ALBERT

Description: We estimated the individual contributions of black carbon (BC) and brown carbon (BrC) to the aerosol absorption coefficient (σap) and absorption aerosol optical depth (AAOD) in the highly polluted Kathmandu Valley, Nepal, by applying the absorption Ångström exponent (AÅE) method to multi-wavelength aethalometer and AERONET sun/sky radiometer measurements. The elevated σap levels observed during the winter and pre-monsoon periods were primarily due to increased usage of diesel generators and low-grade fuel/coal. The AAODBC and AAODBrC values were substantially higher during the pre-monsoon period, almost two-fold higher than winter levels, due to seasonally increased biomass-burning activities from agricultural residue burning and forest fires. The overall contribution of BC to σap was approximately 80%–95%, whereas BrC accounted for 5%–20% at 520 nm. However, the BrC contribution to σap at 370 nm was substantially higher during the winter, ranging from 29.3% to 34.0%. The portioning of AERONET measurements indicates that BC contributed 80% (69%) while BrC contributed 20% (31%) to AAOD of carbonaceous aerosols at 520 nm (370 nm). Although the observation principles and techniques are completely independent, the BC and BrC absorption for simultaneous daytime data points shows a strong correlation between surface aethalometer-based and column AERONET-based estimates. The contributions of BC and BrC to absorption in Kathmandu Valley are similar to those observed under open biomass and garbage-burning conditions; however, the BrC absorption at both 370 and 520 nm is approximately 2- to 3-fold higher than those observed for urban areas in East Asia.

Description: 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 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 in both aerosol scattering (σs; 313  →  577 Mm−1 at 550 nm) and absorption (σa; 98  →  145 Mm−1 at 520 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 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 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 11 m2 g−1 from PM10 measurements, 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 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 08:00 LST (local standard time), respectively but decreased dramatically during the daytime (09:00–18:00 LST), to one-quarter of the morning average (06:00–09:00 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 levels 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 black carbon (or elemental carbon) to sulphate ratio (1.5 ± 1.1).