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

Abstract

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 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 (σ s ; 313 → 577 Mm −1 at 550 nm) and absorption (σ a ; 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 σ s value in the Kathmandu Valley was a factor of 0.5 lower than that in polluted cities in India. The σ a 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 m 2 g −1 from PM 10 measurements in the Kathmandu Valley is similar to that reported in urban areas. However, the aerosol mass absorption efficiency was determined to be 11 m 2 g −1 from PM 10 measurements, which is higher than that reported in the literature for pure soot particles (7.5 ± 1.2 m 2 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 σ s and σ a 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 σ s and σ a 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 atPublished by Copernicus Publications on behalf of the European Geosciences Union. 12618 C. Cho et al.: Wintertime aerosol optical and radiative properties in the Kathmandu Valley mospheric 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).

Aerosol vertical distribution and sources estimation at a site of the Yangtze River Delta region of China

“…Given that aerosol properties vary significantly as a function of height (Cho et al, 2017;Cohen et al, 2018;Guo et al, 2016b), measurements of aerosol vertical distribution are critical for assessing the effects of aerosols on radiative forcing and an important piece of information to improve satellite-based ground PM 2.5 concentration estimation (Han et al, 2015). LiDAR (Light Detection And Ranging) had been widely used as a significant means to detect the vertical distribution and spatio-temporal evolution of aerosols (Baars et al, 2016).…”

Aerosol-induced atmospheric heating rate decreases over South and East Asia as a result of changing content and composition

“…East Asia, in particular the North China Plain (NCP), and South Asia, notably the Indo-Gangetic Plain (IGP) are global air pollution hotspots. Previous studies on aerosol properties and emissions over Asia including the IGP and the NCP regions were based on (i) in-situ data at one or only a few locations for a limited time period or only a select set of aerosol parameters 7 – 10 , (ii) inventories of aerosol emissions 11 , (iii) satellite data on AOD for a limited period, e.g., between 2000 and 2009 12 , and SO 2 emissions between 2005 and 2015 13 , and (iv) multi-model simulations of radiative forcing estimates 14 . Recently, spatial distribution of microphysical and optical properties of aerosols and radiative forcing using the China Aerosol Remote Sensing Network was reported 15 .…”

An overview of airborne measurement in Nepal – Part 1: Vertical profile of aerosol size, number, spectral absorption, and meteorology

“…Consequently, there is a complex interaction among changing emission sources and their interaction with regional and global climate change. Among emitted air pollutants, the chemical and physical properties of aerosols have been linked to significant burdens of disease, to melting of glaciers, to crop loses, to hydrological changes, and to cloud properties (Bollasina et al, 2011;Vinoj et al, 2014;Lau, 2014;Burney and Ramanathan, 2014;Brauer et al, 2012;Cong et al, 2015;Li et al, 2016).…”

“…Studies describing the aerosol-meteorology interaction are often missing in the Himalayan region partly due to lack of surface and airborne measurements of aerosol properties along with meteorology. Most past campaign-mode measurements in the Himalayan regions, to our knowledge, have been ground measurements, which have aided in evaluating aerosol properties and their transformation and transport mechanisms (Shrestha et al, 2013Ramana et al, 2004;Marcq et al, 2010;Panday and Prinn, 2009;Cho et al, 2017). Long-term continuous measurements of aerosols and meteorology are limited to a few stations in the High Himalayas, such as the recently discontinued Nepal Climate Observatory at Pyramid (NCO-P, 27.95 • N, 86.81 • E; 5050 m a.s.l.…”