Aerosol optical properties and radiative forcing in the high Himalaya based on measurements at the Nepal Climate Observatory-Pyramid site (5079 m a.s.l.)

Abstract

Abstract. Intense anthropogenic emissions over the Indian sub-continent lead to the formation of layers of particulate pollution that can be transported to the high altitude regions of the Himalaya-Hindu-Kush (HKH). Aerosol particles contain a substantial fraction of strongly absorbing material, including black carbon (BC), organic compounds (OC), and dust all of which can contribute to atmospheric warming, in addition to greenhouse gases. Using a 3-year record of continuous measurements of aerosol optical properties, we present a time series of key climate relevant aerosol properties including the aerosol absorption (σ ap ) and scattering (σ sp ) coefficients as well as the single-scattering albedo (w 0 ). Results of this investigation show substantial seasonal variability of these properties, with long range transport during the pre-and post-monsoon seasons and efficient precipitation scavenging of aerosol particles during the monsoon season. The monthly averaged scattering coefficients range from 0.1 Mm −1 (monsoon) to 20 Mm −1 while the average absorption coefficients range from 0.5 Mm −1 to 3.5 Mm −1 . Both have their maximum values during the premonsoon period (April) and reach a minimum during Monsoon (July-August). This leads to dry w 0 values from 0.86 (pre-monsoon) to 0.79 (monsoon) seasons. Significant diurnal variability due to valley wind circulation is also reported. Using aerosol optical depth (AOD) measurements, Correspondence to: P. Laj (laj@lgge.obs.ujf-grenoble.fr) we calculated the resulting direct local radiative forcing due to aerosols for selected air mass cases. We found that the presence of absorbing particulate material can locally induce an additional top of the atmosphere (TOA) forcing of 10 to 20 W m −2 for the first atmospheric layer (500 m above surface). The TOA positive forcing depends on the presence of snow at the surface, and takes place preferentially during episodes of regional pollution occurring on a very regular basis in the Himalayan valleys. Warming of the first atmospheric layer is paralleled by a substantial decrease of the amount of radiation reaching the surface. The surface forcing is estimated to range from −4 to −20 W m −2 for small-scale regional pollution events and large-scale pollution events, respectively. The calculated surface forcing is also very dependent on surface albedo, with maximum values occurring over a snow-covered surface. Overall, this work presents the first estimates of aerosol direct radiative forcing over the high Himalaya based on in-situ aerosol measurements, and results suggest a TOA forcing significantly greater than the IPCC reported values for green house gases.

Sunphotometry of the 2006–2007 aerosol optical/radiative properties at the Himalayan Nepal Climate Observatory-Pyramid (5079 m a.s.l.)

“…Determination of the direct radiative forcing of aerosols essentially requires the knowledge of their scattering and absorption optical depths, plus ground albedo and scattering phase function (e.g., Haywood and Boucher, 2000;Schwartz, 1996;Charlson et al, 1992;Nakajima et al, 2007). In this respect, the magnitude of the radiative forcing is proportional to the AOD, and this latter is about an order of magnitude smaller at the high mountain sites with respect to the IG plains (e.g., Ramana et al, 2004, Pant et al, 2006.…”

“…However, some of these aerosols absorb radiation and warm the atmospheric layers they are suspended within. In highly polluted regions, dominance of the absorption effect can lead to a positive climate forcing, that is, a warming effect (Satheesh and Ramanathan, 2000;Ramanathan et al, 2007a, b;Nakajima et al, 2007;Ramanathan and Carmichael, 2008). As commonly intended, the term "forcing" indicates radiative effects of anthropogenic aerosol alone.…”

Chemical characterization of long-range transport biomass burning emissions to the Himalayas: insights from high-resolution aerosol mass spectrometry

“…Numerous aerosol measurements have been conducted in the TPH region in past decades to characterize the physicochemical properties, sources, and transport pathways of ambient aerosol (Liu et al, 2008;Decesari et al, 2010;Marcq et al, 2010;Marinoni et al, 2013;Putero et al, 2014;Xu et al, 2018;R. Zhang et al, 2017).…”

Spring-Time Enhancement in Aerosol Burden over a High-Altitude Location in Western Trans-Himalaya:Results from Long-Term Observations

“…Gobbi et al 51 have reported seasonal mean AOD values varying between 0.04 and 0.08 over the high-altitude location NCO-P (5079 m amsl), while Cong et al 52 have reported an annual mean AOD of ~0.05 at 500 nm over Nam Co (4720 m amsl) in the central Tibetan Plateau. Although the spring-time enhancement is observed in the surface as well as columnar aerosol properties, the magnitude of surface M BC at Hanle is significantly low compared to NCO-P, while the columnar AOD is higher than that is seen at NCO-P 46 , which indicates the possible elevated aerosol layers over Hanle during spring.…”

“…During October, the occurrence of higher fire-count over northwest India (Punjab, Haryana) associated with intense burning activities led to the observed peak in M BC . Interestingly, the October peak in aerosol absorption was noticeable at the NCO-P station as well (figure 4 a in Andrews et al 42 and figure 1 b in Marcq et al 46 ). The peak in M BC observed during October is unique to the high-altitude locations, Hanle and NCO-P. By November, the winter conditions (sub-zero temperatures and appearance of snow caps on the mountain peaks) set in, with shallow local atmospheric boundary layer (ABL) keeping the pollutants close to the surface; and the highaltitude peaks being above this local ABL, will be free from the regional sources of BC.…”