Wintertime aerosol optical and radiative properties in the Kathmandu Valley during the SusKat-ABC field campaign

Chaeyoon Cho; Sang-Woo Kim; Maheswar Rupakheti; Jin-Soo Park; Arnico Panday; Soon-Chang Yoon; Ji-Hyoung Kim; Hyunjae Kim; Haeun Jeon; Minyoung Sung; Bong Mann Kim; Seungkyu K. Hong; Rokjin J. Park; Dipesh Rupakheti; Khadak Singh Mahata; Puppala Siva Praveen; Mark G. Lawrence; Brent Holben

doi:10.5194/acp-17-12617-2017

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 wind circulations. The AERONET measurements from December 2012 to August 2014 revealed a mean aerosol optical depth (AOD) of approximately 0.30 at 675nm during winter, which is similar to that of the post-monsoon but half of that of the

more »

... 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&ndash;February 2013), a noticeable increase in both aerosol scattering (σs; 313 → 577Mm−1 at 550nm) and absorption (σa; 98 → 145Mm−1 at 520nm) coefficients occurred before and after 4 January 2013. This can be attributed to the increase in 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 2.6m2g−1 from PM10 measurements in the Kathmandu Valley is similar to that reported in urban areas. However, the aerosol mass absorption efficiency was determined to be 11m2g−1 from PM10 measurements, which is higher than that reported in the literature for pure soot particles (7.5±1.2m2g−1). This might be due to the fact that most of the carbonaceous aerosols in the Kathmandu Valley were thought to be 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 757Mm−1, 224Mm−1, and 29µgm−3 at 08:00LST (local standard time), respectively but decreased dramatically during the daytime (09:00&ndash;18:00LST), to one-quarter of the morning average (06:00&ndash;09:00LST) 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 levels of 410Mm−1, 130Mm−1, and 17µgm−3, respectively. The average aerosol direct radiative forcings over the intensive measurement period were estimated to be −6.9±1.4Wm−2 (top of the atmosphere) and −20.8±4.6Wm−2 (surface). Therefore, the high atmospheric forcing (i.e., 13.9±3.6Wm−2) and forcing efficiency (74.8±24.2Wm−2τ−1) can be attributed to the high portion of light-absorbing aerosols in the Kathmandu Valley, as indicated by the high black carbon (or elemental carbon) to sulphate ratio (1.5±1.1).

doi:10.5194/acp-17-12617-2017 fatcat:omfpvo33wbdxje5fjsp2hbr4oi

DOAJ Szczepanski

Citation

Chaeyoon Cho, Sang-Woo Kim, Maheswar Rupakheti, Jin-Soo Park, Arnico Panday, Soon-Chang Yoon, Ji-Hyoung Kim, Hyunjae Kim, Haeun Jeon, Minyoung Sung, Bong Mann Kim, Seungkyu K. Hong, Rokjin J. Park, Dipesh Rupakheti, Khadak Singh Mahata, Puppala Siva Praveen, Mark G. Lawrence, Brent Holben. "Wintertime aerosol optical and radiative properties in the Kathmandu Valley during the SusKat-ABC field campaign." Atmospheric Chemistry and Physics 17.20 (2017) 12617-12632

Wintertime aerosol optical and radiative properties in the Kathmandu Valley during the SusKat-ABC field campaign

Preserved Fulltext