Tijian Wang

On the relationship between ozone and its precursors in the Pearl River Delta: application of an observation-based model (OBM)

Background, aim, and scopePhotochemical smog, characterized by high concentrations of O3 and fine particles, is of great concern in the urban areas, in particular megacities and city clusters like the Pearl River Delta.Materials, methods, and resultsAmbient ozone (O3) and its precursors were simultaneously measured at two sites in the Pearl River Delta, namely, Wan Qing Sha (WQS) in Guangzhou and Tung Chung (TC) in Hong Kong, from 23 October to 01 December 2007 in order to explore their potential relationship. Eight high O3 episode days were identified at WQS and two at TC during the sampling campaign, indicating a more serious O3 pollution in Guangzhou than in Hong Kong. An observation-based model was employed to determine the ozone–precursor relationship. At both sites, O3 production was found to be volatile organic compound (VOC)-limited, which is consistent with previous observations. Anthropogenic hydrocarbons played a key role in O3 production, while reducing nitric oxide emissions aided the buildup of O3 concentrations. Among VOC species, the summed relative incremental reactivity (RIR) of the top 12 compounds accounted for 89% and 85% of the total RIR at WQS and TC, respectively, indicating that local photochemical O3 formation can be mainly attributed to a small number of VOC species.Discussion and conclusionsA large increment in both simulated HO2 and O3 concentrations was achieved with additional input of hourly carbonyl data. This suggested that apart from hydrocarbons, carbonyls might significantly contribute to the O3 production in the Pearl River Delta.

Impacts of biogenic emissions of VOC and NOx on tropospheric ozone during summertime in eastern China.

This study is intended to understand and quantify the impacts of biogenic emissions of volatile organic compounds (VOC) and nitrogen oxides (NO(x)) on the formation of tropospheric ozone during summertime in eastern China. The model system consists of the non-hydrostatic mesoscale meteorological model (MM5) and a tropospheric chemical and transport model (TCTM) with the updated carbon-bond chemical reaction mechanism (CBM-IV). The spatial resolution of the system domain is 30 km x 30 km. The impacts of biogenic emissions are investigated by performing simulations (36 h) with and without biogenic emissions, while anthropogenic emissions are constant. The results indicate that biogenic emissions have remarkable impacts on surface ozone in eastern China. In big cities and their surrounding areas, surface ozone formation tends to be VOC-limited. The increase in ozone concentration by biogenic VOC is generally 5 ppbv or less, but could be more than 10 ppbv or even 30 ppbv in some local places. The impacts of biogenic NO(x) are different or even contrary in different regions, depending on the relative availability of NO(x) and VOC. The surface ozone concentrations reduced or increased by the biogenic NO(x) could be as much as 10 ppbv or 20 ppbv, respectively. The impacts of biogenic emissions on ozone aloft are generally restricted to the boundary layer and generally more obvious during the daytime than during the nighttime. This study is useful for understanding the role of biogenic emissions and for planning strategies for surface ozone abatement in eastern China. Due to limitations of the emission inventories used and the highly non-linear nature of zone formation, however, some uncertainties remain in the results.

Temporal characterization and regional contribution to O3 and NOx at an urban and a suburban site in Nanjing, China.

To improve our understanding of the interplay among local and regional photochemical pollutants in the typical city of the Yangtze River Delta (YRD) region, the concurrent observation of O3 and NOx concentrations at an urban and a suburban site in Nanjing during 2008 is presented. In general, the annual mean O3 concentration is 2.35ppbv lower in the downtown than at suburban due to higher NOx pollution levels correlated with heavy traffic. At both sites, O3 shows a distinct seasonality with the spring maximum and the winter minimum, while the minimum concentration of NOx appears in summertime. Besides the chemical processes of O3 sensitivity in the daytime and the NOx titration at night, meteorological conditions also play an essential role in these monthly and diurnal variations. The ozone weekend effect that can be attributed to the weekly routine of human activities is observed in the urban atmosphere of Nanjing as well, with O3 concentrations 2.09ppbv higher and NOx concentrations 6.20ppbv lower on weekends than on weekdays. The chemical coupling of NO, NO2 and O3 is investigated to show that the OX-component (O3 and NO2) partitioning point occurs at about 35ppbv for NOx, with O3 being the dominant form at lower levels and NO2 dominating at higher levels. And it is also discovered that the level of OX is made up of two contributions, including the regional contribution affected by regional background O3 level and the local contribution correlated with the level of primary pollution. The diurnal peak of regional contribution appears 2-5h after the peak of local contribution, implying that OX in Nanjing might prominently affected by the pollutants from a short distance. The highest regional contribution and the second highest local contribution lead to the spring peak of O3 observed in Nanjing, whereas the highest local contribution and the moderate regional contribution make the O3 concentrations in summer higher than those in autumn and winter. Our results reveal the important environment impacts from meteorological conditions and human activities in the YRD region, and can help to understand O3 pollution in these polluted areas by just using the conventional observations.

Analysis of a serious air pollution event resulting from crop residue burning over Nanjing and surrounding regions

During the time period between Oct.28 to Oct.29, 2008, a serious air pollution event took place in Nanjing and surrounding regions, accompanying with sharply increasing of PM10, CO and SO2 in the air. Satellite remote sensing data, surface meteorological observations, air pollution index and the NCEP reanalysis data were used to investigate the atmospheric conditions and planetary boundary layer (PBL) features. Air mass backward trajectory simulation method was employed to analyze the sources of the air pollutants and transport paths of this event. The results show that the transport of gas pollutants released from crop residue burning in the central and north parts of Jiangsu province, combining with unfavorable weather condition, which is the dominating reason of this air pollution episode. It is found there was a high-pressure system with relative uniform pressure pattern, weak vertical velocity, vorticity and divergence below 500hPa, which prevents atmospheric ventilation. The inversion temperature, low mixing height and topographical forcing winds in the PBL are also not favorable for the diffusion and transport of the air pollutants.

Source apportionment of size-fractionated particles during the 2013 Asian Youth Games and the 2014 Youth Olympic Games in Nanjing, China

In this study, samples of size-fractionated particulate matter were collected continuously using a 9-size interval cascade impactor at an urban site in Nanjing, before, during and after the Asian Youth Games (AYG), from July to September of 2013, and the Youth Olympic Games (YOG), from July to September of 2014. First, elemental concentrations, water-soluble ions including Cl-, NO3-, SO42-, NH4+, K+, Na+ and Ca2+, organic carbon (OC) and elemental carbon (EC) were analysed. Then, the source apportionment of the fine and coarse particulate matter was carried out using the chemical mass balance (CMB) model. The average PM10 concentrations were 90.4±20.0μg/m3 during the 2013 AYG and 70.6±25.3μg/m3 during the 2014 YOG. For PM2.1, the average concentrations were 50.0±12.8μg/m3 in 2013 and 34.6±17.0μg/m3 in 2014. Investigations showed that the average concentrations of particles declined significantly from 2013 to 2014, and concentrations were at the lowest levels during the events. Results indicated that OC, EC, sulfate and crustal elements have significant monthly and size-based variations. The major components, including crustal elements, water-soluble ions and carbonaceous aerosol accounted for 75.3-91.9% of the total particulate mass concentrations during the sampling periods. Fugitive dust, coal combustion dust, iron dust, construction dust, soil dust, vehicle exhaust, secondary aerosols and sea salt have been classified as the main emissions in Nanjing. The source apportionment results indicate that the emissions from fugitive dust, which was the most abundance emission source during the 2013 AYG, contributed to 23.0% of the total particle mass. However, fugitive dust decreased to 6.2% of the total particle mass during the 2014 YOG. Construction dust (14.7% versus 7.8% for the AYG and the YOG, respectively) and secondary sulfate aerosol (9.3% versus 8.0% for the AYG and the YOG, respectively) showed the same trend as fugitive dust, suggesting that the mitigation measures of controlling particles from the paved roads, construction and industry worked more efficiently during the YOG.

Impact of aerosols on regional climate in southern and northern China during strong/weak East Asian summer monsoon years

In this work, we mainly simulate the effects of aerosols on regional climate in southern China (SC) and northern China (NC) and compare the differences of aerosol climatic effects in strong/weak summer monsoon years with a modified regional climate model RegCM4. The results show that the total climatic effects of aerosols cause the decline of averaged air temperature and precipitation of SC and NC in summer. In NC, the strength of temperature drop in strong summer monsoon years is higher than that in weak summer monsoon years, indicating the possible impact from the different changes of radiation, circulation, and precipitation. The decrease of precipitation is more significant in NC in weak summer monsoon years, while it is stronger in SC in strong summer monsoon years due to the difference of aerosol distribution as well as the effects on circulation and cloud microphysics processes. Besides, aerosol effects also cause a decrease of zonal wind at 850 hPa in SC and an increase in NC. The cooling center is more northerly and stronger in strong monsoon year, while it is more southerly and weaker in weak summer monsoon years, which results in the differences of vertical circulation anomaly and meridional wind anomaly at 850 hPa. In weak summer monsoon years, meridional wind at 850 hPa is increased in NC, while it is found to be decreased in SC. In strong summer monsoon years, meridional winds at 850 hPa in both NC and SC are weakened. However, the decrease in SC is much more distinct and clear.

Characterization of major natural and anthropogenic source profiles for size-fractionated PM in Yangtze River Delta

Samples of size-fractionated particles were collected from local natural and anthropogenic sources in two industrial cities in the Yangtze River Delta, using either grab/resuspension sampling or ambient air sampling. The chemical characterizations of twenty one source profiles were obtained, based on elements (Al, As, Ca, Cd, Cr, Cu, Fe, K, Mg, Mn, Na, Pb and Zn), soluble ions (Cl-, NO3-, SO42-, NH4+, K+, Na+ and Ca2+) and carbonaceous material such as organic carbon (OC) and elemental carbon (EC). The profiles vary between sources from different areas, as well as between different particulate sizes. Results show that particulate matter that originated as soil dust, construction dust and fugitive dust is typically rich in crustal elements, especially Al, Ca, Fe and Mg, while the presence of calcium is found to be a hallmark of the cement industry and ceramic industry. Dominant compositions of particles from coal-fired power plants are variable, with OC ranging from 2.2%-27%. Garbage-fired power plant and cooking smoke particulate emissions are enriched in Na, Cl-, OC and K due to the presence of salty and organic matter. The element Fe, ranging from 6.1% to 26.0%, is found to be an important marker for steel production dust. Vehicle exhaust profiles are characterized by high abundances of OC (12.1%-53.6%) and EC (3.9%-20.0%). Burning of straw results in K (1.6%-7.2%) and OC (12.4%-25.9%) being the dominant composition. Preliminary work has also been done on establishing local industrial profiles such as textile, ceramic and electroplating industries. This work could be used in receptor model studies to estimate the contributions of different sources to the size-fractionated particulate matter in the Yangtze River Delta.

An agricultural biomass burning episode in eastern China: transport, optical properties and impacts on regional air quality

Agricultural biomass burning (ABB) has been of particular concern due to its influence on air quality and atmospheric radiation, as it produces large amounts of gaseous and aerosol emissions. This paper presents an integrated observation of a significant ABB episode in Nanjing, China during early June 2011, using combined ground-based and satellite sensors (MODIS, AIRS, CALIPSO, and OMI products). The time-height distribution, optical properties, sources and transport of smoke, as well as its impacts on air quality are investigated. Lidar profiles indicate that the smoke aerosols are confined to the planetary-boundary-layer (PBL) and have a depolarization ratio of less than 0.08. The aerosol optical depths (AOD) increase from 0.5 to 3.0 at 500-nm while the extinction-related Angstrom exponent (EAE) increases from 1.1 to 1.6 at the wavelength pair of 440-870 nm. The single-scattering-albedo (SSA) becomes lower at 670-1020 nm following the ABB intrusion, and particularly shows a decreasing tendency between wavelengths of 440 to 1020 nm. The absorption Angstrom exponent (0.7) is smaller than 1.0, which may indicate the aged smoke particles mixed or coated with the urban aerosols. Surface PM10 and PM2.5 show a dramatic increase, reaching hourly mean of 800 µg/m3 and 485 µg/m3, respectively, which results in a heavy air pollution event. The stagnant and high-moisture weather provides favorable conditions for the aerosols to accumulate near the surface. CALIPSO observations also illustrate that the large-scale aerosols are primarily present in the PBL and transported to the ocean; but some dense smoke plumes are misclassified as cloud or polluted dust. By comparing with the observations, we found that the WRF-Chem model captured the accumulation and downwind transport of surface PM2.5 from 20:00 on June 2 to 10:00 on June 3 (Phase-1), but showed a dramatic underestimate from 20:00 on June 3 to 4 June (Phase-2) when dense aerosols are present. Such a discrepancy in the model is associated with the improper vertical apportion of transported smoke and atmospheric diffusion conditions when comparing with the observed aerosol and wind profiles. In addition, the model simulations indicate that the transported smoke can contribute to 50-70% of the ground-level PM2.5 in Nanjing.

Total concentrations and fractionation of heavy metals in road-deposited sediments collected from different land use zones in a large city (Nanjing), China

Abstract In order to investigate heavy metal contamination in an urban environment during urbanization and economic development, 35 road-deposited sediment samples were collected from seven different land-use zones (commercial, residential, traffic, scenic park, educational, industrial and peri-urban) in Nanjing, a large city in P.R. China. The ranges of total metal concentrations found were: 28.7–272 mg kg−1 for Cu; 24.8–268 mg kg−1 for Ni; 37.3–204 mg kg−1 for Pb; 140–798 mg kg−1 for Zn; 0.44–2.19mg kg−1 for Cd; and 60.6–250 mg kg−1 for Cr. Metal fractionation was carried out using a modified three-step European Bureau of References (BCR) sequential extraction procedure. Cadmium and Zn were found predominantly associated with the acid extractable fractions; Ni and Cr were dominant in the residual fraction; Pb was predominantly associated with the residual and reducible fractions; Cu was dominant in the oxidizable and residual metal fractions. Based on the sum of the acid-extractable, reducible, and oxidizable fractions, Cd, Zn and Pb are potentially the most toxic metals in the road-deposited sediment in Nanjing. No significant differences, except for Zn, were found in the metal fractionation pattern for Cu, Ni, Pb, Cd, and Cr in different land use zones.

Modeling of urban heat island and its impacts on thermal circulations in the Beijing–Tianjin–Hebei region, China

Through regulating the land–atmosphere energy balance, urbanization plays an important role in modifying local circulations and cross-border transport of air pollutants. The Beijing–Tianjin–Hebei (BTH) metropolitan area in northern China is frequently influenced by complex atmospheric thermal circulations due to its special topography and geographic position. In this study, the Weather Research and Forecasting (WRF) model combined with remote sensing is used to explore the urbanization impacts on local circulations in the BTH region. The urban heat island (UHI) effect generated around Beijing and Tianjin shows complex interactions with local thermal circulations. Due to the combined effects of UHI and topography, the UHI circulation around Beijing and valley breeze at the southern slopes of Yan Mountain are coupled together to reinforce each other. At the coastal cities, the increased land/sea temperature gradient considerably accelerates the sea breeze along Bohai Bay and moves the sea breeze front further inland to reach as far as Beijing. This study may lay a foundation for the better understanding of air pollutant dispersion on complex terrain.