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 (+6 others)
2017 Atmospheric Chemistry and Physics  
<p><strong>Abstract.</strong> 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 675<span class="thinspace"></span>nm 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&amp;ndash;February 2013), a noticeable increase in both aerosol scattering (<i>σ</i><sub>s</sub>; 313<span class="thinspace"></span> → <span class="thinspace"></span>577<span class="thinspace"></span>Mm<sup>−1</sup> at 550<span class="thinspace"></span>nm) and absorption (<i>σ</i><sub>a</sub>; 98<span class="thinspace"></span> → <span class="thinspace"></span>145<span class="thinspace"></span>Mm<sup>−1</sup> at 520<span class="thinspace"></span>nm) 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 <i>σ</i><sub>s</sub> value in the Kathmandu Valley was a factor of 0.5 lower than that in polluted cities in India. The <i>σ</i><sub>a</sub> 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<span class="thinspace"></span>m<sup>2</sup><span class="thinspace"></span>g<sup>−1</sup> from PM<sub>10</sub> measurements in the Kathmandu Valley is similar to that reported in urban areas. However, the aerosol mass absorption efficiency was determined to be 11<span class="thinspace"></span>m<sup>2</sup><span class="thinspace"></span>g<sup>−1</sup> from PM<sub>10</sub> measurements, which is higher than that reported in the literature for pure soot particles (7.5<span class="thinspace"></span>±<span class="thinspace"></span>1.2<span class="thinspace"></span>m<sup>2</sup><span class="thinspace"></span>g<sup>−1</sup>). 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 <i>σ</i><sub>s</sub> and <i>σ</i><sub>a</sub> values and the equivalent black carbon (EBC) mass concentration reached up to 757<span class="thinspace"></span>Mm<sup>−1</sup>, 224<span class="thinspace"></span>Mm<sup>−1</sup>, and 29<span class="thinspace"></span>µg<span class="thinspace"></span>m<sup>−3</sup> at 08:00<span class="thinspace"></span>LST (local standard time), respectively but decreased dramatically during the daytime (09:00&amp;ndash;18:00<span class="thinspace"></span>LST), to one-quarter of the morning average (06:00&amp;ndash;09:00<span class="thinspace"></span>LST) due to the development of valley winds and an atmospheric bounder layer. The <i>σ</i><sub>s</sub> and <i>σ</i><sub>a</sub> values and the EBC concentration remained almost constant during the night at the levels of 410<span class="thinspace"></span>Mm<sup>−1</sup>, 130<span class="thinspace"></span>Mm<sup>−1</sup>, and 17<span class="thinspace"></span>µg<span class="thinspace"></span>m<sup>−3</sup>, respectively. The average aerosol direct radiative forcings over the intensive measurement period were estimated to be −6.9<span class="thinspace"></span>±<span class="thinspace"></span>1.4<span class="thinspace"></span>W<span class="thinspace"></span>m<sup>−2</sup> (top of the atmosphere) and −20.8<span class="thinspace"></span>±<span class="thinspace"></span>4.6<span class="thinspace"></span>W<span class="thinspace"></span>m<sup>−2</sup> (surface). Therefore, the high atmospheric forcing (i.e., 13.9<span class="thinspace"></span>±<span class="thinspace"></span>3.6<span class="thinspace"></span>W<span class="thinspace"></span>m<sup>−2</sup>) and forcing efficiency (74.8<span class="thinspace"></span>±<span class="thinspace"></span>24.2<span class="thinspace"></span>W<span class="thinspace"></span>m<sup>−2</sup><span class="thinspace"></span><i>τ</i><sup>−1</sup>) 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<span class="thinspace"></span>±<span class="thinspace"></span>1.1).</p>
doi:10.5194/acp-17-12617-2017 fatcat:omfpvo33wbdxje5fjsp2hbr4oi