Distribution and Formation Causes of PM_(2.5) and O_(3) Double High Pollution Events in China during 2013–20

Fine particulate matter(PM_(2.5))and ozone(O_(3))double high pollution(DHP)events have occurred frequently over China in recent years,but their causes are not completely clear.In this study,the spatiotemporal distribution of DHP events in China during 2013–20 is analyzed.The synoptic types affecting DHP events are identified with the Lamb–Jenkinson circulation classification method.The meteorological and chemical causes of DHP events controlled by the main synoptic types are further investigated.Results show that DHP events(1655 in total for China during 2013–20)mainly occur over the North China Plain,Yangtze River Delta,Pearl River Delta,Sichuan Basin,and Central China.The occurrence frequency increases by 5.1%during 2013–15,and then decreases by 56.1%during 2015–20.The main circulation types of DHP events are“cyclone”and“anticyclone”,accounting for over 40%of all DHP events over five main polluted regions in China,followed by southerly or easterly flat airflow types,like“southeast”,“southwest”,and“east”.Compared with non-DHP events,DHP events are characterized by static or weak wind,high temperature(20.9℃ versus 23.1℃)and low humidity(70.0%versus 64.9%).The diurnal cycles of meteorological conditions cause PM_(2.5)(0300–1200 LST,Local Standard Time=UTC+8 hours)and O_(3)(1500–2100 LST)to exceed the national standards at different periods of the DHP day.Three pollutant conversion indices further indicate the rapid secondary conversions during DHP events,and thus the concentrations of NO_(2),SO_(2) and volatile organic compounds decrease by 13.1%,4.7%and 4.4%,respectively.The results of this study can be informative for future decisions on the management of DHP events.

Extreme air pollution events:Modeling and prediction

In order to get prepared for the coming extreme pollution events and minimize their harmful impacts, the first and most important step is to predict their possible intensity in the future. Firstly, the generalized Pareto distribution （GPD） in extreme value theory was used to fit the extreme pollution concentrations of three main pollutants： PM10, NO2 and SO：, from 2005 to 2010 in Changsha, China. Secondly, the prediction results were compared with actual data by a scatter plot. Four statistical indicators： EMA （mean absolute error）, ERMS （root mean square error）, IA （index of agreement） and R2 （coefficient of determination） were used to evaluate the goodness-of-fit as well. Thirdly, the return levels corresponding to different return periods were calculated by the fitted distributions. The fitting results show that the distribution of PM10 and SO2 belongs to exponential distribution with a short tail while that of the NOe belongs to beta distribution with a bounded tail. The scatter plot and four statistical indicators suggest that GPD agrees well with the actual data. Therefore, the fitted distribution is reliable to predict the return levels corresponding to different return periods. The predicted return levels suggest that the intensity of coming pollution events for PM10 and SO2 will be even worse in the future, which means people have to get enough preparation for them.

Urban air quality, meteorology and traffic linkages: Evidence from a sixteen-day particulate matter pollution event in December 2015, Beijing

A heavy 16-day pollution episode occurred in Beijing from December 19, 2015 to January 3,2016. The mean daily AQI and PM2.5 were 240.44 and 203.6 μg/m^3. We analyzed the spatiotemporal characteristics of air pollutants, meteorology and road space speed during this period, then extended to reveal the combined effects of traffic restrictions and meteorology on urban air quality with observational data and a multivariate mutual information model. Results of spatiotemporal analysis showed that five pollution stages were identified with remarkable variation patterns based on evolution of PM2.5 concentration and weather conditions. Southern sites（DX, YDM and DS） experienced heavier pollution than northern ones（DL, CP and WL）. Stage P2 exhibited combined functions of meteorology and traffic restrictions which were delayed peak-clipping effects on PM2.5.Mutual information values of Air quality–Traffic–Meteorology（ATM–MI） revealed that additive functions of traffic restrictions, suitable relative humidity and temperature were more effective on the removal of fine particles and CO than NO2.

Chemical characteristics of PM_(2.5) during dust storms and air pollution events in Chengdu,China

Daily fine particulate （PM2.5） samples were collected in Chengdu from April 2009 to February 2010 to investigate their chemical profiles during dust storms （DSs） and several types of pollution events, including haze （HDs）, biomass burning （BBs）, and fireworks displays （FDs）. The highest PM2.5 mass concentrations were found during DSs （283.3 μg/m^3）, followed by FDs （212.7 μg/m^3）, HDs （187.3 μg/m^3 ）, and BBs （130.1 μ g/m^3）. The concentrations of most elements were elevated during DSs and pollution events, except for BBs. Secondary inorganic ions （NO3^- , SO4^2-, and NH4^＋） were enriched during HDs, while PM2.5 from BBs showed high K^＋ but low SO4^2- , FDs caused increases in K^＋ and enrichment in SO4^2-. Ca^2＋. was abundant in DS samples, Ion-balance calculations indicated that PM2.5 from HDs and FDs was more acidic than on normal days, but DS and BB particles were alkaline. The highest organic carbon （OC） concentration was 26.1 μg/m^3 during FDs, followed by BBs （23.6 μg/m^3 ）, HDs （19.6 μg/m^3 ）, and DSs （18.8 μg/m^3 ）. In contrast, elemental carbon （EC） concentration was more abundant during HDs （10.6μg/m^3） and FDs （9.5 μg/m^3） than during BBs （6.2μg/m^3） and DSs （6.0 μg/m^3）. The highest OC/EC ratios were obtained during BBs, with the lowest during HDs. SO4^2＋ /K^＋ and TCA/SO4^2- ratios proved to be effective indicators for differentiating pollution events. Mass balance showed that organic matter, SO4^2-, and NO3^- were the dominant chemical components during pollution events, while soil dust was dominant during DSs.

TOF–SIMS surface analysis of chemical components of size–fractioned urban aerosols in a typical heavy air pollution event in Beijing

Size–fractioned atmospheric aerosol particles were collected during a typical heavy air pollution event in Beijing. The organic and inorganic components on the surfaces of the samples were analyzed using time–of–flight secondary ion mass spectrometry（TOF–SIMS）.The variation characteristics of the surface chemical composition and influencing factors were studied, and the possible sources of these chemical compositions were identified through principal component analysis. The results showed that inorganic components such as crustal elements and sulfate, and organic components such as aliphatic hydrocarbons and oxygen–containing organic groups were present. Some surface components, such as polycyclic aromatic hydrocarbons, heavy metals and fluorides may exert adverse effects on human health. The species and relative percentages of the chemical components varied with particle size, diurnal and pollution progress. During a heavy pollution event, the species and relative percentages of secondary components such as oxygen–containing organic groups and sulfurous compounds increased, indicating that particles aged during this event. The surface chemical composition of the aerosol particles was affected mainly by emissions from coal combustion and motor vehicles. In addition, air pollution, meteorological factors, and air mass transport also exerted a significant effect on the surface chemical composition of aerosol particles.

High-precision parallel computing model of solute transport based on GPU acceleration

The scenario simulation analysis of water environmental emergencies is very important for risk prevention and control,and emergency response.To quickly and accurately simulate the transport and diffusion process of high-intensity pollutants during sudden environmental water pollution events,in this study,a high-precision pollution transport and diffusion model for unstructured grids based on Compute Unified Device Architecture(CUDA)is proposed.The finite volume method of a total variation diminishing limiter with the Kong proposed r-factor is used to reduce numerical diffusion and oscillation errors in the simulation of pollutants under sharp concentration conditions,and graphics processing unit acceleration technology is used to improve computational efficiency.The advection diffusion process of the model is verified numerically using two benchmark cases,and the efficiency of the model is evaluated using an engineering example.The results demonstrate that the model perform well in the simulation of material transport in the presence of sharp concentration.Additionally,it has high computational efficiency.The acceleration ratio is 46 times the single-thread acceleration effect of the original model.The efficiency of the accelerated model meet the requirements of an engineering application,and the rapid early warning and assessment of water pollution accidents is achieved.

Differences in Meteorological Conditions between Days with Persistent and Non-Persistent Pollution in Beijing, China

We compared the regional synoptic patterns and local meteorological conditions during persistent and non-persistent pollution events in Beijing using US NCEP–Department of Energy reanalysis outputs and observations from meteorological stations. The analysis focused on the impacts of high-frequency（period 〈 90 days） variations in meteorological conditions on persistent pollution events（those lasting for at least 3 days）. Persistent pollution events tended to occur in association with slow-moving weather systems producing stagnant weather conditions, whereas rapidly moving weather systems caused a dramatic change in the local weather conditions so that the pollution event was short-lived. Although Beijing was under the influence of anomalous southerly winds in all four seasons during pollution events, notable differences were identified in the regional patterns of sea-level pressure and local anomalies in relative humidity among persistent pollution events in different seasons. A region of lower pressure was present to the north of Beijing in spring, fall, and winter, whereas regions of lower and higher pressures were observed northwest and southeast of Beijing, respectively, in summer. The relative humidity near Beijing was higher in fall and winter, but lower in spring and summer. These differences may explain the seasonal dependence of the relationship between air pollution and the local meteorological variables. Our analysis showed that the temperature inversion in the lower troposphere played an important part in the occurrence of air pollution under stagnant weather conditions.Some results from this study are based on a limited number of events and thus require validation using more data.