Ammonia emission control in China would mitigate haze pollution and nitrogen deposition, but worsen acid rain.

China has been experiencing fine particle (i.e., aerodynamic diameters ≤ 2.5 µm; PM2.5) pollution and acid rain in recent decades, which exert adverse impacts on human health and the ecosystem. Recently, ammonia (i.e., NH3) emission reduction has been proposed as a strategic option to mitigate haze pollution. However, atmospheric NH3 is also closely bound to nitrogen deposition and acid rain, and comprehensive impacts of NH3 emission control are still poorly understood in China. In this study, by integrating a chemical transport model with a high-resolution NH3 emission inventory, we find that NH3 emission abatement can mitigate PM2.5 pollution and nitrogen deposition but would worsen acid rain in China. Quantitatively, a 50% reduction in NH3 emissions achievable by improving agricultural management, along with a targeted emission reduction (15%) for sulfur dioxide and nitrogen oxides, can alleviate PM2.5 pollution by 11-17% primarily by suppressing ammonium nitrate formation. Meanwhile, nitrogen deposition is estimated to decrease by 34%, with the area exceeding the critical load shrinking from 17% to 9% of China's terrestrial land. Nevertheless, this NH3 reduction would significantly aggravate precipitation acidification, with a decrease of as much as 1.0 unit in rainfall pH and a corresponding substantial increase in areas with heavy acid rain. An economic evaluation demonstrates that the worsened acid rain would partly offset the total economic benefit from improved air quality and less nitrogen deposition. After considering the costs of abatement options, we propose a region-specific strategy for multipollutant controls that will benefit human and ecosystem health.

Enhanced atmospheric oxidation toward carbon neutrality reduces methane's climate forcing.

The hydroxyl radical (OH), as the central atmospheric oxidant, controls the removal rates of methane, a powerful greenhouse gas. It is being suggested that OH levels would decrease with reductions of nitrogen oxides and ozone levels by climate polices, but this remains unsettled. Here, we show that driven by the carbon neutrality pledge, the global-mean OH concentration, derived from multiple chemistry-climate model simulations, is projected to be significantly increasing with a trend of 0.071‒0.16% per year during 2015-2100. The leading cause of this OH enhancement is dramatic decreases in carbon monoxide and methane concentrations, which together reduce OH sinks. The OH increase shortens methane's lifetime by 0.19‒1.1 years across models and subsequently diminishes methane's radiative forcing. If following a largely unmitigated scenario, the global OH exhibits a significant decrease that would exacerbate methane's radiative forcing. Thus, we highlight that targeted emission abatement strategies for sustained oxidation capacity can benefit climate change mitigation in the Anthropocene.

The effect of cross-regional transport on ozone and particulate matter pollution in China: A review of methodology and current knowledge.

China is currently one of the countries impacted by severe atmospheric ozone (O3) and particulate matter (PM) pollution. Due to their moderately long lifetimes, O3 and PM can be transported over long distances, cross the boundaries of source regions and contribute to air pollution in other regions. The reported contributions of cross-regional transport (CRT) to O3 and fine PM (PM2.5) concentrations often exceed those of local emissions in the major regions of China, highlighting the important role of CRT in regional air pollution. Therefore, further improvement of air quality in China requires more joint efforts among regions to ensure a proper reduction in emissions while accounting for the influence of CRT. This review summarizes the methodologies employed to assess the influence of CRT on O3 and PM pollution as well as current knowledge of CRT influence in China. Quantifying CRT contributions in proportion to O3 and PM levels and studying detailed CRT processes of O3, PM and precursors can be both based on targeted observations and/or model simulations. Reported publications indicate that CRT contributes by 40-80 % to O3 and by 10-70 % to PM2.5 in various regions of China. These contributions exhibit notable spatiotemporal variations, with differences in meteorological conditions and/or emissions often serving as main drivers of such variations. Based on trajectory-based methods, transport pathways contributing to O3 and PM pollution in major regions of China have been revealed. Recent studies also highlighted the important role of horizontal transport in the middle/high atmospheric boundary layer or low free troposphere, of vertical exchange and mixing as well as of interactions between CRT, local meteorology and chemistry in the detailed CRT processes. Drawing on the current knowledge on the influence of CRT, this paper provides recommendations for future studies that aim at supporting ongoing air pollution mitigation strategies in China.

Complexity of carbon dioxide flux in urban areas: A comparison with natural surfaces.

The characteristics of turbulent CO2 transport and its dissimilarity with heat and water vapor are investigated over both natural and urban areas. A novel index TS is proposed to effectively quantify the transport similarity between two scalars. By comparison, it is found that the transport of CO2 shows great complexity in urban areas. It is ideal in natural areas that heat, water vapor, and CO2 are efficiently transported by thermal plumes (i.e., the dominant coherent structures under unstable conditions), and that the transport similarity among them becomes increasingly evident with the increase of atmospheric instability. However, in urban areas, the transport of CO2 shows significant dissimilarity from that of heat and water vapor, and it is hard to detect the role of thermal plumes. Furthermore, it is observed that the sector-average CO2 flux in urban areas changes largely with the wind blowing from different urban functional areas. Specially, for a given direction, there might be contrasting characteristics in CO2 transport under different unstable conditions. These features can be explained by the flux footprint. Since the CO2 sources and sinks are distributed heterogeneously in urban areas, the variation of footprint areas with wind direction or atmospheric instability, causes the alternation between source-dominated (i.e., upward) and sink-dominated (i.e., downward) CO2 transport. Therefore, the role of coherent structures in CO2 transport is substantially confused by spatially-confined sources/sinks in urban areas, leading to significant transport dissimilarity between CO2 and heat or water vapor and thus the great complexity in CO2 transport. The findings in this study are helpful to promote the understanding of the global carbon cycle in depth.

Mechanism of haze pollution in summer and its difference with winter in the North China Plain.

Heavy haze pollution usually occurs in winter. However, according to the enhanced atmospheric boundary layer (ABL) field experiments conducted in the North China Plain (NCP) from 17 June to 6 July 2019, heavy haze pollution may also occur in summer, although with a lower probability. Winter haze pollution is significantly affected by adverse boundary layer meteorological conditions, whereas our study shows different mechanisms of summer haze pollution from that of winter. In summer, PM2.5 is distributed uniformly as a thick layer at a lighter pollution level; however, the PM2.5 column content in summer exceeds that in winter, suggesting that the better air quality in summer is mainly due to improved diffusion conditions. In summer, even under haze conditions, the ABL can develop over 1000 m and has a large ventilation similar to clean periods, which indicates both favourable vertical diffusion conditions and advection capability of the summer ABL. Unlike in winter, the heavy haze pollution in summer is often caused by regional transport which is related to local circulation. To explore the influence of different scale systems on summer haze pollution, we applied the spectral analysis method to surface PM2.5 concentrations. Strong periodicity of PM2.5 concentrations is found in 4-9 d and 1 d, corresponding to the impacts of large-scale synoptic system changes and the ABL evolution, respectively. The influence of weather change is much stronger than that of the ABL evolution on PM2.5 concentrations in summer. The resulting changes in PM2.5 concentrations are approximately 45 µg/m3 and 15 µg/m3, respectively. There has been a consensus on the importance of emission control in winter. And this study shows that heavy haze pollution can also occur in summer and regional joint emission control should also be emphasized in summer.

Regional PM2.5 pollution confined by atmospheric internal boundaries in the North China Plain: Analysis based on surface observations.

There are plenty of mesoscale meteorological discontinuities in the atmosphere, acting as atmospheric internal boundaries (AIBs). In conjunction with the atmospheric boundary layer in the vertical direction, they form confined three-dimensional structures that significantly affect air pollution. However, the role of AIBs in regional pollution has not been systematically elucidated. Based on surface observations, this study investigates PM2.5 pollution distributions under the forcing of various AIBs in the North China Plain. A total of 98 regional pollution episodes are identified during the autumn and winter of 2014-2020, and are further classified according to the impact of AIBs. In the pollution formation-maintenance stage, there are three categories. The frontal category (with a frequency of 41%), including the frontal trough type and frontal inverted trough type, displays the most polluted air masses along the mountains. The frontal AIB defines the lateral border of the pollution zone and forms a frontal inversion above, creating a closed and stable structure wherein the highest concentration of PM2.5 accumulates. The wind shear category (29%) is decided by the dynamic convergence AIB, which causes lighter PM2.5 pollution with diverse spatial patterns corresponding to west-southwest shear, southeast-east shear, and south-north shear. The topographic obstruction category (14%) presents as a narrow arc-shaped pollution belt at the foot of the windward mountains, resulting from the cold air damming AIB with dynamical obstruction and thermal stratification. Pollution diffuses in three ways: northwest, west, and northeast, respectively. The first one is the strongest and most frequent (42%), with both strong horizontal wind and vertical mixing. The second category is relatively rare (17%), characterized by foehn-induced active vertical ventilation. The last one is frequent (41%), but relatively weak, mainly relying on horizontal diffusion. Some evolution details of the AIB affecting PM2.5 pollution are also illustrated by a typical case.

Mechanism of the effect of vertically propagating internal gravity waves on turbulence barrier and pollutant diffusion during heavy haze episodes.

During heavy haze episodes, especially the cumulative stage (CS) of pollutants, strong turbulence intermittency and the resulting turbulence barrier effect (i.e., a phenomenon that turbulence at certain heights may disappear forming a laminar flow as if there is a barrier layer impeding the vertical turbulent exchange) suppress the vertical diffusion of pollutants, leading to high PM2.5 concentrations. However, there are still some short-time removal processes of pollutants occurring at different heights in the CS, accompanied with interesting non-simultaneous drop or opposite variation of PM2.5 concentrations at different heights. The ubiquitous internal gravity waves (IGWs) in the stable boundary layer (SBL) may play a critical role in the above situation, as they are closely related to the intermittent turbulence bursts appearing in the persistent weak turbulent motions. In this study, two representative heavy haze pollution cases were chosen to demonstrate the above speculation using five layers of turbulence data, two layers of pressure fluctuations and three layers of PM2.5 concentrations. Results showed that the non-simultaneous drop or opposite variation of PM2.5 concentrations was associated with the destruction of turbulence barrier by the vertical propagation of IGWs. IGWs generated by some certain mechanisms, such as nonhomogeneous terrain and wind shear around the low-level jet (LLJ), can propagate upward or downward with the upward or downward development of the temperature inversion layer. The vertically propagating IGWs then triggered intermittent increasing turbulence layer by layer. Turbulence between layers reconnected sequentially and turbulence barriers were broken in turn. The enhanced turbulent exchange expedited the pollutant diffusion, thus the PM2.5 concentrations at different heights varied non-simultaneously even inversely. This study provides a good explanation for the positive effects of sub-mesoscale motions such as IGWs on triggering intermittent increasing turbulence and facilitating the diffusion of pollutants during heavy haze pollution events.

Unexpected response of nitrogen deposition to nitrogen oxide controls and implications for land carbon sink.

Terrestrial ecosystems in China receive the world's largest amount of reactive nitrogen (N) deposition. Recent controls on nitrogen oxides (NOx = NO + NO2) emissions in China to tackle air pollution are expected to decrease N deposition, yet the observed N deposition fluxes remain almost stagnant. Here we show that the effectiveness of NOx emission controls for reducing oxidized N (NOy = NOx + its oxidation products) deposition is unforeseen in Eastern China, with one-unit reduction in NOx emission leading to only 55‒76% reductions in NOy-N deposition, as opposed to the high effectiveness (around 100%) in both Southern China and the United States. Using an atmospheric chemical transport model, we demonstrate that this unexpected weakened response of N deposition is attributable to the enhanced atmospheric oxidizing capacity by NOx emissions reductions. The decline in N deposition could bear a penalty on terrestrial carbon sinks and should be taken into account when developing pathways for China's carbon neutrality.

Temporal and spatial characteristics of turbulent transfer and diffusion coefficient of PM2.5.

The temporal and spatial characteristics of turbulent transfer and diffusion coefficient of PM2.5 (KC) were investigated by determining the deviation, turbulent flux and form of universal function of PM2.5 mass concentrations. Turbulence and sounding observations from December 8-25, 2019, of three sites, Tuonan, Baoding, and Renqiu stations in the North China Plain were selected. Mean PM2.5 mass fluxes during the intensive observational period of three stations were negative. The spatial distribution of PM2.5 mass flux of three stations showed no obvious tendency. Then, the fact that PM2.5 mass concentrations satisfied the Monin-Obukhov similarity were reconfirmed by examining the relationship between the normalized standard deviation of PM2.5 mass concentrations and stability factor ζ. Thus, the universal functions in the three stations were achieved. The time series and profiles of KC in the three stations were also shown. There was a good inverse correlation between KC and PM2.5 mass concentration which suggested that the influence of turbulent diffusion is remarkably important during observational time even the emission, deposition, secondary transformation can all affect the change of mass concentrations of PM2.5 in the ABL. Changes in KC obviously presented diurnal characteristics. The comparisons of KC and KM and KH suggested that the strength of turbulent PM2.5 mass flux exchange could be weaker or stronger than the strength of turbulent momentum and heat flux exchange at different stations. The magnitude relationship between KC and KH could not be completely determined, so there were limitations in using KH to replace directly KC in the existing numerical weather or climate models. Finally, the spatial distribution of KC at the three stations presented almost symmetrical characteristics from east to west (Tuonan to Renqiu); that is, Baoding always had the lowest KC, and Tuonan and Renqiu stations had higher KCs. The spatial distribution of KC at the three stations corresponded well with that of PM2.5 mass concentrations, and Baoding always had the largest PM2.5 mass concentrations.

Turbulence barrier effect during heavy haze pollution events.

Under low wind speed conditions, the frequent intermittent turbulence phenomenon in the atmospheric boundary layer (ABL) greatly weakens the turbulent diffusion of pollutants to cause the heavy haze events. Turbulence may disappear at certain heights forming a laminar flow as if there is a barrier layer hindering the transmission up and down during the heavy haze periods. The turbulent data at five layers and PM2.5 (fine particular matter with a diameter smaller than 2.5 µm) concentration at two levels were used to discuss the barrier that is called the barrier effect vividly. The results revealed that the changes in the PM2.5 concentration at different heights corresponded excellently with the change in vertical turbulence barrier effect. This work explains the physical mechanism responsible for the accumulation of pollutants in heavy pollution events and the influence of turbulent diffusion conditions on the distribution of the PM2.5 concentration.

Electroresistance effect in oxygen-deficient La0.8Ba0.2MnO3-δ thin films.

Electroresistance (ER) has been intensively studied in low- and intermediate-bandwidth manganites, which possess phase separation characteristics. As for the Sr- and Ba-doped large-bandwidth manganites, however, few results about ER have been reported so far. Here we report ER effect in oxygen-deficient La0.8Ba0.2MnO3-δ thin films, which were obtained by applying a large electric current (33 mA) to the pristine films in vacuum. While the pristine film displays a negligible change in resistivity with respect to the test current, the oxygen-deficient film shows significant ER effect, i.e. ER ratio of -22% at 260 K under a test current of 0.3 mA. By gradually restoring oxygen content in the films, it is found that the ER effect is closely related to the residual resistivity at low temperatures, demonstrating the key role of grain boundaries. Furthermore, the residual resistivity can readily be tuned by heating the oxygen-deficient films in air, suggesting strong oxygen activity in the grain boundaries. The magnetoresistance (MR) data show current dependent feature, also revealing the role of grain boundaries. At 40 K, the MR ratio of the 100 °C restored film under 30 kOe increases from -15% to -25% when decreasing the test current from 1 to 10-3 mA. The large ER effect in the oxygen-deficient films is discussed based upon the conductive filament picture in grain boundaries. Our approach to controlling the ER effect through oxygen deficiency makes oxide films more promising for potential applications in the memristive devices and neuomorphic computing.

Determining the fluctuation of PM2.5 mass concentration and its applicability to Monin-Obukhov similarity.

In this manuscript, a method based on the nonlinear relationship between mass concentration of fine-particle and visibility is introduced to obtain the high-frequency mass concentration and vertical mass flux of PM2.5 during a haze pollution process. The high-frequency sampling was obtained using a visibility meter upgraded by our research group. To make the results more reliable, we restricted the service conditions of this method, that is, RH < 80% and the limited visibility value (15 km herein). The method was applied to a period of pollution from 27 December 2018 to 7 January 2019 at the Pingyuan site, located in the North China Plain. A 1-Hz fluctuation of the mass concentration of PM2.5 was obtained, as was the vertical turbulent flux of the mass concentration of PM2.5. The mean flux of the mass concentration of PM2.5 for the entire 13 days was 0.022 µg m-2 s-1. The turbulent fluxes in five short-term pollution cases lasting for several hours or several days were -0.015 µg m-2 s-1, -0.03 µg m-2 s-1, 0.053 µg m-2 s-1, 0.023 µg m-2 s-1 and 0.075 µg m-2 s-1. Then, the relationship between the absolute value of the vertical flux of PM2.5 mass concentration w'c'¯ and many turbulent variables showed that the vertical flux decreases exponentially with the increase of variables such as turbulent kinetic energy, standard deviation σu, σw, horizontal mean wind U, momentum flux τ and heat flux H. However, the absolute value of the vertical flux w'c'¯ was not affected by the value of the mass concentration of PM2.5. The turbulent characteristics of PM2.5 were also analysed. Under unstable conditions, the normalized standard deviation of PM2.5σc/C∗=20.07-zL-1/3, where c is mass concentration of PM2.5, C∗ is scale of PM2.5. The normalized spectra (nSPM2.5(n)/σc2) followed the Kolmogorov form, n-2/3, where n is frequency, and the co-spectra of deviation of vertical wind speed and mass concentration of PM2.5 (nCwPM2.5(n)/σwσc) was n-4/3. Finally, the universal function of the mass concentration of PM2.5 was calculated. The fitted curves for unstable and stable conditions were φczL=4.01-9.8zL-1/2 and φczL=4.01+1.2zL. Although the observed values of the universal function of the mass concentration of PM2.5 were widely scattered with respect to z/L, the trend is obvious. The measurements show that PM2.5 has similar properties to regular scalars such as potential temperature or water vapor. Particularly, the measurement shows a good trend to satisfy the traditional Monin-Obukhov similarity theory (MOST). However, further experiments should be implemented for verification and to provide a more accurate coefficient.

Temperature inversions in severe polluted days derived from radiosonde data in North China from 2011 to 2016.

Temperature inversion tends to inhibit the transfer of momentum, heat and moisture in the atmospheric boundary layer, which is often accompanied by severe air pollution. Recently, severe haze pollution has frequently occurred in North China. In this study, the characteristics of temperature inversion on severe polluted days (SPDs) in Beijing were investigated by using radiosonde data with standard pressure levels from 2011 to 2016. Both surface-based inversion (SI) and elevated inversion (EI) were analyzed. 93% of the SPDs were accompanied by temperature inversion, most of which occurred in wintertime. Annual frequency of SI (FSI) and EI (FEI) showed slight fluctuations with mean value of 0.18 and 0.67, respectively. Overall, the annual SI was stronger and deeper than annual EI. Seasonally, the SI was most frequent (0.39) in autumn, in contrast to EI that occurred most frequently (0.95) in summer. Both SI and EI were weakest in summer and strongest in winter. Average monthly SI strength was about 0.38 °C in summer and 2.40 °C in winter, average monthly EI strength was about 0.64 °C in summer and 2.20 °C in winter. The average monthly SI and EI were deepest in winter and shallowest in summer. SI depth were 778 m and 221 m in winter and summer, EI were 630 m and 336 m in winter and summer. The substantially strong liner relationship was found between seasonal inversion strength and PM2.5 concentration, and the inversion strength was found to be better compared with the inversion depth at predicting the PM2.5 concentration during SPDs. Obvious lower air outflow and turbulent kinetic energy were found in SPDs compared to non-SPDs, which indicated weaker turbulence in SPDs. Future efforts should focus on accurate model simulations of temperature inversions in SPDs.

Measurements of atmospheric aerosol vertical distribution above North China Plain using hexacopter.

In this study, a novel setup was developed to measure the vertical profile of particle number size distribution (PNSD) and meteorological parameters by using hexacopter equipped with a portable instrument package including a custom-built scanning mobility particle sizer, an optical particle counter and temperature and relative humidity (RH) sensors. By using this setup, a field experiment was carried out to investigate the vertical profiles of RH, temperature, PNSD ranged from 8 to 245 nm, and particle number concentration with the diameter ranges of 0.3-0.5 µm and 0.5-1.0 µm from the ground up to 300 m above ground level in a rural site of the North China Plain. New particle formation (NPF) event in a vertical scale was observed during the daytime cruises. The newly formed particles showed a heterogeneous vertical distribution, indicating the inhomogeneous occurrence of the NPF event vertically. During the daytime, the vertical variations in number concentration of particles larger than 0.3 µm was not obvious, while, showed a tendency to decrease associated with the increasing altitude in the evening. The newly-developed unmanned aerial vehicle research platform could be applied to study the vertical NPF events and transport processes of air pollutants within the atmospheric boundary layer and urban canopy, and to monitor source emissions with the purpose of environment management.

Identifying anthropogenic and natural influences on extreme pollution of respirable suspended particulates in Beijing using backward trajectory analysis.

In China, daily respirable suspended particulate (RSP, particles with aerodynamic diameters less than 10 microm) concentrations exceeding 420 microg m(-3) are considered "hazardous" to health. These can lead to the premature onset of certain diseases and premature death of sick and elderly people; even healthy people are warned to avoid outdoor activity when RSP concentrations are high. Such high pollution levels are defined as extreme RSP pollution events. Recent epidemiological studies have shown that a distinct difference exists between the health effects caused by natural sources and anthropogenic sources, mandating knowledge of the source of extreme RSP pollution. Twenty-six extreme RSP pollution events were recorded in Beijing from January 2003 to December 2006. The HYSPLIT4 (Hybrid Single Particle Lagrangian Integrated Trajectory) model (Version 4) was used to discriminate the sources of these extreme RSP pollution events. The model found that twelve events were caused from natural sources (dust storms), nine events from anthropogenic sources (e.g., vehicles and industrial activities, etc.) under quasi-quiescent weather, and five events were from mixed causes. Identifying such events will be valuable in epidemiological studies on air pollution in Beijing.

A health-based assessment of particulate air pollution in urban areas of Beijing in 2000-2004.

Particulate air pollution is a serious problem in Beijing. The annual concentration of particulate matter with aerodynamic diameter less than 10 microm (PM(10)), ranging from 141 to 166 microg m(-3) in 2000-2004, could be very harmful to human health. In this paper, we presented the mortality and morbidity effects of PM(10) pollution based on statistical data and the epidemiological exposure-response function. The economic costs to health during the 5 years were estimated to lie between US$1670 and $3655 million annually, accounting for about 6.55% of Beijing's gross domestic product each year. The total costs were apportioned into two parts caused by: the local emissions and long-range transported pollution. The contribution from local emissions dominated the total costs, accounting on average for 3.60% of GDP. However, the contributions from transported pollution cannot be neglected, and the relative percentage to the total costs from the other regions could account for about 45%. An energy policy and effective measures should be proposed to reduce particulate matter, especially PM(2.5) pollution in Beijing to protect public health. The Beijing government also needs to cooperate with the other local governments to reduce high background level of particulate air pollution.

Analysis of the transport pathways and potential sources of PM10 in Shanghai based on three methods.

In this study, we investigated the transport pathways and potential sources of PM(10) in Shanghai based on PM(10) monitoring data recorded from 2006 to 2009 using three methods: backward trajectory cluster analysis, trajectory sector analysis (TSA) and potential source contribution function (PSCF). Seven clusters were generated from the backward trajectory cluster, and two potential sources were identified from the PSCF method. Among the seven clusters, three northerly clusters corresponded to the winter monsoon. The northerly air flow transported high-concentration PM(10) that had been emitted from northwestern sources, including Hebei, Shandong, Anhui and Jiangsu to Shanghai in winter and spring. The other three southerly clusters were associated with the summer monsoon caused by the Indian low and the Subtropical high over the western Pacific Ocean controlling the weather patterns of the eastern coastal area in summer. Corresponding to the southerly path, the PSCF method also identified a southwestern source including Zhejiang, Jiangxi and Fujian. The remaining eastern cluster, which represented the transition of monsoons, did not contribute much to the PM(10) concentration in Shanghai. According to the results of TSA, the relative PM(10) contribution to Shanghai of the northwestern source was approximately twice that of the southwestern source.

Economic assessment of the health effects related to particulate matter pollution in 111 Chinese cities by using economic burden of disease analysis.

With China's rapid economic development, environmental problems have become more and more serious. Particulate air pollution is terrible in cities with large and dense population. It may lead to adverse health effects and economic costs. In this study, we calculated the health effects of pollution caused by particulate matter with aerodynamic diameters less than 10 microm (PM(10)) in 111 Chinese cities in 2004 according to statistical data and epidemiological exposure-response functions. Using economic burden of disease analysis, an economic assessment of these health risks was also presented. In contrast to many previous studies that have examined individual cities, this study covered most large and medium-sized cities in China, which accounted for more than 70% gross domestic product (GDP) of China in 2004. The total economic cost caused by PM(10) pollution was estimated as approximately USD 29,178.7 million. Mega cities such as Beijing, Shanghai, and Tianjin contribute relatively more to the total costs. The results will help policy makers in formulating more effective countermeasures and increasing public awareness to enhance environmental protection.

[A backward trajectory inversion model for methane emission over Beijing area].

A Backward trajectory inversion model was established for estimating methane emission over Beijing area, with in situ data of methane measurement. The model was systematically tested for its ability on reflecting diffusion processes in atmospheric boundary layers. In specific condition, the model acted excellently on calculating emission strength and distribution over an area of 100 km scale. By applying to Beijing area with 9 days real methane data and practice meteorological data, the model showed source patches in this area with emission rate 0.0066-0.026 mg/(m2.s), which was correspond well to rice fields emission, the major methane source, in this region. Model results of source patches also showed certain spatial correlation with rice fields distribution.