4.7 Article

Absorption coefficient of urban aerosol in Nanjing, west Yangtze River Delta, China

Journal

ATMOSPHERIC CHEMISTRY AND PHYSICS
Volume 15, Issue 23, Pages 13633-13646

Publisher

COPERNICUS GESELLSCHAFT MBH
DOI: 10.5194/acp-15-13633-2015

Keywords

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Funding

  1. National Key Basic Research Development Program of China [2014CB441203, 2011CB403406]
  2. Young Scientists Fund of the National Natural Science Foundation of China [41205111]
  3. New Teachers' Fund for Doctor Stations, Ministry of Education [20120091120031]
  4. Fundamental Research Funds for the Central Universities [20620140744]
  5. FP7 project: REQUA [PIRSES-GA-2013-612671]
  6. Priority Academic Program Development of the Jiangsu Higher Education Institutions (PAPD)

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Absorbing aerosols can significantly modulate short-wave solar radiation in the atmosphere, affecting regional and global climate. The aerosol absorption coefficient (AAC) is an indicator that assesses the impact of absorbing aerosols on radiative forcing. In this study, the near-surface AAC and absorption angstrom ngstrom exponent (AAE) in the urban area of Nanjing, China, are characterized on the basis of measurements in 2012 and 2013 using the seven-channel Aethalometer (model AE-31, Magee Scientific, USA). The AAC is estimated with direct and indirect corrections, which result in consistent temporal variations and magnitudes of AAC at 532 nm. The mean AAC at 532 nm is about 43.23 +/- 28.13 M m(-1) in the urban area of Nanjing, which is much lower than that in Pearl River Delta and the same as in rural areas (Lin'an) in Yangtze River Delta. The AAC in the urban area of Nanjing shows strong seasonality (diurnal variations); it is high in cold seasons (at rush hour) and low in summer (in the afternoon). It also shows synoptic and quasi-2-week cycles in response to weather systems. Its frequency distribution follows a typical log-normal pattern. The 532 nm AAC ranging from 15 to 65 M m(-1) dominates, accounting for more than 72% of the total data samples in the entire study period. Frequent high pollution episodes, such as those observed in June 2012 and in winter 2013, greatly enhanced AAC and altered its temporal variations and frequency distributions. These episodes are mostly due to local emissions and regional pollution. Air masses flowing from northern China to Nanjing can sometimes be highly polluted and lead to high AAC at the site. AAE at 660/470 nm from the Schmid correction (Schmid et al., 2006) is about 1.56, which might be more reasonable than from the Wein-gartner correction (Weingartner et al., 2003). Low AAEs mainly occur in summer, likely due to high relative humidity (RH) in the season. AAC increases with increasing AAE at a fixed aerosol loading. The RH-AAC relationship is more complex. Overall, AAC peaks at RH values of around 40% (1.3 < AAE < 1.6), 65% (AAE < 1.3 and AAE > 1.6), and 80% (1.3 < AAE < 1.6).

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