Weather Research And Forecasting-Community Multiscale Air Quality Research Articles

Analyses and Simulations of PM2.5 Pollution Characteristics under the Influence of the New Year’s Day Effects in China

Regional haze often occurs after the New Year holiday. To explore the characteristics of PM2.5 pollutions under the influence of the New Year’s Day effect, this study analyzed the spatiotemporal changes relating to PM2.5 during and around the New Year’s Day holiday in China from 2015 to 2022, and used the Weather Research and Forecasting-Community Multiscale Air Quality (WRF-CMAQ) model to study the effects of human activities and meteorological factors on PM2.5 pollutions, as well as the differences in the contributions of different industries to PM2.5 pollutions. The results show that for the entire study period (i.e., before, during, and after the New Year’s Day holiday) from 2015 to 2022, the average concentrations of PM2.5 in China decreased by 41.9% overall. In 2019~2022, the New Year’s Day effect was significant, meaning that the average concentrations of PM2.5 increased by 18.9~46.8 μg/m3 from before to after the New Year’s Day holiday, with its peak occurring (64.3~74.9 μg/m3) after the holiday. In terms of spatial differences, the average concentrations of PM2.5 were higher in the Beijing–Tianjin–Hebei region, the Yangtze River Delta, and central China. Moreover, the Beijing–Tianjin–Hebei region and its surrounding areas, the Chengdu–Chongqing region, the Fenwei Plain, and the middle reaches of the Yangtze River region were greatly affected by the New Year’s Day effect. Human activities led to higher increases in PM2.5 in Henan, Hubei, Hebei, and Anhui on 3 and 4 January 2022. If the haze was accompanied by cloudy days or weak precipitation, the accumulation of surface water vapor and atmospheric aerosols further increased the possibility of heavy pollution. It was found that, for the entire study period, PM2.5 generated by residential sources contributed the vast majority (60~100 μg/m3) of PM2.5 concentrations, and that the main industry sources that caused changes in time distributions were industrial and transportation sources.

Significant reductions of urban daytime ozone by extremely high concentration NOX from ship’s emissions: A case study

In this paper, a four nested Weather Research and Forecasting-Community Multiscale Air Quality (WRF-CMAQ) model was applied to analyze ozone (O3) concentration reductions under the influence of extremely high NOx concentrations emitted from ships at the Hankou Jiangtan (HJ) station adjacent to the Yangtze River in Wuhan, China. By adding NO emissions along the waterway for the sensitivity experiments, the model successfully reproduced two cases of high NOx concentrations on April 9 and April 28, 2020, and one case of persistent and extremely high NOx concentrations on the night of May 3, 2020. During daytime, strong photochemical reactions generated new O3, while the high concentration NOx from ship emissions continuously reduced O3 through catalytic cycling. This led to the O3 concentrations at the HJ station being consistently lower than the surrounding stations in the afternoon, with the high NOx plume dropping O3 concentrations by 51 ppb in 1 h, and NO2 concentrations up to 110 ppb. The presence of large amounts of fresh NO (up to 265 ppb) in the ship emission plumes rapidly reduces O3 concentrations through titration reactions during night, resulting in a wide area of low O3 concentrations (2–4 ppb) over a long period of time. This study demonstrates that the pollution and impact of emissions from ships traveling in waterways on the atmosphere of the surrounding area cannot be ignored.

The WRF-CMAQ Simulation of a Complex Pollution Episode with High-Level O3 and PM2.5 over the North China Plain: Pollution Characteristics and Causes

The problem of atmospheric complex pollution led by PM2.5 and O3 has become an important factor restricting the improvement of air quality in China. In drawing on observations and Weather Research and Forecasting-Community Multiscale Air Quality (WRF-CMAQ) model simulations, this study analyzed the characteristics and causes of a regional PM2.5-O3 complex pollution episode in North China Plain, in the period from 3 to 5 April 2019. The results showed that in static and stable weather conditions with high temperature and low wind speed, despite photochemical reactions of O3 near the ground being weakened by high PM2.5 concentrations, a large amount of O3 generated through gas-phase chemical reactions at high altitudes was transported downwards and increased the O3 concentrations at the ground level. The high ground-level O3 could facilitate both the conversion of SO2 and NO2 into secondary inorganic salts and volatile organic compounds into secondary organic aerosols, thereby amplifying PM2.5 concentrations and exacerbating air pollution. The contributions of transport from outside sources to PM2.5 (above 60%) and O3 (above 46%) increased significantly during the episode. This study will play an instrumental role in helping researchers to comprehend the factors that contribute to complex pollution in China, and also offers valuable references for air pollution management.

Impact and pathway of halogens on atmospheric oxidants in coastal city clusters in the Yangtze River Delta region in China

Halogens (chlorine, bromine, and iodine) are known to profoundly influence atmospheric oxidants (hydroxyl radical (OH), hydroperoxyl radical (HO2), ozone (O3), and nitrate radical (NO3)) in the troposphere and subsequently affecting air quality. However, their impact on atmospheric oxidation and air pollution in coastal areas in China is poorly characterized. In this study, we use the WRF-CMAQ (Weather Research and Forecasting-Community Multiscale Air Quality) model with full halogen chemistry and process analysis to assess the influences and pathways of halogens on atmospheric oxidants in the Yangtze River Delta (YRD) region, a typical coastal city cluster in China. Halogens cause the annual OH radical increase by up to 16.4% and NO3 decrease by up to 45.3%. O3 increases by 2.0% in the YRD but decreases by 3.3% in marine environment. Halogen induced changes in atmospheric oxidants lead to a general increase of atmospheric oxidation capacity by 5.1% (maximum 48.4%). The production rate of OH (POH) in the YRD is enhanced by anthropogenic chlorine through both increased HO2 pathway and hypohalous acid photolysis pathway, while POH over ocean is enhanced by oceanic halogens through converting HO2 into hypohalous acid. Anthropogenic chlorine enhances both O3 and NO3 production (PNO3) rates through influencing their precursors while oceanic halogens reduce PNO3 and directly destroy ozone. Iodine contributed most (on average of 91% in oceanic halogens) in reducing production rates of oxidants. Thus, halogen emissions and potential effects of halogens on air quality need to be considered in air quality policies and regulations in the YRD region.

Interpretation of the effects of anthropogenic chlorine on nitrate formation over northeast Asia during KORUS-AQ 2016

The Weather Research and Forecasting–Community Multiscale Air Quality (WRF-CMAQ) model, implemented with anthropogenic chlorine (Cl) emissions, was evaluated against ground and NASA DC-8 aircraft measurements during the Korea-United States Air Quality (KORUS-AQ) 2016 campaign. The latest anthropogenic Cl emissions, including gaseous HCl and particulate chloride (pCl−) emissions from the Anthropogenic Chlorine Emissions Inventory of China (ACEIC-2014) (over China) and a global emissions inventory (Zhang et al., 2022) (over outer China), were used to examine the impacts of Cl emissions and the role of nitryl chloride (ClNO2) chemistry in N2O5 heterogeneous reactions on secondary nitrate (NO3−) formation across the Korean Peninsula. The model results against aircraft measurements clearly showed significant Cl- underestimations due mainly to the high gas-particle (G/P) partitioning ratios at aircraft measurement altitudes such as 700–850 hPa, but the ClNO2 simulations were reasonable. Several simulations of CMAQ-based sensitivity experiments against ground measurements indicated that although addition of Cl emission did not significantly alter NO3− formation, the activated ClNO2 chemistry with Cl emissions showed the best model performance with the reduced normalized mean bias (NMB) of 18.7 % compared to a value of 21.1 % for the Cl emissions-free case. In our model evaluation, ClNO2 accumulated during the night but quickly produced Cl radical due to ClNO2 photolysis at sunrise, which modulated other oxidation radicals (e.g., ozone [O3] and hydrogen oxide radicals [HOx]) in the early morning. In the morning hours (0800–1000 LST), the HOx were the dominant oxidants, contributing 86.6 % of the total oxidation capacity (sum of major oxidants such as O3 and HOx species), while oxidability was enhanced by up to ∼6.4 % (increase in 1 h HOx average of 2.89 × 106 molecules·cm−3) in the early morning mainly due to the changes in OH (+7.2 %), hydroperoxyl radical (HO2)(+10.0 %), and O3 (+4.2 %) over the Seoul Metropolitan Area, during the KORUS-AQ campaign. Our results improve understanding of the atmospheric changes in the PM2.5 formation pathway caused by ClNO2 chemistry and Cl emissions over northeast Asia.

Estimation the effect of accumulated long-range transported pollutants during a PM2.5 event in Taiwan

Fine particulate matter (PM2.5) pollution has attracted much attention in recent years due to its adverse impact on human health and the environment. This study examined a major PM2.5 event observed in Taiwan on March 02–03, 2022, using the Weather Research and Forecasting - Community Multiscale Air Quality (WRF-CMAQ) models. This period also coincided with an outage caused by the power failure of three power plants in Southern Taiwan on March 03, 2022, which created a unique situation to distinguish the impacts of long-range transported (LRT) pollutants and the operation of power plants on the air quality in Taiwan during the high-air-pollution event.Four numerical experiments, Baseline (real case), Replace (Baseline with power plants), No_LRT (Baseline without LRT), and No_0303_LRT (Baseline without LRT on March 03, 2022), were conducted with different initial emission scenarios. The simulation results revealed that the PM2.5 concentrations emitted from three power plants only counted a relatively small proportion in total, while the LRT pollutants were the key factor contributing to the high-concentration PM2.5 during the event. Besides, we figured out that the high PM2.5 concentrations on March 03, 2022 were dominated by the contribution of accumulated LRT. In addition, the results also suggested that the existence of a lee-side vortex, which is considered a potential key issue of the mesoscale system contributes to extreme air pollutant events in Taiwan, causing encouraging conditions for the upward diffusion and northward transport of the PM2.5 concentrations over the southern part of Taiwan, as a result, the distribution of pollution was modulated.

Impacts of land use and land cover changes on local meteorology and PM2.5 concentrations in Changchun, Northeast China

Land use and land cover (LULC) changes have a considerable influence on the surface energy balance, altering regional meteorology and air quality. However, this impact is not quantified in Changchun, an important city in the old industrial base of Northeast China. In this study, based on the Weather Research and Forecasting-Community Multiscale Air Quality (WRF-CMAQ) model, the LULC2017 (LULC data in 2017) and LULC2001 (LULC data in 2001) scenarios were simulated for January and July 2017, respectively, to assess the impact of LULC changes on meteorology and fine particulate matter (PM2.5) concentrations in Changchun. The results show that the sensible heat flux in the urban expansion area (UEA) increased during the daytime, reaching a maximum value of 154 W/m2 and 162 W/m2, respectively, while the latent heat flux decreased during the daytime, reaching a maximum value of 22.84 W/m2 and 180.75 W/m2, respectively. Consequently, 2 m temperature (T2) increased by 4 °C and 3 °C, respectively; 10 m wind speed (WS10) increased by 1.05 m/s and 1.60 m/s, respectively; and planetary boundary layer height (PBLH) increased by 100 m and 117 m, respectively. These variations in meteorological factors can substantially impact the spatial distribution of air pollutants. In the UEA, PM2.5 concentrations decreased by 34 μg/m3 and 20 μg/m3 in January and July, respectively. The change in SO42− accounted for approximately 25% of the total concentration change of PM2.5, with a decrease of approximately 5–6 μg/m3 during the nighttime in January. Secondary organic aerosol (SOA) formed from biogenic volatile organic compounds (BVOC) precursors (BSOA) slightly decreased owing to the reduction in croplands dominated by green vegetation. Meanwhile, PM2.5 concentrations in the surrounding areas of the UEA increased significantly in January. The results of the process analysis based on the CMAQ model indicate that the main reason for the spatial variation of PM2.5 concentrations is the enhancement of transport and diffusion in the horizontal and vertical directions in the UEA. In January, the negative contribution of vertical advection (ZADV) and horizontal advection (HADV) processes to PM2.5 in the UEA increased by 25 μg/m3 and 40 μg/m3, respectively. Vertical diffusion (VDIF) process caused an increase in PM2.5 diffusion by 40 μg/m3 and 16 μg/m3 during the daytime and nighttime in the UEA, respectively. In July, the negative contribution of VDIF and HADV processes to PM2.5 increased by 40 μg/m3 and 32 μg/m3 during the nighttime in the UEA, respectively.

Investigating Southeast Asian biomass burning by the WRF-CMAQ two-way coupled model: Emission and direct aerosol radiative effects

Southeast Asia (SEA) is one of the world's major sources of biomass burning (BB). In this study, the recently released Weather Research and Forecasting-Community Multiscale Air Quality (WRF-CMAQ) two-way coupled model was used with the Global Fire Emissions Database Version 4, to investigate the effect of BB emissions on the meteorology and air quality over SEA in March and April 2015. The results show that the model performance was improved by considering BB emissions. When BB pollutants reach the free troposphere, they can be transported by westerly and southwesterly winds to downstream regions. The contributions of BB were up to 48.4 and 35.5 ppbv to surface O3 concentrations and 251.0 and 104.4 μg/m3 to surface PM2.5 concentrations in March and April, respectively. The BB emission caused aerosol direct radiative effect (DRE) on the monthly mean clear-sky downward shortwave flux at the ground surface to decrease by −65.4 and −33.6 W/m2 in March and April, respectively. The surface temperature decreased over the land (by a maximum of −0.24 °C in March) and increased over the sea, while the temperature at higher levels tended to increase (to a maximum of 0.12 °C in March). The BB aerosol DRE caused the planetary boundary layer height (PBLH) to decrease by a maximum of −91.2 m in March. Surface O3 concentrations decreased generally owing to variations in the shortwave flux and temperature. Moreover, the decreased PBLH worsened the diffusion condition within the PBL but also caused larger amounts of BB emissions to penetrate the free atmosphere. This led to a decrease in surface PM2.5 concentrations in southern Indochina and an increase in Myanmar. These findings highlight the key effects of BB emissions on local and downwind meteorology and air quality over SEA and demonstrate the practical applications of the WRF-CMAQ coupled model.

Impact of urban heat island on inorganic aerosol in the lower free troposphere: a case study in Hangzhou, China

Abstract. Urban heat island (UHI) and urban air pollution are two major environmental problems faced by many metropolises. The UHI affects air pollution by changing the local circulation and the chemical reaction environment, e.g. air temperature and relative humidity. In this study, the WRF-CMAQ (Weather Research and Forecasting Community Multiscale Air Quality) model was used to investigate the impact of an UHI on the vertical distribution of aerosol particles, especially secondary inorganic aerosol (SIA), taking the strong UHI in Hangzhou, China, as an example. The results show that during the day with the strongest UHI case of the 8 d simulation, the UHI effect resulted in a decrease in the PM2.5 concentrations in the boundary layer (BL) by about 33 %, accompanied by an increase in the lower free troposphere (LFT) by about 19 %. This is mostly attributed to the UHI circulation (UHIC) effect, which accounted for 91 % of the UHI-induced variations in PM2.5, rather than the UHI temperature or humidity effects, which contributed only 5 % and 4 %, respectively. The UHIC effect plays a dominant role, ranging from 72 % to 93 %, in UHI-induced PM2.5 variation in all eight UHI cases. The UHIC not only directly transports aerosol particles from ground level to the LFT but also redistributes aerosol precursors. During the strongest UHI case, about 80 % of the UHIC-induced increase in the aerosol particles in the LFT is due to direct transport of aerosol particles, whereas the other 20 % is due to secondary aerosol formation resulting from the transport of aerosol precursor gases. Of this 20 %, 91 % is contributed by SIA, especially ammonium nitrate aerosol formed from ammonia and nitric acid. In the atmosphere, ammonium nitrate is in equilibrium with ammonia and nitric acid, and the equilibrium depends on the ambient temperature. In the lower urban BL, the temperature is higher than in the LFT, and the ammonium nitrate equilibrium in the lower BL is more toward the gas phase than in the LFT; when these gases are transported by the UHIC into the colder LFT, the equilibrium shifts to the aerosol phase. Hence, the UHIC changes the vertical distribution of SIA, which may have potential implications on the radiation budget, cloud formation, and precipitation in the urban and surrounding areas.

Impacts of land-surface forcing on local meteorology and ozone concentrations in a heavily industrialized coastal urban area

In this study, the Weather Research and Forecasting/Community Multiscale Air Quality (WRF-CMAQ) model was used to investigate the interaction of urban land-surface forcing with local circulations and the impact on boundary layer ozone concentrations in southern Taiwan at an urban-scale resolution. Two simulations were performed with the same emissions but different land cover designations. URBAN was the baseline simulation representing the current urbanized condition, while NO-URBAN replaced all urban grid cells with cropland. The interaction of the sea-breeze with the urban-heat-island (UHI) convergent flow during the daytime in URBAN transports near-surface O3 precursors to the upper planetary boundary layer (PBL). When the UHI convergent flow stalls over the city center, a circulation flow is formed and traps the pollutants at an elevated height, increasing the reaction rate of hydroxyl radical with volatile organic compounds by 2.0–4.0 ppbv h−1 at 1000–1500 m. At nighttime, the deeper boundary layer of URBAN diluted the NOx mixing ratio by ~17 ppbv and weakened the titration effect, contributing to higher O3 by +15 ppbv in the urban area. However, once the daytime vertical mixing diminished, the O3 aloft also diffused downward to the surface level and further degraded the nighttime air quality.

Evaluation of Long-Term Modeling Fine Particulate Matter and Ozone in China During 2013–2019

Air quality in China has been undergoing significant changes due to the implementation of extensive emission control measures since 2013. Many observational and modeling studies investigated the formation mechanisms of fine particulate matter (PM2.5) and ozone (O3) pollution in the major regions of China. To improve understanding of the driving forces for the changes in PM2.5 and O3 in China, a nationwide air quality modeling study was conducted from 2013 to 2019 using the Weather Research and Forecasting/Community Multiscale Air Quality (WRF/CMAQ) modeling system. In this study, the model predictions were evaluated using the observation data for the key pollutants including O3, sulfur dioxide (SO2), nitrogen dioxide (NO2), and PM2.5 and its major components. The evaluation mainly focused on five major regions, that is , the North China Plain (NCP), the Yangtze River Delta (YRD), the Pearl River Delta (PRD), the Chengyu Basin (CY), and the Fenwei Plain (FW). The CMAQ model successfully reproduced the air pollutants in all the regions with model performance indices meeting the suggested benchmarks. However, over-prediction of PM2.5 was noted in CY. NO2, O3, and PM2.5 were well simulated in the north compared to the south. Nitrate (NO3−) and ammonium (NH4+) were the most important PM2.5 components in heavily polluted regions. For the performance on different pollution levels, the model generally over-predicted the clean days but underpredicted the polluted days. O3 was found increasing each year, while other pollutants gradually reduced during 2013–2019 across the five regions. In all of the regions except PRD (all seasons) and YRD (spring and summer), the correlations between PM2.5 and O3 were negative during all four seasons. Low-to-medium correlations were noted between the simulated PM2.5 and NO2, while strong and positive correlations were established between PM2.5 and SO2 during all four seasons across the five regions. This study validates the ability of the CMAQ model in simulating air pollution in China over a long period and provides insights for designing effective emission control strategies across China.

Impacts of Chemical Initial Conditions in the WRF-CMAQ Model on the Ozone Forecasts in Eastern China

Ozone (O3) has become the major factor for exceeding air pollution standards in many Chinese cities, especially in the more economically developed and densely populated regions, such as eastern China. In this study, we applied the Weather Research and Forecasting/Community Multiscale Air Quality (WRF/CMAQ) model to predict the air quality, and evaluated the influences of different chemical initial conditions on the O3 forecasts with observations in Tai’an and other 13 cities in eastern China in June 2021. The influences of different chemical initial conditions on the O3 forecasts are presented by using two sets of meteorological data (NCEP Final Operational Global Analysis [FNL] and Global Forecast System [GFS]) as initial conditions (IC) and boundary conditions (BC) to drive the WRF/CMAQ model. It was found that the O3 concentrations forecasted by FNL-GFS, in which the chemical IC derived from the CMAQ simulation results by using the FNL data as IC and BC, were closer to observations in all cities than GFS-GFS, in which the chemical IC derived from the CMAQ simulation results by using the GFS data as IC and BC. The normalized mean bias (NMB) values of FNL-GFS for O3 met the benchmark (±15%), while the NMB values of GFS-GFS in Hangzhou and Shijiazhuang did not meet the benchmark. The model performances in Tai’an city were similar to those in 13 cities with better results for FNL-GFS than GFS-GFS. The comparisons of contributions of source regions to O3 in the receptor Tai’an city indicate that different episodes had different relative contributions of source regions and that the simulations of FNL-GFS were more similar to the retrospective simulations than GFS-GFS. The comparisons of contributions of different source sectors to O3 in Tai’an city show that industry emissions are the largest contributor, followed by transportation, power plants and residential emissions.

Contributions of Traffic and Industrial Emission Reductions to the Air Quality Improvement after the Lockdown of Wuhan and Neighboring Cities Due to COVID-19.

Wuhan was locked down from 23 January to 8 April 2020 to prevent the spread of the novel coronavirus disease 2019 (COVID-19). Both public and private transportation in Wuhan and its neighboring cities in Hubei Province were suspended or restricted, and the manufacturing industry was partially shut down. This study collected and investigated ground monitoring data to prove that the lockdowns of the cities had significant influences on the air quality in Wuhan. The WRF-CMAQ (Weather Research and Forecasting-Community Multiscale Air Quality) model was used to evaluate the emission reduction from transportation and industry sectors and associated air quality impact. The results indicate that the reduction in traffic emission was nearly 100% immediately after the lockdown between 23 January and 8 February and that the industrial emission tended to decrease by about 50% during the same period. The industrial emission further deceased after 9 February. Emission reduction from transportation and that from industry was not simultaneous. The results imply that the shutdown of industry contributed significantly more to the pollutant reduction than the restricted transportation.

Characterization of ambient volatile organic compounds, source apportionment, and the ozone–NO<sub>x</sub>–VOC sensitivities in a heavily polluted megacity of central China: effect of sporting events and emission reductions

Abstract. The implementation of strict emission control during the 11th National Minority Games (NMG) in September 2019 provided a valuable opportunity to assess the impact of such emission controls on the characteristics of VOCs and other air pollutants. Here, we investigated the characteristics of VOCs and the O3–NOx–VOC sensitivity comprehensively in Zhengzhou before, during, and after the NMG by delivering field measurements combined with WRF-CMAQ (Weather Research and Forecasting Community Multiscale Air Quality) model simulations. The average mixing ratios of VOCs during the control periods were 121 ± 55 µg m−3 and decreased by about 19 % and 11 % before and after emission reduction. The ozone precursors (NOx) also decreased significantly during the control period; however, the ozone pollution was severe during the entire observation period. Positive matrix factorization analysis indicated seven major sources of ambient VOCs, including coal combustion, biomass burning, vehicle exhausts, industrial processes, biogenic emissions, solvent utilization, and liquefied petroleum gas (LPG). The results show that the major source emissions, such as coal combustion and solvent utilization, were significantly reduced during the control period. As for ozone formation potential (OFP), the value during the control period was 183 ± 115 µg m−3, which was 0.23 and 0.17 times lower than before and after the control period, respectively. Solvent utilization and combustion controls were the most important measures taken to reduce OFP during the NMG period. Control policies can effectively reduce carcinogenic risk. However, non-cancer risks of ambient VOC exposures were all exceeding the safe level (hazard quotient = 1) during the sampling periods, and emphasis on the reduction of acrolein emissions was needed. In addition, the WRF-CMAQ model simulation indicated that O3 formation was controlled by VOCs in Zhengzhou. The results of the Empirical Kinetic Modeling Approach showed that the NOx reduction in Zhengzhou might lead to higher ozone pollution. It is suggested that reduction ratios of the precursors (VOCs : NOx) should be more than 2, which can effectively alleviate ozone pollution.

Separating emission and meteorological contributions to long-term PM<sub>2.5</sub> trends over eastern China during 2000–2018

Abstract. The contribution of meteorology and emissions to long-term PM2.5 trends is critical for air quality management but has not yet been fully analyzed. Here, we used the combination of a machine learning model, statistical method, and chemical transport model to quantify the meteorological impacts on PM2.5 pollution during 2000–2018. Specifically, we first developed a two-stage machine learning PM2.5 prediction model with a synthetic minority oversampling technique to improve the satellite-based PM2.5 estimates over highly polluted days, thus allowing us to better characterize the meteorological effects on haze events. Then we used two methods to examine the meteorological contribution to PM2.5: a generalized additive model (GAM) driven by the satellite-based full-coverage daily PM2.5 retrievals and the Weather Research and Forecasting/Community Multiscale Air Quality (WRF/CMAQ) modeling system. We found good agreements between GAM estimations and the CMAQ model estimations of the meteorological contribution to PM2.5 on a monthly scale (correlation coefficient between 0.53–0.72). Both methods revealed the dominant role of emission changes in the long-term trend of PM2.5 concentration in China during 2000–2018, with notable influence from the meteorological condition. The interannual variabilities in meteorology-associated PM2.5 were dominated by the fall and winter meteorological conditions, when regional stagnant and stable conditions were more likely to happen and when haze events frequently occurred. From 2000 to 2018, the meteorological contribution became more unfavorable to PM2.5 pollution across the North China Plain and central China but were more beneficial to pollution control across the southern part, e.g., the Yangtze River Delta. The meteorology-adjusted PM2.5 over eastern China (denoted East China in figures) peaked in 2006 and 2011, mainly driven by the emission peaks in primary PM2.5 and gas precursors in these years. Although emissions dominated the long-term PM2.5 trends, the meteorology-driven anomalies also contributed −3.9 % to 2.8 % of the annual mean PM2.5 concentrations in eastern China estimated from the GAM. The meteorological contributions were even higher regionally, e.g., −6.3 % to 4.9 % of the annual mean PM2.5 concentrations in the Beijing-Tianjin-Hebei region, −5.1 % to 4.3 % in the Fenwei Plain, −4.8 % to 4.3 % in the Yangtze River Delta, and −25.6 % to 12.3 % in the Pearl River Delta. Considering the remarkable meteorological effects on PM2.5 and the possible worsening trend of meteorological conditions in the northern part of China where air pollution is severe and population is clustered, stricter clean air actions are needed to avoid haze events in the future.

Origin of regional springtime ozone episodes in the Sichuan Basin, China: Role of synoptic forcing and regional transport.

The Sichuan Basin (SCB) located in southwestern China has long been considered the most polluted city cluster with exposure to unhealthy levels of ozone (O3) at times. However, the features of O3 regional transport and source contributions in SCB are poorly understood. In this study, ambient measurements, ERA5 reanalysis dataset, IASI O3 column, and the Weather Research and Forecasting-Community Multiscale Air Quality (WRF-CMAQ) modeling system coupled with the Integrated Source Apportionment Method (ISAM) module were used to investigate the formation mechanism and sources of a severe O3 episode in spring 2020 over the SCB. In the first stage of the O3 episode, a high-pressure system persisted over the western SCB and caused northeasterly wind fields, leading to enhanced regional transport from the northern boundary with the O3 contribution from the boundary exceeding 50% across the SCB. As the synoptic pattern evolved, southeasterly winds dominated the SCB and the stagnant zone over the Chengdu Plain confined O3 originating from the southern SCB and Chongqing city, leading to the accumulation of precursors and elevated O3 levels. During the O3 episode, transportation and industrial sources were major contributors to O3 formation especially for the Chengdu Plain and Chongqing city. In addition, the O3-rich air mass in the nocturnal residual layer that formed over Chongqing city was transported to the Chengdu Plain through southeastern corridor at 400-1600m above ground-level under the prevailing southeasterly winds. With sunrise and the development of the atmospheric boundary layer, the O3-rich air mass in the residual layer (RL) was entrained to the ground-level via vertical mixing, which further enhanced O3 pollution across the Chengdu Plain. Our results revealed the mechanism of regional transport via northeastern and southeastern corridors during an O3 episode and demonstrated the need for joint emission regulation across the SCB to mitigate O3 pollution.

Numerical simulation and cause analysis for a severe mixed pollution of smog and haze in the Pearl River Delta in Winter

Based on the simulation results of the Weather Research and Forecasting-Community Multiscale Air Quality (WRF-CMAQ), the meteorological elements, boundary layer characteristics and temporal and spatial distribution characteristics of particulate matter during the heavy haze-dense fog-haze pollution process from 1 to 12 January 2017 were analyzed with the aid of the data from conventional meteorological observation and environmental monitoring station. Also, the causes of this pollution have been presented considering the terrain effects. The results show that, during the first process (day 1 to day 3), the upwind transports pollutants to Shaoguan, Qingyuan and other provinces, and the local surface wind velocity is lower than 3 m·s−1, which increased concentration of pollutants in the Pearl River Delta region and formed moderate smog and haze pollution. During the second process (day 4 to day 6), the average surface wind speed is lower, and the static wind is lower than 2 m·s−1 with the influence of terrain effect (even lasted for 5-16 hours). Besides, the influence of the strong convergence field of small wind increases the accumulation and convergence of pollutants on the ground, and simultaneously, the thickness of the ground inversion layer is further intensified and the height of the boundary layer is as low as 100 meters. At the same time, the relative humidity increases to nearly 100%, resulting in continuous visibility of fewer than 500 meters for 10 hours. Additionally, it was found that on the day of heavy pollution in Guangzhou, the main source is coarse nuclear particles, and the corresponding actual situation is that the concentration of NO2 was higher than that in other places, which reflects the influence of high concentration of human activities on air pollution. As the northeasterly winds continued to strengthen, pollutants across the region continued to be transported to the Pearl River Delta. Through the convergence of wind farms in Zhaoqing, Foshan, and Guangzhou, cross-regional pollutants and local pollutants emitted by the Pearl River Delta are again accumulated in the pearl river delta central area, forming a third moderate haze pollution process. It is concluded that the influence of strong convergence field of small wind under 2 m·s−1 (even 5-16 hours static wind), the increase of ground temperature inversion, the height of boundary layer as low as 100 m, the increase of near-ground humidity are superposed, and the combined effect of topography are the main meteorological causes of mixed pollution of fog and haze in the Pearl River Delta in winter.

Novel index to comprehensively evaluate air cleanliness: the Clean aIr Index (CII)

Abstract. Air quality on our planet has been changing in particular since the industrial revolution (1750s) because of anthropogenic emissions. It is becoming increasingly important to realize air cleanliness, since clean air is as valuable a resource as clean water. A global standard for quantifying the level of air cleanliness is swiftly required, and we defined a novel concept, namely the Clean aIr Index (CII). The CII is a simple index defined by the normalization of the amount of a set of individual air pollutants. A CII value of 1 indicates completely clean air (no air pollutants), and 0 indicates the presence of air pollutants that meet the numerical environmental criteria for the normalization. In this time, the air pollutants used in the CII were taken from the Air Quality Guidelines (AQG) set by the World Health Organization (WHO), namely O3, particulate matters, NO2, and SO2. We chose Japan as a study area to evaluate CII because of the following reasons: (i) accurate validation data, as the in situ observation sites of the Atmospheric Environmental Regional Observation System (AEROS) provide highly accurate values of air pollutant amounts, and (ii) fixed numerical criteria from the Japanese Environmental Quality Standards (JEQS) as directed by the Ministry of the Environment (MOE) of Japan. We quantified air cleanliness in terms of the CII for the all 1896 municipalities in Japan and used data from Seoul and Beijing to evaluate Japanese air cleanliness. The amount of each air pollutant was calculated using a model that combined the Weather Research and Forecasting (WRF) and Community Multiscale Air Quality (CMAQ) models for 1 April 2014 to 31 March 2017. The CII values calculated by the WRF–CMAQ model and the AEROS measurements showed good agreement. The mean of the correlation coefficient for the CII values of 498 municipalities where the AEROS measurements operated was 0.66±0.05, which was higher than that of the Air Quality Index (AQI) of 0.57±0.06. The CII values averaged for the study period were 0.67, 0.52, and 0.24 in Tokyo, Seoul, and Beijing, respectively; thus, the air in Tokyo was 1.5 and 2.3 times cleaner, i.e., lower amounts of air pollutants, than the air in Seoul and Beijing, respectively. The average CII value for the all Japanese municipalities was 0.72 over the study period. The extremely clean air, CII ≈0.90, occurred in the southern remote islands of Tokyo and to the west of the Pacific coast, i.e., Kochi, Mie, and Wakayama prefectures during summer, with the transport of clean air from the ocean. We presented the top 100 clean air cities in Japan as one example of an application of CII in society. We confirmed that the CII enabled the quantitative evaluation of air cleanliness. The CII can be useful and valuable in various scenarios such as encouraging sightseeing and migration, investment and insurance business, and city planning. The CII is a simple and fair index that can be applied to all nations.

Aircraft landing and takeoff emission impacts on surface O3 and PM2.5 through aerosol direct feedback effects estimated by the coupled WRF-CMAQ model

The aerosol direct feedback effects (ADFEs) are neglected in traditional air quality modeling studies (where meteorology is used as input and not affected by the chemistry and aerosol microphysics) for estimating the impacts of aircraft emissions on air quality. In this study, aircraft landing and take-off (LTO) attributable change of O3 and PM2.5 concentrations through ADFEs for the year 2005 within the contiguous United States (CONUS) are quantified by a coupled meteorology-chemistry modeling system: Weather Research and Forecasting – Community Multi-scale Air Quality (WRF-CMAQ) model. We first quantified the effects of ADFEs of all aerosols within the CONUS (not the effects of aircraft LTO emissions) on surface meteorology and air quality and found that ADFEs changed on average the downward short-wave radiation (SWR), 2-m temperature (T2), planetary boundary layer (PBL) height, O3 and PM2.5 by −7.38 W/m2, −0.47 K, −20.72 m, −0.41 ppb and +0.28 μg/m3 in 2005. We also found a seasonal influence where ADFE-influenced change (decrease) of SWR, T2, PBL, O3 and change (increase) of PM2.5 were higher in summer than in winter. We found that the aircraft LTO emissions’ contribution to domain average surface concentration of O3 and PM2.5 were +0.0065 ppb and +0.0022 μg/m3 respectively when ADFEs are accounted for. The ADFEs decrease aircraft LTO attributable surface O3 and PM2.5 change by 21% and 23% respectively comparing with that without ADFE in 2005. We also found that in both without-and-with ADFE cases, the aircraft LTO emissions increases domain average of O3 from April to October and decreases from November to March showing a strong seasonal pattern. Our modeling study revealed that use of a coupled model with ADFE shows localized changes in air quality by aircraft LTO emissions across the domain which were masked when looking at domain averages for both O3 and PM2.5, and which may be important for accurately quantifying health risk due to air pollution exposures in densely populated areas.

Effects of cool roofs on turbulent coherent structures and ozone air quality in Seoul

Abstract The cool-roof strategy is an effective way of mitigating severe urban heat islands, and its impacts on urban flow and air quality deserve in-depth investigations. This study examines the effects of cool roofs on turbulent coherent structures and ozone air quality in Seoul, South Korea using the Weather Research and Forecasting-Community Multiscale Air Quality (WRF-CMAQ) model with a 50 m horizontal grid spacing. Cool roofs decrease the daily average air temperature, planetary boundary layer (PBL) height, and wind speed in the urban area of Seoul by 0.80 °C, 230 m, and 0.17 m s−1, respectively. Due to the lowered air temperature by cool roofs, the sea breeze and convective structures weaken and eddies at the PBL top appear less frequently. Since high O3 concentration air flows into Seoul by the sea breeze, the weakened sea breeze decreases the daily average O3 concentration near the surface by 3.3 ppb. Air at lower level is transported upward across the PBL top by convective structures and eddies at the PBL top. The transported air at lower level has lower O3 concentration and higher concentrations of O3 precursors than air at upper level. Therefore, the weakened convective structures and less-frequent appearance of eddies at the PBL top by cool roofs weaken the upward transport of O3 precursors at lower level across the PBL top. As a result, the chemical production of O3 and O3 concentration slightly above the PBL top are decreased. An integrated process rate analysis shows that cool roofs weaken the effects of turbulent coherent structures on O3 concentration.