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Atmospheric Chemistry and Physics An interactive open-access journal of the European Geosciences Union
© Author(s) 2017. This work is distributed
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Research article
09 May 2017
Review status
This discussion paper is under review for the journal Atmospheric Chemistry and Physics (ACP).
Wintertime Aerosol Optical and Radiative Properties in the Kathmandu Valley during the SusKat-ABC Field Campaign
Chaeyoon Cho1, Sang-Woo Kim1, Maheswar Rupakheti2, Jinsoo Park3, Arnico Panday4, Soon-Chang Yoon1, Ji-Hyoung Kim1, Hyunjae Kim3, Haeun Jeon3, Minyoung Sung3, Bong Mann Kim5, Seunkyu Hong1, Rokjin J. Park1, Dipesh Rupakheti6, Khadak Singh Mahata2, Puppala Siva Praveen4, Mark G. Lawrence2, and Brent Holben7 1School of Earth and Environmental Science, Seoul National University, Seoul 08826, South Korea
2Institute for Advanced Sustainability Studies, Berliner Str. 130, 14467 Potsdam, Germany
3National Institute of Environmental Research, Incheon 22689, South Korea
4International Centre for Integrated Mountain Development, Kathmandu 44700, Nepal
5iGBu, Corona, CA 92882, USA
6Key Laboratory of Tibetan Environment Changes and Land Surface Processes, Institute of Tibetan Plateau Research, Chinese Academy of Sciences, Beijing 100101, China
7NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA
Abstract. Particulate air pollution in the Kathmandu Valley has reached severe levels that are mainly due to uncontrolled emissions and the location of the urban area in a bowl-shaped basin with associated local circulations. The AERONET measurements from December 2012 to August 2014 revealed a mean aerosol optical depth (AOD) of approximately 0.3 at 675 nm during winter, which is similar to that of the post-monsoon but half of that of the pre-monsoon AOD (0.63). The distinct seasonal variations are closely related to regional-scale monsoon circulations over South Asia and emissions in the Kathmandu Valley. During the SusKat-ABC campaign (December 2012–February 2013), a noticeable increase of both aerosol scattering (σs; 313 → 522 Mm−1 at 550 nm) and absorption (σa; 98 → 145 Mm−1 at 520 nm) coefficients occurred before and after January 4, 2013. This can be attributed to the increase of wood-burned fires due to a temperature drop and the start of firing at nearby brick kilns. The σs value in the Kathmandu Valley was a factor of 0.5 lower than that in polluted cities in India. The σa value in the Kathmandu Valley was approximately 2 times higher than that at severely polluted urban sites in India. The aerosol mass scattering efficiency of 2.6 m2 g−1 in the Kathmandu Valley is similar to that reported in urban areas. However, the aerosol mass absorption efficiency was determined to be 11 m2 g−1, which is higher than that reported in the literature for pure soot particles (7.5 ± 1.2 m2 g−1). This might be due to the fact that most of the carbonaceous aerosols in the Kathmandu Valley were thought to be fresh aerosols, mostly externally mixed with other aerosols under dry conditions due to a short travel time from their sources. The σs and σa values and the equivalent black carbon (EBC) mass concentration reached up to 757 Mm−1, 224 Mm−1, and 29 µg m−3 at 08 LST, respectively but decreased dramatically during the daytime (09–18 LST), to one quarter of the morning average (06–09 LST) due to the development of valley winds and an atmospheric bounder layer. The σs and σa values and the EBC concentration remained almost constant during the night at the level of 410 Mm−1, 130 Mm−1, and 17 µg m−3, respectively. The average aerosol direct radiative forcings over the intensive measurement period were estimated to be −6.9 ± 1.4 W m−2 (top of the atmosphere) and −20.8 ± 4.6 W m−2 (surface). Therefore, the high atmospheric forcing (i.e., 13.9 ± 3.6 W m−2) and forcing efficiency (74.8 ± 24.2 W m−2 τ−1) can be attributed to the high portion of light-absorbing aerosols in the Kathmandu Valley, as indicated by the high BC (or elemental carbon) to sulphate ratio (1.5 ± 1.1).

Citation: Cho, C., Kim, S.-W., Rupakheti, M., Park, J., Panday, A., Yoon, S.-C., Kim, J.-H., Kim, H., Jeon, H., Sung, M., Kim, B. M., Hong, S., Park, R. J., Rupakheti, D., Mahata, K. S., Praveen, P. S., Lawrence, M. G., and Holben, B.: Wintertime Aerosol Optical and Radiative Properties in the Kathmandu Valley during the SusKat-ABC Field Campaign, Atmos. Chem. Phys. Discuss., doi:10.5194/acp-2017-239, in review, 2017.
Chaeyoon Cho et al.
Chaeyoon Cho et al.
Chaeyoon Cho et al.


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Short summary
We investigated the optical, chemical properties and direct radiative effects of aerosols in the Kathmandu Valley(KV). This study concluded that the ratio light-absorbing aerosols to scattering ones as well as the concentration of light-absorbing aerosols is much higher at Kathmandu than those at other comparable regions and it contributes to great atmospheric absorption efficiency. This study provides unprecedented insights into aerosol optical properties and their radiative forcings in the KV.
We investigated the optical, chemical properties and direct radiative effects of aerosols in the...