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Quantification and source apportionment of brown carbon aerosols in the high Himalayas

Final Report Summary - HIMALAYABROWNCARBON (Quantification and source apportionment of brown carbon aerosols in the high Himalayas)

The Himalayan glaciers that determine the albedo of the mountains and provide the water supply that feeds large south Asian rivers are very sensitive to climate change. Anthropogenic light-absorbing atmospheric aerosol can reach very high concentrations in South-East Asia ("brown clouds"), affecting atmospheric transparency and generating spatial gradients of temperature over land with a possible impact on atmospheric dynamics and monsoon circulation. Besides black carbon (BC), an important light-absorbing component of anthropogenic aerosols is the organic carbon component known as ‘brown carbon’ (BrC). The gaining of new knowledge about the sources and the composition of the light-absorbing aerosol components is essential for developing an effective mitigation strategy for impacts of the climate warming on the Himalayan glacial region. In this research, we provide the first quantification and source apportionment of BrC in atmospheric aerosols in the high Himalayas during different seasons and linking BrC concentrations in the high Himalayas to the vertical transport from potential source areas in the lower Nepalese valleys (Kathmandu) and from the Indo-Gangetic plain. The specific objectives of this project are: a) quantify the BrC and its seasonal variability at a high altitude site in the Himalayas, b) compare with the BrC levels quantified at a potential source region (an urban site in Nepal), c) determine the sources of BrC in the Himalayas.
Research activities performed: The researcher analysed BrC on set of PM10 samples collected at “Nepal Climate Observatory – Pyramid” (NCO-P) station (the high Khumbu Valley, 5079 m a.s.l.) during 2013-2014. Water-soluble BrC (WS-BrC) and methanol-soluble BrC (MeS-BrC) were extracted and analyzed by the researcher using a UV/VIS spectrophotometer equipped with a 50 cm liquid waveguide capillary cell, which allows ultra-sensitive absorbance measurements for low sample concentrations. Concentrations of carbonaceous aerosol components, organic and inorganic ions have been measured. Proton Nuclear Magnetic Resonance (NMR) analysis of the water- and methanol-soluble organic components was used to characterize the chemical composition of BrC and for source attribution of organic aerosol. Carbon isotope-based source apportionment has been performed by the researcher in collaboration with Stockholm University. Finally, selected aerosol filter samples collected during the SusKat-ABC (Sustainable Kathmandu – Asian Brown Cloud) field campaign held in Kathmandu during 2013 – 2014 were also analyzed in order to compare the properties of BrC at a source site (Kathmandu) and in a remote high altitude site (NCO-P).
Main results: For the first time, BrC measurements were carried out in the high Himalayan environment, and for different seasons of the year. BrC concentrations were correlated to total carbonaceous aerosol concentrations and were the highest during the pre-monsoon season and the lowest during monsoon. Light-absorbing aerosols in the Himalayas are considerably diluted respect to source regions (the BrC concentrations in Kathmandu are on average 50 times higher than at NCO-P). However, the relative absorption of BrC respect to BC is two to ten times higher at NCO-P respect to Kathmandu. The fractional solar radiation absorption by BrC compared to BC considering the full solar spectrum is 9% for MeS-BrC at NCO-P (or 17% when accounting for correction factors to convert absorption coefficients in bulk solutions into light absorption by accumulation mode aerosol particles). These results confirm the importance of BrC in contributing to light-absorbing aerosols in this region of the world. Carbonaceous aerosols at NCO-P are contributed by both fossil fuel sources (25% of total carbon) and modern carbon sources (the remaining 75%). BrC, in particular, enriches in carbonaceous aerosols when the contribution of biomass burning (hence of modern carbon) is greater. These results indicate that the sources of BrC in the high Himalayas are mainly anthropogenic and related to biomass combustion.
Potential impact and use: ”HimalayaBrownCloud” provides the very first results on quantification and source apportionment of brown carbon in particulate matter in the high Himalayan environment. Light-absorbing aerosols exert a potential great impact on global and regional climate warming, with strongest effects in glaciated regions (UNEP ”Integrated Assessment of Black Carbon and Tropospheric Ozone” 2011). At the same time, reducing aerosol emissions from combustion sources is strategic for both mitigating human impact on climate and improving air quality (Shindell et al., Science 335, 183, 2012). This study provides the first inequivocal attribution of organic light-absorbing aerosols - the brown carbon - in the Himalayas to anthropogenic activities (biomass burning). At the same time, it delivers a first accurate quantification of the light absorpting potential of BrC, contributing to elucidate the actual extent of biomass burning climate impact in this region of the world. These results contribute to the scientific debate on the anthropogenic climate forcing agents, and can be useful for policy-makers in evaluating possible climate benefits from air quality measures (e.g. from regulating biomass burning in urban and rural areas).