Wenkang Gao

The Campaign on Atmospheric Aerosol Research Network of China: CARE-China

AbstractBased on a network of field stations belonging to the Chinese Academy of Sciences (CAS), the Campaign on Atmospheric Aerosol Research network of China (CARE-China) was recently established as the country’s first monitoring network for the study of the spatiotemporal distribution of aerosol physical characteristics, chemical components, and optical properties, as well as aerosol gaseous precursors. The network comprises 36 stations in total and adopts a unified approach in terms of the instrumentation, experimental standards, and data specifications. This ongoing project is intended to provide an integrated research platform to monitor online PM2.5 concentrations, nine-size aerosol concentrations and chemical component distributions, nine-size secondary organic aerosol (SOA) component distributions, gaseous precursor concentrations (including SO2, NOx, CO, O3, and VOCs), and aerosol optical properties. The data will be used to identify the sources of regional aerosols, the relative contributions fr...

Investigating the evolution of summertime secondary atmospheric pollutants in urban Beijing.

Understanding the formation of tropospheric ozone (O3) and secondary particulates is essential for controlling secondary pollution in megacities. Intensive observations were conducted to investigate the evolution of O3, nitrate (NO3-), sulfate (SO42-) and oxygenated organic aerosols ((OOAs), a proxy for secondary organic aerosols) and the interactions between O3, NOx oxidation products (NOz) and OOA in urban Beijing in August 2012. The O3 concentrations exhibited similar variations at both the urban and urban background sites in Beijing. Regarding the O3 profile, the O3 concentrations increased with increasing altitude. The peaks in O3 on the days exceeding the 1h or 8h O3 standards (polluted days) were substantially wider than those on normal days. Significant increases in the NOz mixing ratio (i.e., NOy - NOx) were observed between the morning and early afternoon, which were consistent with the increasing oxidant level. A discernable NO3- peak was also observed in the morning on the polluted days, and this peak was attributed to vertical mixing and strong photochemical production. In addition, a SO42- peak at 18:00 was likely caused by a combination of local generation and regional transport. The OOA concentration cycle exhibited two peaks at approximately 10:00 and 19:00. The OOA concentrations were correlated well with SO42- ([OOA]=0.55×[SO42-]+2.1, r2=0.69) because they both originated from secondary transformations that were dependent on the ambient oxidization level and relative humidity. However, the slope between OOA and SO42- was only 0.35, which was smaller than the slope observed for all of the OOA and SO42- data, when the RH ranged from 40 to 50%. In addition, a photochemical episode was selected for analysis. The results showed that regional transport played an important role in the evolution of the investigated secondary pollutants. The measured OOA and Ox concentrations were well correlated at the daily scale, whereas the hourly OOA and Ox concentrations were insignificantly correlated in urban Beijing. The synoptic situation and the differences in the VOC oxidation contributing to O3 and SOAs may have resulted in the differences among the correlations between OOA and Ox at different time scale. We calculated OOA production rates using the photochemical age (defined as -log10(NOx/NOy)) in urban plumes. The CO-normalized OOA concentration increased with increasing photochemical age, with production rates ranging from 1.1 to 8.5μgm-3ppm-1h-1 for the plume from the NCP.

The observation‐based relationships between PM2.5 and AOD over China

This is the first investigation of the generalized linear regressions of PM2.5 and aerosol optical depth (AOD) with the Campaign on atmospheric Aerosol Research-China network over the large high-concentration aerosol region during the period from 2012 to 2013. The map of the PM2.5 and AOD levels showed large spatial differences in the aerosol concentrations and aerosol optical properties over China. The ranges of the annual mean PM2.5 and AOD were 10–117 µg/m3 and 0.12–1.11 from the clean regions to seriously polluted regions, from the almost “arctic” and the Tibetan Plateau to tropical environments. There were significant spatial agreements and correlations between the PM2.5 and AOD. However, the linear regression functions (PM2.5 = A*AOD + B) exhibited large differences in different regions and seasons. The slopes (A) were from 13 to 90, the intercepts (B) were from 0.8 to 33.3, and the correlation coefficients (R2) ranged from 0.06 to 0.75. The slopes (A) were much higher in the north (41–99) than in the south (13–64) because the extinction efficiency of hygroscopic aerosol was rapidly increasing with the increasing humidity from the dry north to the humid south. Meanwhile, the intercepts (B) were generally lower, and the correlation coefficients (R2) were much higher in the dry north than in the humid south. There was high consistency of AOD versus PM2.5 for all sites in three ranges of the atmospheric column precipitable water vapor (PWV). The segmented linear regression functions were y = 84.66x + 9.85 (PWV   2.5). The correlation coefficients (R2) were high from 0.64 to 0.70 across China.

Characterization of fine particles during the 2014 Asia-Pacific economic cooperation summit: Number concentration, size distribution and sources

Abstract To study the impacts of emission controls on aerosol physical and chemical properties, real-time measurements of size-resolved aerosol number concentration and chemical composition were conducted in urban Beijing during the 2014 Asia-Pacific Economic Cooperation (APEC) summit, in a period that a series of measures, for example shutting down or halting production from factories and power plants, and restricting the number of vehicles on the roads were implemented in Beijing and surrounding regions. Significantly, reductions in particle mass concentration (55% for PM2.5 and 48% for PM10) were observed during the APEC summit. A clear decrease in secondary inorganic aerosols (SIA), such as sulphate, nitrate and ammonium, was found during APEC, with the reduction ranged from 65.7 to 72.2% for PM1, in which sulphate showed the largest decrease compared with periods before APEC. As a comparison, organics showed a much smaller decrease of 44.3% for PM1 during APEC. These changes were mainly caused by large reductions in accumulation mode particles, which decreased by 36% compared with 19% for Aitken mode particles. The results from the positive matrix factorization (PMF) of particle number concentration indicate that regionally transported aerosols showed significant decreases (70%), similar to those of SIA during APEC, whereas primary factors from traffic and local combustion sources presented much smaller decreases, with the reduction ranged from 4 to 40%. The elevated contributions of these sources indicated the presence of strong local source emissions. The changes in particle chemical composition, size distribution and sources during the evolution of pollution episodes with and without emission controls are further illustrated. Our results highlight the importance of regional atmospheric transport in the formation of severe pollution episodes in Beijing, indicating that reducing the precursors of secondary aerosol over regional scales represent the key steps to reduce the urban particulate pollution. However, stricter emission controls on local source emissions are needed to further mitigate air pollution in Beijing.

Characteristics of air quality in Tianjin during the Spring Festival period of 2015

Abstract To better understand the characteristics of air quality and the relationship between the chemical composition evolution and source variation, an intensive atmospheric campaign was conducted in Tianjin, a megacity of the North China Plain, from 10 February to 6 March 2015. There were 20 days exceeding the threshold value of secondary Chinese Ambient Air Quality Standards for PM2.5 (75 μg m−3, daily average over 24 h) during the study period. Five air pollution episodes were selected for investigation. During the pre-holiday pollution episode, NH4+, NO3−, and SO42− were more abundant, indicating that air pollution was caused by motor vehicle exhaust emissions and coal consumption under unfavorable meteorological conditions. During Chinese Lunar New Year’s Eve, widespread use of fireworks resulted in extremely high aerosol concentrations. Firework displays caused increases in K+ and also enrichment of SO42− relative to NO3−. The holiday pollution episode was caused by regional transport, characterized by abundant SO42− and NH4+. In addition, the aging processes of the particles from fireworks discharge played an important role in the formation of NO3− and SO42−. The Lantern Festival episode was characterized by a transition from the enrichment of K+ to secondary inorganic ions (NO3−, SO42−, and NH4+). The results of this study are useful for a detailed understanding of the variation in atmospheric compositions and sources caused by anthropogenic activity, and highlight the importance of controlling intensive fireworks discharge.

Characteristics of complex air pollution in typical cities of North China

Abstract The Beijing–Tianjin–Hebei urban agglomeration is currently facing severe complex air pollution. In this paper, simultaneous observations conducted in 2014 show that the annual mean concentration of fine particulate matter (PM2.5) was 84 ± 70, 86 ± 60, and 118 ± 95 μg m−3 in Beijing, Tianjin, and Shijiazhuang, respectively. The mean O3_8 h max in the summer was 171 ± 43, 147 ± 45, and 146 ± 44 μg m−3, respectively. This research indicates that PM2.5 and O3 are positively correlated when the temperature exceeds 20 °C, and the urban agglomeration shows characteristics of complex air pollution consisting of superimposed O3 and PM2.5. In summer, when the humidity was less than 55%, secondary particles and O3 also increased in a coordinated manner (y = 1.35x + 29.85; R2 = 0.61), which demonstrates severe complex pollution. However, the mean PM2.5 (y) and mean O3_8 h max (x) in summer showed a negative correlation (y = −1.3x + 245; R2 = 0.61) in the three regions, indicating high concentrations of PM2.5 pollution partially inhibit O3 generation.

Characteristics of PM 2.5 mass concentrations and chemical species in urban and background areas of China: emerging results from the CARE-China network

The “Campaign on Atmospheric Aerosol Research” network of China (CARE-China) is a long-term project for the study of the spatio-temporal distributions of physical aerosol characteristics as well as the chemical components and optical properties of aerosols over China. This study presents the first long-term data sets from this project, including 3 years of observations of online PM2.5 mass concentrations (2012–2014) and 1 year of observations of PM2.5 compositions (2012–2013) from the CAREChina network. The average PM2.5 concentration at 20 urban sites is 73.2 μg m−3 (16.8–126.9 μg m−3), which was 3 times higher than the average value from the 12 background sites (11.2–46.5 μg m−3). The PM2.5 concentrations are generally higher in east-central China than in the other parts of the country due to their relatively large particulate matter (PM) emissions and the unfavourable meteorological conditions for pollution dispersion. A distinct seasonal variability in PM2.5 is observed, with highs in the winter and lows during the summer at urban sites. Inconsistent seasonal trends were observed at the background sites. Bimodal and unimodal diurnal variation patterns were identified at both urban and background sites. The chemical compositions of PM2.5 were analysed at six paired urban and background sites located within the most polluted urban agglomerations – North China Plain (NCP), Yangtze River delta (YRD), Pearl River delta (PRD), North-east China region (NECR), South-west China region (SWCR) – and the cleanest region of China – the Tibetan Autonomous Region (TAR). The major PM2.5 constituents across all the urban sites are organic matter (OM, 26.0 %), SO2− 4 (17.7 %), mineral dust (11.8 %), NO−3 (9.8 %), NH + 4 (6.6 %), elemental carbon (EC) (6.0 %), Cl (1.2 %) at 45 % RH and unaccounted matter (20.7 %). Similar chemical compositions of PM2.5 were observed at background sites but were associated with higher fractions of OM (33.2 %) and lower fractions of NO−3 (8.6 %) and EC (4.1 %). Significant variations of the chemical species were observed among the sites. At the urban sites, the OM ranged from 12.6 μg m−3 (Lhasa) to 23.3 μg m−3 (Shenyang), the SO2− 4 ranged from 0.8 μg m−3 (Lhasa) to 19.7 μg m−3 (Chongqing), the NO−3 Published by Copernicus Publications on behalf of the European Geosciences Union. 8850 Z. Liu et al.: Characteristics of PM2.5 mass concentrations and chemical species ranged from 0.5 μg m−3 (Lhasa) to 11.9 μg m−3 (Shanghai) and the EC ranged from 1.4 μg m−3 (Lhasa) to 7.1 μg m−3 (Guangzhou). The PM2.5 chemical species at the background sites exhibited larger spatial heterogeneities than those at urban sites, suggesting different contributions from regional anthropogenic or natural emissions and from long-range transport to background areas. Notable seasonal variations of PM2.5-polluted days were observed, especially for the megacities in east-central China, resulting in frequent heavy pollution episodes occurring during the winter. The evolution of the PM2.5 chemical compositions on polluted days was consistent for the urban and nearby background sites, where the sum of sulfate, nitrate and ammonia typically constituted much higher fractions (31–57 %) of PM2.5 mass, suggesting fine-particle pollution in the most polluted areas of China assumes a regional tendency, and the importance of addressing the emission reduction of secondary aerosol precursors including SO2 and NOx . Furthermore, distinct differences in the evolution of [NO−3 ]/[SO 2− 4 ] ratio and OC/EC ratio on polluted days imply that mobile sources and stationary (coal combustion) sources are likely more important in Guangzhou and Shenyang, respectively, whereas in Beijing it is mobile emission and residential sources. As for Chongqing, the higher oxidation capacity than the other three cities suggested it should pay more attention to the emission reduction of secondary aerosol precursors. This analysis reveals the spatial and seasonal variabilities of the urban and background aerosol concentrations on a national scale and provides insights into their sources, processes and lifetimes.