Mengmeng Li

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.

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.

Improved meteorology and ozone air quality simulations using MODIS land surface parameters in the Yangtze River Delta urban cluster, China

Land surface parameters play an important role in the land-atmosphere coupling and thus are critical to the weather and dispersion of pollutants in the atmosphere. This work aims at improving the meteorology and air quality simulations for a high-ozone (O3) event in the Yangtze River Delta urban cluster of China, through incorporation of satellite-derived land surface parameters. Using Moderate Resolution Imaging Spectroradiometer (MODIS) input to specify the land cover type, green vegetation fraction, leaf area index, albedo, emissivity, and deep soil temperature provides a more realistic representation of surface characteristics. Preliminary evaluations reveal clearly improved meteorological simulation with MODIS input compared with that using default parameters, particularly for temperature (from −2.5 to −1.7°C for mean bias) and humidity (from 9.7% to 4.3% for mean bias). The improved meteorology propagates through the air quality system, which results in better estimates for surface NO2 (from 11.5 to 8.0 ppb for mean bias) and nocturnal O3 low-end concentration values (from −18.8 to −13.6 ppb for mean bias). Modifications of the urban land surface parameters are the main reason for model improvement. The deeper urban boundary layer and intense updraft induced by the urban heat island are favorable for pollutant dilution, thus contributing to lower NO2 and elevated nocturnal O3. Furthermore, the intensified sea-land breeze circulation may exacerbate O3 pollution at coastal cities through pollutant recirculation. Improvement of mesoscale meteorology and air quality simulations with satellite-derived land surface parameters will be useful for air pollution monitoring and forecasting in urban areas.

Urban heat island impacted by fine particles in Nanjing, China

Atmospheric aerosol particles (especially particles with aerodynamic diameters equal to or less than 2.5 μm, called PM2.5) can affect the surface energy balance and atmospheric heating rates and thus may impact the intensity of urban heat islands. In this paper, the effect of fine particles on the urban heat island intensity in Nanjing was investigated via the analysis of observational data and numerical modelling. The observations showed that higher PM2.5 concentrations over the urban area corresponded to lower urban heat island (UHI) intensities, especially during the day. Under heavily polluted conditions, the UHI intensity was reduced by up to 1 K. The numerical simulation results confirmed the weakening of the UHI intensity due to PM2.5 via the higher PM2.5 concentrations present in the urban region than those in the suburban areas. The effects of the fine particles on the UHI reduction were limited to the lowest 500–1000 m. The daily range of the surface air temperature was also reduced by up to 1.1 K due to the particles’ radiative effects. In summary, PM2.5 noticeably impacts UHI intensity, which should be considered in future studies on air pollution and urban climates.

Characteristics of ozone and particles in the near-surface atmosphere in urban area of the Yangtze River Delta, China

11 To improve the understanding of the interactions between particles and trace gases in a typical city 12 of the YRD region, continuous measurements of particles and trace gases were made at an urban 13 site in Nanjing during cold seasons in 2016 in this study. The average of particles, including black 14 carbon (BC), PM2.5, and PM10 are 2.602 1.720 μg/m, 58.2 36.8 μg/m, and 86.3 50.8 μg/m, 15 respectively, while the average of trace gases, which contain CO, O3, NOx, and NOy, are 850.9 16 384.1, 37.7 33.5, 23.5 14.7, and 32.8 22.3 ppb, respectively. Compared to National Ambient 17 Air Quality Standards in China (NAAQS-CN), we found 48 days excess of PM2.5, 14 days excess 18 of PM10, and 40 days excess of O3. The particles, CO, and nitrogen oxide concentrations shared a 19 similar pattern of seasonality and diurnal cycles, which are different from O3. The former ones are 20 all high in DJF and at rush hours, while the latter one had high loadings in the daytime, especially 21 Atmos. Chem. Phys. Discuss., https://doi.org/10.5194/acp-2018-927 Manuscript under review for journal Atmos. Chem. Phys. Discussion started: 15 October 2018 c

Influence of Atmospheric Particulate Matter on Ozone in Nanjing, China: Observational Study and Mechanistic Analysis

Particulate matter with diameters of 2.5 μm or smaller (PM2.5) and ozone (O3) are major pollutants in the urban atmosphere. PM2.5 can affect O3 by altering the photolysis rate and heterogeneous reactions. However, these two processes and their relative importance remain uncertain. In this paper, with Nanjing in China as the target city, we investigate the characteristics and mechanism of interactions between particles and O3 based on ground observations and numerical modeling. In 2008, the average concentrations of PM2.5 and O3 at Caochangmen station are 64.6±47.4 μg m−3 and 24.6±22.8 ppb, respectively, while at Pukou station they are 94.1 ± 63.4 μg m−3 and 16.9 ± 14.9 ppb. The correlation coefficient between PM2.5 and O3 is −0.46. In order to understand the reaction between PM2.5 and O3, we construct a box model, in which an aerosol optical property model, ultraviolet radiation model, gas phase chemistry model, and heterogeneous chemistry model, are coupled. The model is employed to investigate the relative contribution of the aforementioned two processes, which vary under different particle concentrations, scattering capability and VOCs/NOx ratios (VOCs: volatile organic compounds; NOx: nitric oxide and nitrogen dioxide). Generally, photolysis rate effect can cause a greater O3 reduction when the particle concentrations are higher, while heterogeneous reactions dominate O3 reduction with low-level particle concentrations. Moreover, in typical VOC-sensitive regions, O3 can even be increased by heterogeneous reactions. In Nanjing, both processes lead to O3 reduction, and photolysis rate effect is dominant. Our study underscores the importance of photolysis rate effect and heterogeneous reactions for O3, and such interaction processes should be fully considered in future atmospheric chemistry modeling.摘要PM2.5是指大气中空气动力学直径小于2.5 µm的细颗粒物. 细颗粒物(PM2.5)和臭氧(O3)是城市大气中的重要污染物. PM2.5可以通过影响光解反应过程和非均相化学反应过程来影响O3的浓度. 本文基于地面观测和数值模拟, 以中国南京为例研究了PM2.5与O3相互作用的特征和机理. 研究发现, 南京草场门站PM2.5和O3的平均浓度分别为64.6 ± 47.4 μg m-3和24.6 ± 22.8 ppb, 浦口站PM2.5和O3的平均浓度分别为94.1 ± 63.4 μg m-3和16.9 ± 14.9 ppb. PM2.5和O3之间的相关系数为-0.46. 为了深入理解PM2.5与O3之间的相互作用, 构建了一个包含气溶胶光学性质模块, 紫外辐射传输模块, 气相化学模块和非均相化学模型块的零维箱型模式, 用于研究在不同的PM2.5浓度, 大气散射能力和VOC/NOx(VOC: 挥发性有机化合物; NOx: 一氧化氮和二氧化氮)情况下, 光解反应和非均相化学反应过程的相对贡献及变化. 通常情况下, 当PM2.5浓度较低时, 颗粒物主要通过其表面发生的非均相反应导致O3减少; 当PM2.5浓度较高时, 颗粒物主要通过影响光解反应导致O3减少. 此外, 在典型的VOC敏感区域中, 非均相化学反应还可能导致O3浓度的增加. 在南京地区, PM2.5通过上述两个过程导致O3减少, 其中光解反应的影响占主导. 本研究强调了光解反应和非均相反应影响O3的重要性, 在未来的大气化学模型中应充分考虑PM2.5与O3之间的相互作用.

Impacts of aerosol-radiation feedback on local air quality during a severe haze episode in Nanjing megacity, eastern China

Abstract Severe haze events and their radiation feedbacks exert a profound impact on the weather and tropospheric chemistry. Using the on-line-coupled Weather Research and Forecasting with Chemistry (WRF-Chem) model, this study investigates the impacts of direct aerosol-radiation feedbacks on local air quality (i.e. particulate matter and ozone photochemistry) during a severe autumn haze episode in Nanjing megacity, eastern China. Pronounced radiation feedbacks are found for the predictions of meteorological and chemical variables. In response to the negative radiative forcing of scattering-dominant anthropogenic haze aerosols, the instantaneous irradiance and temperature at the surface lower by 130 W m−2 and 1.1–1.4 °C, respectively, leading to a reduction of boundary layer height by 103.2–232.6 m (11–38%) and vertical wind speed by 0.1–0.8 mm s−1 (2–30% at mid-day) during this haze event. Such a stable atmosphere favours the accumulation of fine particles (30.5 μg m−3, 28.7%) and NO2 (6.0 ppb, 23.7%) in the urban pollution plume. The weaker turbulent mixing and photochemical activity associated with the enhanced titration loss, and reduced downward radiation and photolysis rate result in a 0.1−5.0 ppb (12.0%) reduction of near-surface ozone. The simulations highlight that the aerosol-radiation feedbacks play an important role in the atmospheric transport and chemistry of large urban pollution plumes.