Seasonal and Intraseasonal Variations of East Asian Summer Monsoon Precipitation Simulated by a Regional Air-Sea Coupled Model

The performance of a regional air-sea coupled model, comprising the Regional Integrated Environment Model System （RIEMS） and the Princeton Ocean Model （POM）, in simulating the seasonal and intraseasonal variations of East Asian summer monsoon （EASM） rainfall was investigated. Through comparisons of the model results among the coupled model, the uncoupled RIEMS, and observations, the impact of air-sea coupling on simulating the EASM was also evaluated. Results showed that the regional air sea coupled climate model performed better in simulating the spatial pattern of the precipitation climatology and produced more realistic variations of the EASM rainfall in terms of its amplitude and principal EOF modes. The coupled model also showed greater skill than the uncoupled RIEMS in reproducing the principal features of climatological intraseasonal oscillation （CISO） of EASM rainfall, including its dominant period, intensity, and northward propagation. Further analysis indicated that the improvements in the simulation of the EASM rainfall climatology and its seasonal variation in the coupled model were due to better simulation of the western North Pacific Subtropical High, while the improvements of CISO simulation were owing to the realistic phase relationship between the intraseasonal convection and the underlying SST resulting from the air-sea coupling.

Does regional air–sea coupling improve the simulation of the summer monsoon over the western North Pacific in the WRF4 model?

A new regional coupled ocean–atmosphere model,WRF4-LICOM,was used to investigate the impacts of regional air–sea coupling on the simulation of the western North Pacific summer monsoon(WNPSM),with a focus on the normal WNPSM year 2005.Compared to WRF4,WRF4-LICOM improved the simulation of the summer mean monsoon rainfall,circulations,sea surface net heat fluxes,and propagations of the daily rainband over the WNP.The major differences between the models were found over the northern South China Sea and east of the Philippines.The warmer SST reduced the gross moist stability of the atmosphere and increased the upward latent heat flux,and then drove local ascending anomalies,which led to the increase of rainfall in WRF4-LICOM.The resultant enhanced atmospheric heating drove a low-level anomalous cyclone to its northwest,which reduced the simulated circulation biases in the stand-alone WRF4 model.The local observed daily SST over the WNP was a response to the overlying summer monsoon.In the WRF4 model,the modeled atmosphere exhibited passive response to the underlying daily SST anomalies.With the inclusion of regional air–sea coupling,the simulated daily SST–rainfall relationship was significantly improved.WRF4-LICOM is recommended for future dynamical downscaling of simulations and projections over this region.

Effect of Meteorological Data Assimilation on Regional Air Quality Forecasts over the Korean Peninsula

The Weather Research and Forecasting model coupled with Chemistry(WRF-Chem),a type of online coupled chemistry-meteorology model(CCMM),considers the interaction between air quality and meteorology to improve air quality forecasting.Meteorological data assimilation(DA)can be used to reduce uncertainty in meteorological field,which is one factor causing prediction uncertainty in the CCMM.In this study,WRF-Chem and three-dimensional variational DA were used to examine the impact of meteorological DA on air quality and meteorological forecasts over the Korean Peninsula.The nesting model domains were configured over East Asia(outer domain)and the Korean Peninsula(inner domain).Three experiments were conducted by using different DA domains to determine the optimal model domain for the meteorological DA.When the meteorological DA was performed in the outer domain or both the outer and inner domains,the root-mean-square error(RMSE),bias of the predicted particulate matter(PM)concentrations,and the RMSE of predicted meteorological variables against the observations were smaller than those in the experiment where the meteorological DA was performed only in the inner domain.This indicates that the improvement of the synoptic meteorological fields by DA in the outer domain enhanced the meteorological initial and boundary conditions for the inner domain,subsequently improving air quality and meteorological predictions.Compared to the experiment without meteorological DA,the RMSE and bias of the meteorological and PM variables were smaller in the experiments with DA.The effect of meteorological DA on the improvement of PM predictions lasted for approximately 58-66 h,depending on the case.Therefore,the uncertainty reduction in the meteorological initial condition by the meteorological DA contributed to a reduction of the forecast errors of both meteorology and air quality.

A Stackelberg game model with tax for regional air pollution control

The command-and-control regulation is likely inefficient and costly.This study investigates a regional pollution control scheme with tax(RPCST)under which the central government sets the tax rate under a given pollutant reduction quota and local governments determine their pollution removal rates based on the central government’s policy.First,a one-leader-multi-follower(OLMF)Stackelberg game model is formulated,in which the central government is the leader and the local governments are the followers.Then,a procedure based on bilevel programming and relaxation method is applied to solve the OLMF model.Finally,a case study analyzing the SO2 reduction of the Yangtze River Delta in China is conducted to demonstrate the effectiveness of the RPCST.The results show that RPCST works better than the current command-andcontrol scheme.Our analysis provides a guideline for governments to design optimal tax schemes to effectively solve the regional air pollution crisis.

Environmental options of local governments for regional air pollution joint control:application of evolutionary game theory

Apanage management is currently the main method used to control air pollution in China,but it has proved to be inefficient for controlling transboundary air pollution.As a result,China’s central government is demanding joint control of regional air pollution.From the perspective of cooperation benefits,we adopt the evolutionary game theory(EGT)to analyse evolutionary trends of regional authorities’behaviours and their stable strategy in the campaign for joint control of regional air pollution.A case study,the intergovernmental cooperation management for‘APEC Blue’,is taken to illustrate the intergovernmental game.The result shows that an evolutionarily stable strategy(ESS)of‘joint control’for local governments depends on individual region’s benefits and collaboration revenues.Local governments should be encouraged in collaborating with their neighbouring governments,because a certain amount of transaction costs will not undermine their cooperation.With regards to the case study,joint control through executive orders is unpractical in the Beijing–Tianjin–Hebei region.‘APEC Blue’can only be temporary and the failure of such collaboration for long-term regional air pollution control is inevitable because of its high control costs,economic loss,transaction costs and low common profits.

Spatial-temporal variations in near-surface soil freeze-thaw cycles in the source region of the Yellow River during the period 2002–2011 based on the Advanced Microwave Scanning Radiometer for the Earth Observing System(AMSR-E) data

Detecting near-surface soil freeze-thaw cycles in high-altitude cold regions is important for understanding the Earth＇s surface system, but such studies are rare. In this study, we detected the spatial-temporal variations in near-surface soil freeze-thaw cycles in the source region of the Yellow River（SRYR） during the period 2002–2011 based on data from the Advanced Microwave Scanning Radiometer for the Earth Observing System（AMSR-E）. Moreover, the trends of onset dates and durations of the soil freeze-thaw cycles under different stages were also analyzed. Results showed that the thresholds of daytime and nighttime brightness temperatures of the freeze-thaw algorithm for the SRYR were 257.59 and 261.28 K, respectively. At the spatial scale, the daily frozen surface（DFS） area and the daily surface freeze-thaw cycle surface（DFTS） area decreased by 0.08% and 0.25%, respectively, and the daily thawed surface（DTS） area increased by 0.36%. At the temporal scale, the dates of the onset of thawing and complete thawing advanced by 3.10（±1.4） and 2.46（±1.4） days, respectively; and the dates of the onset of freezing and complete freezing were delayed by 0.9（±1.4） and 1.6（±1.1） days, respectively. The duration of thawing increased by 0.72（±0.21） day/a and the duration of freezing decreased by 0.52（±0.26） day/a. In conclusion, increases in the annual minimum temperature and winter air temperature are the main factors for the advanced thawing and delayed freezing and for the increase in the duration of thawing and the decrease in the duration of freezing in the SRYR.

Long-term variation and evaluation of air quality across Hong Kong region

Study of Air Quality Objectives(AQOs)and long-term changes of air pollution plays a decisive role in formulating and refining pollution control strategies.In this study,10-year variations of six major air pollutants were analyzed at seven monitoring sites in Hong Kong region.The continuous decrease of annual averaged concentrations of NO_(2),SO_(2),CO,PM_(2.5)and PM_(10)and numbers of days with severe pollution conditions validated the efficiency of the series of air pollution control schemes implemented by the Hong Kong region government.However,there is still a big gap to meet the ultimate targets described by the World Health Organization.Besides,the concentration of O_(3)at roadside and urban stations increased by 135%±25%and 37%±18%from 2011 to 2020,respectively,meanwhile the highest 8 hr averaged O_(3)concentration was observed as 294μg/m^(3)at background station in 2020,which pointed out the increasing ozone pollution in Hong Kong region.There was a great decrease in the annual times of air quality health index(AQHI)laying in“high”,“very high”and“serious”categories from 2011 to 2020 with the decrease rate of 89.70%,91.30%and 89.74%at roadside stations,and 79.03%,95.98%and 72.73%at urban stations,respectively.Nevertheless,the number of days categorized as“high”or above at roadside station was twice more than that in the urban station during the past ten years.Thus,more policies and attentions should be given to the roadside air quality and its adverse health effect to pedestrians on street.

Air quality management in China:Issues,challenges,and options

This article analyzed the control progress and current status of air quality,identified the major air pollution issues and challenges in future,proposed the long-term air pollution control targets,and suggested the options for better air quality in China.With the continuing growth of economy in the next 10–15 years,China will face a more severe situation of energy consumption,electricity generation and vehicle population leading to increase in multiple pollutant emissions.Controlling regional air pollution especially fine particles and ozone,as well as lowering carbon emissions from fossil fuel consumption will be a big challenge for the country.To protect public health and the eco-system,the ambient air quality in all Chinese cities shall attain the national ambient air quality standards （NAAQS） and ambient air quality guideline values set by the World Health Organization （WHO）.To achieve the air quality targets,the emissions of SO 2,NOx,PM 10,and volatile organic compounds （VOC） should decrease by 60%,40%,50%,and 40%,respectively,on the basis of that in 2005.A comprehensive control policy focusing on multiple pollutants and emission sources at both the local and regional levels was proposed to mitigate the regional air pollution issue in China.The options include development of clean energy resources,promotion of clean and efficient coal use,enhancement of vehicle pollution control,implementation of synchronous control of multiple pollutants including SO 2,NOx,VOC,and PM emissions,joint prevention and control of regional air pollution,and application of climate friendly air pollution control measures.

Developing chemical signatures of particulate air pollution in the Pearl River Delta region, China

PM 2.5 samples were collected in a regional sampling network with three sites in Hong Kong and four sites in the adjacent inland Pearl River Delta （PRD） or Guangdong Province during four months/seasons from 2002–2003. Trans-boundary transport between Hong Kong and the inland PRD is inevitable under the influence of Asian monsoon. In summer, Hong Kong serves as the upwind site of the inland PRD while during other seasons it is under the influence of continental emissions. Previous studies have recognized the importance of using chemical signatures to differentiate local vs. regional contributions to air pollutants in Hong Kong such as the CO/NOx ratio, ratios of different VOC species. In this study, detailed chemical speciation by gas chromatography-mass spectrometry was performed with PM 2.5 samples to identify new chemical signatures to distinguish aerosols in Hong Kong from those from the inland PRD. Since Hong Kong is not influenced by the continental emissions from the inland PRD during summer, comparison focused on chemical data obtained from this season for chemical signatures. The new ratios developed from the current study include LCPI/HCPI ratio of alkanes （0.39 ± 0.02 in Hong Kong vs. 0.78 ± 0.08 in the inland PRD）, pyrene to benzo[ghi]perylene ratio （0.97 ± 0.21 in Hong Kong compared to 0.20 ± 0.06 in the inland PRD）, and the ratio of 1,2-benzenedioic acid to 1,4-benzenedioic acid （1.8 ± 0.1 in Hong Kong vs. 0.6 ± 0.05 in the inland PRD）. Results from this study also revealed that Hong Kong was impacted by ship emissions as reflected by substantially high V/Ni ratio （9 ± 2） while this ratio was about 1–2 at all sites in the inland PRD, which is very close to typical ratios from residual oil combustion.

Space view of the decadal variation for typical air pollutants in the Pearl River Delta （PRD） region in China

The Pearl River Delta （PRD） is one of the most industrialized, urbanized and populated regions in China, and thus has been long suffering from severe air pollutions. Space data provide a unique perspective for investigating the atmospheric environment at a regional scale. By utilizing multiple satellite retrievals from 2005 to 2013, this study presented, for the first time, the spatial patterns and temporal trends of typical air pollutants over PRD and its vicinity. As viewed from space, aerosol optical depth （AOD）, NO2 and SO2 all had their higher values at the central part of PRD, and showed clear descending gradients as moving to the outskirt of this region. As to the inter-annual variation, all these pollutants had decreasing trends in PRD during the study period, which generally agreed with the relevant in situ measurements. However, the satellite retrievals differed from ground measurements when addressing NO2 and SO2 in the vicinity of PRD. This work also provides the inter-comparison among PRD and three other metropolitan clusters in China： PRD had relatively high AOD, moderate NO2 and low SO2 levels, and it was the only region achieving the effective reduction of NO2 and SO2 during last decade. Unlike the previous three pollutants, HCHO observed by satellite showed very special patterns： it had a relatively homogeneous spatial distribution over both of PRD and its vicinity, and presented an opposite increasing trend from 2005 to 2010. Moreover, PRD had the highest HCHO level among all the metropolitan clusters, hinting a considerable contribution of biogenic origins of HCHO in PRD.

Multiyear analyses of ground-level air contaminants over Paris metropolitan region using real-time observations and air mass backward trajectories

For the years 2008-2013, particles of diameter 〈 10 and 2.5 μm （PM10 and PM2.5, respectively）, NOx, SO2, and O3 concentrations at urban, suburban, rural, and traffic sites in the Paris metropolitan area were analyzed. Strong spatial variability at traffic and rural sites and relatively uniform profiles at urban and suburban sites for PM10, PM2.5, and O3 were observed. The O3 weekend effect was induced by lower NOx emissions during the weekend, and favored volatile organic compounds （VOCs）-limited atmospheric conditions. In conJunction with low ambient temperature, these conditions could also favor increased formation of secondary particulate nitrates in winter. Winter air pollution events were associated with multiple pollutants, whereas those observed in spring were caused by high PM10 and PM2.5 levels. Back- ward trajectory analyses showed the contribution of sources in Western and Central Europe on days with high PM10, PM2.5, and O3, and a local/national comvonent for NOx and SO2.

Synergistic impacts of anthropogenic and biogenic emissions on summer surface O_3 in East Asia

A factor separation technique and an improved regional air quality model （RAQM） were applied to calculate synergistic contributions of anthropogenic volatile organic compounds （AVOCs）,biogenic volatile organic compounds （BVOCs） and nitrogen oxides （NOx） to daily maximum surface O3（O3DM） concentrations in East Asia in summer （June to August 2000）.The summer averaged synergistic impacts of AVOCs and NOx are dominant in most areas of North China,with a maximum of 60 ppbv,while those of BVOCs and NOx are notable only in some limited areas with high BVOC emissions in South China,with a maximum of 25 ppbv.This result implies that BVOCs contribute much less to summer averaged O3DM concentrations than AVOCs in most areas of East Asia at a coarse spatial resolution （1×1） although global emissions of BVOCs are much greater than those of AVOCs.Daily maximum total contributions of BVOCs can approach 20 ppbv in North China,but they can reach 40 ppbv in South China,approaching or exceeding those in some developed countries in Europe and North America.BVOC emissions in such special areas should be considered when O3 control measures are taken.Synergistic contributions among AVOCs,BVOCs and NOx significantly enhance O3 concentrations in the Beijing-Tianjin-Tangshan region and decrease them in some areas in South China.Thus,the total contributions of BVOCs to O3DM vary significantly from day to day and from location to location.This result suggests that O3 control measures obtained from episodic studies could be limited for long-term applications.