitrate more.Such extra benefits will be reduced if the synergistic NH_(3) and VOC reduction is delayed,and thus reducing emission of multiple precursors is urgently required for the effective control of increasingly severe winter nitrate pollution in“2+26”cities.

Mechanism for the formation of the January 2013 heavy haze pollution episode over central and eastern China

In January 2013, a long-lasting episode of severe haze occurred in central and eastern China, and it attracted attention from all sectors of society. The process and evolution of haze pollution episodes were observed by the ＂Forming Mechanism and Con- trol Strategies of Haze in China＂ group using an intensive aerosol and trace gases campaign that simultaneously obtained data at 11 ground-based observing sites in the CARE-China network. The characteristics and formation mechanism of haze pollu- tion episodes were discussed. Five haze pollution episodes were identified in the Beijing-Tianjin-Hebei （Jing-Jin-Ji） area; the two most severe episodes occurred during 9-15 January and 25-31 January. During these two haze pollution episodes, the maximum hourly PMz5 mass concentrations in Beijing were 680 and 530 ~tg m-3, respectively. The process and evolution of haze pollution episodes in other major cities in the Jing-Jin-Ji area, such as Shijiazhuang and Tianjin were almost the same as those observed in Beijing. The external cause of the severe haze episodes was the unusual atmospheric circulation, the depres- sion of strong cold air activities and the very unfavorable dispersion due to geographical and meteorological conditions. How- ever, the internal cause was the quick secondary transformation of primary gaseous pollutants to secondary aerosols, which contributed to the ＂explosive growth＂ and ＂sustained growth＂ of PM2.5. Particularly, the abnormally high amount of nitric ox- ide （NOx） in the haze episodes, produced by fossil fuel combustion and vehicle emissions, played a direct or indirect role in the quick secondary transformation of coal-burning sulphur dioxide （SO2） to sulphate aerosols. Furthermore, gaseous pollutants were transformed into secondary aerosols through heterogeneous reactions on the surface of fine particles, which can change the particle＇s size and chemical composition. Consequently, the proportion of secondary inorganic ions, such as sulphate and nitrate, gradually increased, which enhances particle hygroscopicity and thereby accelerating formation of the haze pollution.

Intensified wintertime secondary inorganic aerosol formation during heavy haze pollution episodes(HPEs) in Beijing,China

Air pollution in China is complex,and the formation mechanism of chemical components in particulate matter is still unclear.This study selected three consecutive heavy haze pollution episodes(HPEs)during winter in Beijing for continuous field observation,including an episode with heavy air pollution under red alert.Clean days during the observation period were selected for comparison.The HPE characteristics of Beijing in winter were:under the influence of adverse meteorological conditions such as high relative humidity,temperature inversion and low wind speed;and strengthening of secondary transformation reactions,which further intensified the accumulation of secondary aerosols and other pollutants,promoting the explosive growth of PM_(2.5).PM_(2.5)/CO values,as indicators of the contribution of secondary transformation in PM_(2.5),were approximately 2 times higher in the HPEs than the average PM_(2.5)/CO during the clean period.The secondary inorganic aerosols(sulfate nitrate and ammonium salt)were significantly enhanced during the HPEs,and the conversion coefficients were remarkably improved.In addition,it is interesting to observe that the production of sulfate tended to exceed that of nitrate in the late stage of all three HPEs.The existence of aqueous phase reactions led to the explosive growth sulfur oxidation ratio(SOR)and rapid generation of sulfate under high relative humidity(RH>70%).

Heavy haze pollution during the COVID-19 lockdown in the Beijing-Tianjin-Hebei region,China

To investigate the characteristics of particulate matter with an aerodynamic diameter less than 2.5μm(PM_(2.5))and its chemical compositions in the Beijing-Tianjin-Hebei(BTH)region of China during the novel coronavirus disease(COVID-19)lockdown,the ground-based data of PM_(2.5),trace gases,water-soluble inorganic ions,and organic and elemental carbon were analyzed in three typical cities(Beijing,Tianjin,and Baoding)in the BTH region of China from 5-15 February 2020.The PM_(2.5)source apportionment was established by combining the weather research and forecasting model and comprehensive air quality model with extensions(WRF-CAMx).The results showed that the maximum daily PM_(2.5)concentration reached the heavy pollution level(>150μg/m^(3))in the above three cities.The sum concentration of SO_(4)^(2-),NO_(3)^(-)and NH_(4)^(+)played a dominant position in PM_(2.5)chemical compositions of Beijing,Tianjin,and Baoding;secondary transformation of gaseous pollutants contributed significantly to PM_(2.5)generation,and the secondary transformation was enhanced as the increased PM_(2.5)concentrations.The results of WRF-CAMx showed obviously inter-transport of PM_(2.5)in the BTH region;the contribution of transportation source decreased significantly than previous reports in Beijing,Tianjin,and Baoding during the COVID-19 lockdown;but the contribution of industrial and residential emission sources increased significantly with the increase of PM_(2.5)concentration,and industry emission sources contributed the most to PM_(2.5)concentrations.Therefore,control policies should be devoted to reducing industrial emissions and regional joint control strategies to mitigate haze pollution.

Influence of haze pollution on water-soluble chemical species in PM2.5 and size-resolved particles at an urban site during fall

To investigate the influence of haze on the chemical composition and formation processes of ambient aerosol particles,PM_（2.5） and size-segregated aerosol particles were collected daily during fall at an urban site of Gwangju,Korea.During the study period,the total concentration of secondary ionic species（SIS） contributed an average of 43.9% to the PM_（2.5） ,whereas the contribution of SIS to the PM_（2.5） during the haze period was 62.3%.The NO_3 and SO^（2-）_4 concentrations in PM_（2.5） during the haze period were highly elevated,being 13.4 and 5.0 times higher than those during non-haze period,respectively.The PM,NO^-_3,SO^（2-）_4,oxalate,water-soluble organic carbon（WSOC）,and humic-like substances（HULIS） had tri-modal size distributions peaks at 0.32,1.0,and 5.2 μm during the non-haze and haze periods.However,during the non-haze period they exhibited dominant size distributions at the condensation mode peaking at 0.32 μm,while on October 21 when the heaviest haze event occurred,they had predominant droplet mode size distributions peaking at 1.00 μm.Moreover,strong correlations of WSOC and HULIS with SO^（2-）_4,oxalate,and K＋at particle sizes of 〈 1.8 μm indicate that secondary processes and emissions from biomass burning could be responsible for WSOC and HULIS formations.It was found that the factors affecting haze formation could be the local stable synoptic conditions,including the weak surface winds and high surface pressures,the long-range transportation of haze from eastern China and upwind regions of the Korean peninsula,as well as the locally emitted and produced aerosol particles.

Characterization and source identification of submicron aerosol dur ing ser ious haze pollution periods in Beijing

Submicron aerosol is of extensive concern not only due to its significant impact on air quality but also because it is detrimental to human health.In this study,we investigated the characteristics,sources and chemical processes of submicron aerosol based on realtime online measurements of submicron aerosols(NR-PM 1)during December 2015 at an urban site in Beijing.The average mass concentration of NR-PM 1 was 92.5±84.9μg/m^(3),the hourly maximum was 459.1μg/m^(3) during the entire observation.The organic aerosol(OA)(55%)was the largest contributor to NR-PM 1.The average mass concentration of PAHs was 0.217±0.247μg/m^(3),exhibiting the highest concentration at night and the lowest levels in the daytime.The average mass concentration of organic nitrate was 2.52±2.36μg/m^(3) and that of inorganic nitrate was 7.62±8.22μg/m^(3),accounting for 36%and 64%,respectively,of the total nitrate mass.Positive matrix factorization(PMF)differentiated the OA into five chemical components including LV-OOA,SV-OOA,COA,HOA and CCOA,accounting for 22%,16%,13%,25%and 24%respectively,of the total OA.The average NR-PM 1 mass concentration on the heavy polluted days(HPD)was 182.8±70.2μg/m^(3),which was approximately 9 times that on clean days(CD).The enhanced secondary formation of SNA was evident on HPD,especially the rapid increase of sulfate(23%)and nitrate(19%).

Benefit of aerosol reduction to winter wheat during China's clean air action:A case study of Henan Province

A strongly declining aerosol radiative effect has been observed in China since 2013 after implementing the clean air action,yet its impact on wheat(Triticum aestivum L.)production remains unclear.We use satellite measures and a biophysical crop model to assess the impact of aerosol-induced radiative perturbations on winter wheat production in the agricultural belt of Henan province from 2013 to 2018.After calibrating parameters with the extended Fourier Amplitude Sensitivity Test(EFAST)and the generalized likelihood uncertainty estimation(GLUE)method,the DSSAT CERES-Wheat model was able to simulate crop biomass and yield more accurately.We found that the aerosol negatively impacted wheat biomass by 21.87%and yield by 22.48%from 2006 to 2018,and the biomass effects from planting to anthesis were more significant compared to anthesis to maturity.Due to the strict clean air action,under all-sky conditions,the surface solar shortwave radiation(SSR)in 2018 increased by about 7.08%over 2006-2013 during the wheat growing seasons.As a result of the improvement of crop photosynthesis,winter wheat biomass and yield increased by an average of 5.46%and 2.9%,respectively.Our findings show that crop carbon uptake and yield will benefit from the clean air action in China,helping to ensure national food and health security.

Characteristics of atmospheric single particles during haze periods in a typical urban area of Beijing：A case study in October,2014

To investigate the composition and possible sources of particles, especially during heavy haze pollution, a single particle aerosol mass spectrometer（SPAMS） was deployed to measure the changes of single particle species and sizes during October of 2014, in Beijing. A total of 2,871,431 particles with both positive and negative spectra were collected and characterized in combination with the adaptive resonance theory neural network algorithm（ART-2a）. Eight types of particles were classified： dust particles（dust, 8.1%）, elemental carbon（EC, 29.0%）, organic carbon（OC, 18.0%）, EC and OC combined particles（ECOC, 9.5%）,Na-K containing particles（Na K, 7.9%）, K-containing particles（K, 21.8%）, organic nitrogen and potassium containing particles（KCN, 2.3%）, and metal-containing particles（metal,3.6%）. Three haze pollution events（P1, P2, P3） and one clean period（clean） were analyzed,based on the mass and number concentration of PM_（2.5）and the back trajectory results from the hybrid single particle Lagrangian integrated trajectory model（Hysplit-4 model）. Results showed that EC, OC and K were the major components of single particles during the three haze pollution periods, which showed clearly increased ratios compared with those in the clean period. Results from the mixing state of secondary species of different types of particles showed that sulfate and nitrate were more readily mixed with carbon-containing particles during haze pollution episodes than in clean periods.

Variation of airborne quartz in air of Beijing during the Asia-Pacific Economic Cooperation Economic Leaders' Meeting

Quartz particles are a toxic component of airborne paniculate matter（PM）.Quartz concentrations were analyzed by X-ray diffraction in eighty-seven airborne PM samples collected from three locations in Beijing before,during,and after the Asia-Pacific Economic Cooperation（APEC） Leaders＇ Meeting in 2014.The results showed that the mean concentrations of quartz in PM samples from the two urban sites were considerably higher than those from the rural site.The quartz concentrations in samples collected after the APEC meeting,when the pollution restriction lever was lifted,were higher than those in the samples collected before or during the APEC meeting.The quartz concentrations ranged from 0.97 to 13.2 μg/m^3,which were among the highest values amid those reported from other countries.The highest quartz concentration exceeded the Californian Office of Environmental Health Hazard Assessment reference exposure level and was close to the occupational threshold limit values for occupational settings.Moreover,a correlation analysis showed that quartz concentrations were positively correlated with concentrations of pollution parameters PM10,PM2.5,SO2 and NOx,but were negatively correlated with O3 concentration.The results suggest that the airborne quartz particles may potentially pose health risks to the general population of Beijing.