Contribution of Northeastern Asian stratospheric warming to subseasonal prediction of the early winter haze pollution in Sichuan Basin, China.

Predicting the interannual (IA) variability of the early winter haze pollution (HP) in Sichuan Basin (SCB) and analyzing the contribution of the complex terrain are of great significance and challenge as well. For one thing, this study finds that the second mode of early winter IA variations of the HP in Central Eastern China (CEC) is dominated by the pollutant pattern of SCB, characterized by an extreme value center located over SCB. For another, the mode accounts for 15.1% of the total variance and well represents the IA variability of the areal mean HP in SCB. Furthermore, the anomalous circulations conducive to the severe HP in SCB consist of the strengthened (weakened) north (south) branch of Tibetan Plateau (TP) bypassing westerlies at 700 hPa, enhanced descents over SCB, and a strong inversion layer at 750 hPa, suppressing the precipitation and the development of the planetary boundary layer (PBL) in SCB. As a result, dispersion and wet deposition conditions are poor, favorable for the occurrence of the HP in SCB. Observed evidences show that the regional warming in the upper stratosphere over Northeastern Asia in November can be a possible subseasonal precursory signal for the SCB mode. Above all, the warming at upper stratosphere will propagate downward and arrive at the lower stratosphere in 3-4 weeks. Followed by the southward and downward propagation through the longitudinal tilted isothermal surface, the warming arrives at the tropospheric mid-low latitudes in 2-3 weeks, leading to anomalous warming and descents over TP and SCB. Consequently, SCB sees the aforementioned three-dimensional anomalous circulations. For this reason, a physical-empirical subseasonal prediction model for the IA variability of the early winter HP in SCB is established in terms of the regional stratospheric temperature in November. Apparently, the hindcast shows a promising prediction skill.

A comparison of haze pollution variability in China using haze indices based on observations.

Defining the occurrence and degree of haze pollution (HP) in China has been extremely controversial in recent years. This study elaborates on the meanings of ten most frequently used HP indices based on observation and compares their performance in measuring the HP variability in China concerning the annual cycle, interannual variations and interdecadal trends for the period of 1980-2015. It is found that these indices exhibit a high consistency in South China, while apparent discrepancies exist in North Chinam, Sichuan Basin and the Yangtze River Delta. Observed surface PM2.5 concentration is utilized to assess the performance of the ten HP indices. Results show that some of these indices can well capture primary features of the HP in some specific area, only these defined by a daily averaged value or 14:00 single threshold value have relatively good performance in the whole Eastern China. Such four indices (HPI2/HPI3/HPI4/HPI5) exhibit high consistence in depicting spatial-temporal variations (from annual cycle to interannual variability) of surface PM2.5 concentration over China, meaning they can highlight the significance of high aerosol concentration in triggering HP events. Moreover, further analysis suggest that the contributions of weather conditions to the occurrence and degree of the HP is becoming more in China, especially in the NC, SCB, YRD regions. These results provide useful guidance for monitoring variations of the HP in China and for making long-term environmental regulatory and controlling policy.

The impact of Climate Change on the Western Pacific Subtropical High and the related ozone pollution in Shanghai, China.

Severe ozone (O3) episodes occur frequently in Shanghai during late-summers. We define geopotential height averaged over the key area region (122.5°E-135°E, 27.5°N -35°N) at 500 hPa as a WPSH_SHO3 index which has high positive correlation with surface O3 concentration in Shanghai. In addition, the index has a significant long-term increasing trend during the recent 60 years. Analysis shows the meteorological conditions under the strong WPSH_SHO3 climate background (compared to the weak background) have several important anomalies: (1) A strong WPSH center occurs over the key area region. (2) The cloud cover is less, resulting in high solar radiation and low humidity, enhancing the photochemical reactions of O3. (3) The near-surface southwesterly winds are more frequent, enhancing the transport of upwind pollutants and O3 precursors from polluted regions to Shanghai and producing higher O3 chemical productions. This study suggests that the global climate change could lead to a stronger WPSH in the key region, enhancing ozone pollution in Shanghai. A global chemical/transport model (MOZART-4) is applied to show that the O3 concentrations can be 30 ppbv higher under a strong WPSH_SHO3 condition than a weak condition, indicating the important effect of the global climate change on local air pollution in Shanghai.

Role of climate anomalies on decadal variation in the occurrence of wintertime haze in the Yangtze River Delta, China.

The wintertime haze day (HD) in the Yangtze River Delta (YRD) region of China shows a significant upward trend during the past decades due to the rapid industrialization and urbanization. Besides the enhanced anthropogenic emission, climate change also plays the important role in the long term HD variations. In this study, the significant decadal variation of wintertime HD during the period 1960-2012 in YRD is examined by the empirical orthogonal function (EOF) analysis, featured as less HD occurrence before 1980 and more occurrence after 2000. The numerical simulations by the global transport and chemical model (Model for Ozone and Related chemical Tracers, MOZART) with the same emission inventory suggest 8.4% enhancement of wintertime PM2.5 (particulate matter with the equivalent diameter of air dynamics less than or equal to 2.5µm) mass concentration in YRD during 2001-2009 compared with that during 1971-1979 attributed to meteorological changes, indicating the significant effect of climate anomaly on the decadal variations of wintertime HD. Through the composite analysis on the atmospheric dynamical and thermal conditions based on the reanalysis data, the faster warming in the lower and middle troposphere over the continent in the recent decade is suggested to be important for the out-of-phase decadal HD variation in YRD. The thermal anomaly not only reverses the zonal thermal difference of land-sea to stimulate the anomalous southerlies over YRD leading to reduced prevailing north wind in winter, but also develops the deep inversion below the mid-troposphere to enhance the atmospheric stability. As a result, more frequent and persistent air stagnations in recent decade are expected for the reduction of atmospheric horizontal dispersion and vertical diffusion capacity leading to more occurrence of wintertime HD in YRD.

Long-term trend of O3 in a mega City (Shanghai), China: Characteristics, causes, and interactions with precursors.

In recent years, ozone (O3) is often the major pollutant during summertime in China. In order to better understand this problem, a long-term measurement of ozone (from 2006 to 2015) and its precursors (NOx and VOCs) as well as the photochemical parameter (UV radiation) in a mega city of China (Shanghai) is analyzed. The focus of this study is to investigate the trend of O3 and the causes of the O3 trend in large cities in China. In order to understand the relationship between the O3 precursors and O3 formation, two distinguished different sites of measurements are selected in the study, including an urbanization site (XJH-Xujiahui) and a remote site (DT-Dongtan). At the XJH site, there are high local emissions of ozone precursors (such as VOCs and NOx), which is suitable to study the effect of O3 precursors on O3 formation. In contrast, at the DT site, where there are low local emissions, the measured result can be used to analyze the background conditions nearby the city of Shanghai. The analysis shows that there were long-term trends of O3 and NOx concentrations at the urban site (XJH) from 2006 to 2015 (O3 increasing 67% and NOx decreasing 38%), while there were very small trends of O3 and NOx concentrations at the background site (DT). The analysis for causing the O3 trend suggests that (1) the large O3 increase at the urban area (XJH) was not due to the regional transport of O3; (2) the measurement of solar radiation had not significant trend during the period, and was not the major cause for the long-term O3 trend; (3) the measurement of VOCs had small change during the same period, suggesting that the trend in NOx concentrations at the urban site (XJH) was a major factor for causing the long-term change of O3 at the urban area of Shanghai. As a result, the O3 and NOx concentrations from 2006 to 2015 at the urban area of Shanghai were strongly anti-correlated, suggesting that the extremely high NOx concentration in the urban area depressed the O3 concentrations. It is interesting to note that the anti-correlation between O3 and NOx was in an un-linearly relationship. Under high O3 concentration condition, the ratio of Δ[O3]/Δ[NOx] was as large as -1.5. In contrast, under low O3 concentrations, the ratio of Δ[O3]/Δ[NOx] was only -0.2. This result suggested that when O3 concentration was high, it was more sensitive to NOx concentration, while when O3 concentration was low, it was less sensitive to NOx concentration. This study provides useful insights for better understanding the causes of the long-term-trend of regional O3 pollution nearby Shanghai, and has important implication for air pollution control in large cities in China. Due to the fact that NOx and VOCs are not only precursors for O3, but also are important precursors for particular matter (PM). If reduction of NOx leads to decrease in PM, but increase in O3, the NOx emission control become a very complicated issue and need to carefully design a comprehensive control method.

Development of an on-line source-tagged model for sulfate, nitrate and ammonium: A modeling study for highly polluted periods in Shanghai, China.

An on-line source-tagged model coupled with an air quality model (Nested Air Quality Prediction Model System, NAQPMS) was applied to estimate source contributions of primary and secondary sulfate, nitrate and ammonium (SNA) during a representative winter period in Shanghai. This source-tagged model system could simultaneously track spatial and temporal sources of SNA, which were apportioned to their respective primary precursors in a simulation run. The results indicate that in the study period, local emissions in Shanghai accounted for over 20% of SNA contributions and that Jiangsu and Shandong were the two major non-local sources. In particular, non-local emissions had higher contributions during recorded pollution periods. This suggests that the transportation of pollutants plays a key role in air pollution in Shanghai. The temporal contributions show that the emissions from the "current day" (emission contribution from the current day during which the model was simulating) contributed 60%-70% of the sulfate and ammonium concentrations but only 10%-20% of the nitrate concentration, while the previous days' contributions increased during the recorded pollution periods. Emissions that were released within three days contributed over 85% averagely for SNA in January 2013. To evaluate the source-tagged model system, the results were compared by sensitivity analysis (emission perturbation of -30%) and backward trajectory analysis. The consistency of the comparison results indicated that the source-tagged model system can track sources of SNA with reasonable accuracy.

Impact of the 2015 El Nino event on winter air quality in China.

During the winter of 2015, there was a strong El Nino (ENSO) event, resulting in significant anomalies for meteorological conditions in China. Analysis shows that the meteorological conditions in December 2015 (compared to December 2014) had several important anomalies, including the following: (1) the surface southeasterly winds were significantly enhanced in the North China Plain (NCP); (2) the precipitation was increased in the south of eastern China; and (3) the wind speeds were decreased in the middle-north of eastern China, while slightly increased in the south of eastern China. These meteorological anomalies produced important impacts on the aerosol pollution in eastern China. In the NCP region, the PM2.5 concentrations were significantly increased, with a maximum increase of 80-100 µg m-3. A global chemical/transport model (MOZART-4) was applied to study the individual contribution of the changes in winds and precipitation to PM2.5 concentrations. This study suggests that the 2015El Nino event had significant effects on air pollution in eastern China, especially in the NCP region, including the capital city of Beijing, in which aerosol pollution was significantly enhanced in the already heavily polluted capital city of China.

The meteorological modulation on PM2.5 interannual oscillation during 2013 to 2015 in Shanghai, China.

Since the Action Plan for Air Pollution Prevention and Control (the Action Plan) was implemented at the end of 2013, the ambient air quality in China is significantly improved. However, PM2.5 (particles with diameter≤2.5µm) levels in some cities still exhibit clear interannual oscillations. For example, the annual mean PM2.5 levels in Shanghai decreased by 16.1% in 2014, while increased by 2.2% in 2015 according to year-on-year comparisons. To better understand the corresponding causes, the obliquely rotated T-mode principal component analysis (PCA) method and WRF-Chem model are jointly employed in this study. Results show that the west wind frequency and the accumulative wind (<1.8m/s) duration are the key indicators affecting local PM2.5 transport and dispersion significantly. Moreover, four typical synoptic patterns conductive to PM2.5 pollution are illustrated as mid-ward path cold (CM), eastward path cold (CE), L-share high (GL) and near high center (GC), in which GL is the most adverse circulation pattern. The year-on-year changes of meteorology have positive effects on PM2.5 year-on-year variations. The significant decline of PM2.5 levels in 2014 compared with those in 2013, as well as the obvious increase in 2015 compared with 2014, both well correspond to year-on-year variations of meteorological indicators. Model results present that PM2.5 interannual variations result from the changes of meteorology during 2013 to 2015, are consistent with measured oscillations. By comparing measured and modeled PM2.5 year-on-year variabilities, a greater PM2.5 decreasing at 9.4% is estimated under favorable meteorological conditions, while a less increasing at 6% under unfavorable meteorological conditions due to emission reductions, indicating the initial improvement has been achieved by the Action Plan. Otherwise, since the current Action Plan has difficulties in completely offsetting the PM2.5 rise attributed to the adverse weather, more stringent program should be drawn up for unfavorable meteorological conditions.

[A Numerical Study of Typical Heavy Air Pollution Episode of PM2.5 in Shanghai].

To analyze the characteristics and formation mechanism of a heavy air pollution episode in Shanghai City from January 23th to January 24th, 2013, the observed data of PM2.5 concentration and ground meteorological data and the WRF-Chem model were collected. The analysis revealed that the synoptic necessary mechanism of the heavy air pollution episode could be characterized by the following patterns: Accompanied with weak cold front activities, the city experienced weak winds (i. e. stable atmosphere) at first and then northerly winds (i. e. pollutant transport process ), causing the continuous increase and maintaining of pollutant concentration. The detailed results are shown as follows: Firstly, the stable atmosphere circulation pattern which lasted for 10 hours was not good for air pollution dispersion, as a result, local PM2.5 concentrations continued to increase and reached severe pollution level and the high concentrations maintained for 7 hours caused by the stable boundary layer (e. g. static surface winds and low level temperature inversion) during nighttime, and the average PM2.5 concentrations during the stable weather process was 172.4 µg · m⁻³. Secondly, the dispersion condition was slightly improved later on with the arrival of a weak cold front, the upstream pollution transportation occurred at the same time, leading to further increase of PM2.5 concentration (up to 280 µg · m⁻³), and the average PM2.5 concentration during the upstream transportation process was 213.6 µg · m⁻³. Numerical simulation with the WRF-Chem model showed that, average contribution of upstream transportation to local PM2.5 concentrations during the episode was 23% . Among them, the contribution during the stable weather and upstream transportation stage was 17.2% and 32.2% . Our results suggested that there were significant differences in the contribution of upstream transportation to the local PM2.5 concentration of Shanghai due to variation of weather conditions. Therefore, the government can design effective emission control strategy in advance taking pollution weather forecasting into account.