2:30 PM - 2:45 PM
[AAS09-16] Estimating Real-time Brown Carbon Absorption: Insights from a High-Altitude Station in Eastern Himalaya, India
Keywords:Brown Carbon, Eastern Himalaya, Carbonaceous aerosol
Light-absorbing aerosols, particularly Brown Carbon (BrC), play a significant role in Earth's radiative balance, impacting both regional and global climate change. BrC, a subset of organic carbon, efficiently absorbs solar radiation in the UV-vis range (200-400 nm), affecting the oxidizing capacity of photochemically active gases. BrC has primary sources such as biomass burning emissions and fossil fuel combustion, and secondary sources through the oxidation of volatile organic compounds (VOCs). Characterizing BrC is challenging due to its complex composition, emitted as mixtures containing black carbon (BC), BrC, non-absorbing organic aerosols, and inorganic compounds. Global radiative forcing by BrC is reported to be one-fourth of that of BC. In regions with intense combustion activities, the regional radiative forcing by BrC surpasses the global average, emphasizing its role in regional climate change. Characterizing BrC in an urban Eastern Himalayan region with significant combustion activities contributes to understanding aerosol-climate interactions.
In the present study, the absorption coefficient (babs,BrC,370) of Brown Carbon at 370 nm was derived from continuous monitoring of light absorption properties of ambient aerosol at seven wave lengths (ranging from UV to IR) by an Aethalometer over an eastern Himalayan hill station Darjeeling (27.01 deg. N, 88.15 deg. E, 2200 m asl). Simultaneous measurement of carbonaceous aerosol concentration was conducted using an online OC-EC analyzer. Both these measurements were conducted during the month Mar-May in the year 2019. The wavelength-dependent improved AAE method (following the work by Wang et al., 2018 and Zhang et al., 2020) was used to derive the absorption coefficient of BrC (babs,BrC,370). The contribution of BrC to the total absorption and the Mass Absorption Cross section (MAC) at 370 nm were calculated
The AAE represents the Absorption Angstrom Exponent at different wavelengths. AAEratio is the ratio of AAE of BC at the wavelength range (370-660) and (660-880) nm. In the present study, the AAEratio value used was 0.64 following the previous work by Kapoor et al., 2022.
A high-altitude location like Darjeeling shows a unique feature in terms of pollutant concentration variation which is markedly different from the plan land regions. At Nighttime, a shallow mixing layer traps pollutants in the valley region, resulting in low OC-EC/BC concentrations at high altitudes (Figure 1a). As the morning sun rises, pollutants are transported upward, creating a peak in OC and EC/BC. Concentrations decrease in the afternoon, spiking again before decreasing at night, potentially due to up slope valley wind transporting polluted air. The diurnal pattern of BrC absorption (babs,BrC,370) aligns with OC-EC/BC, suggesting a common source of primary anthropogenic origin. %BrC and MACBrC,370 remain high during late night, with MAC reaching maximum just before sunrise, indicating highly absorbing BrC at night. Daytime photochemical processes bleach nighttime BrC, leading to reduced MACBrC,370. Evening peaks in MACBrC,370 coincide with OC concentration peaks, suggesting the transportation of brown carbon from low-land regions during low solar intensity.
This study is one of the very few studies that show the formation and transformation of atmospheric brown carbon using real-time observation over a high-altitude Himalayan site which indicates that solar radiation intensity and nighttime chemistry may play a complex role in the nature and absorbing properties of BrC aerosol.
In the present study, the absorption coefficient (babs,BrC,370) of Brown Carbon at 370 nm was derived from continuous monitoring of light absorption properties of ambient aerosol at seven wave lengths (ranging from UV to IR) by an Aethalometer over an eastern Himalayan hill station Darjeeling (27.01 deg. N, 88.15 deg. E, 2200 m asl). Simultaneous measurement of carbonaceous aerosol concentration was conducted using an online OC-EC analyzer. Both these measurements were conducted during the month Mar-May in the year 2019. The wavelength-dependent improved AAE method (following the work by Wang et al., 2018 and Zhang et al., 2020) was used to derive the absorption coefficient of BrC (babs,BrC,370). The contribution of BrC to the total absorption and the Mass Absorption Cross section (MAC) at 370 nm were calculated
The AAE represents the Absorption Angstrom Exponent at different wavelengths. AAEratio is the ratio of AAE of BC at the wavelength range (370-660) and (660-880) nm. In the present study, the AAEratio value used was 0.64 following the previous work by Kapoor et al., 2022.
A high-altitude location like Darjeeling shows a unique feature in terms of pollutant concentration variation which is markedly different from the plan land regions. At Nighttime, a shallow mixing layer traps pollutants in the valley region, resulting in low OC-EC/BC concentrations at high altitudes (Figure 1a). As the morning sun rises, pollutants are transported upward, creating a peak in OC and EC/BC. Concentrations decrease in the afternoon, spiking again before decreasing at night, potentially due to up slope valley wind transporting polluted air. The diurnal pattern of BrC absorption (babs,BrC,370) aligns with OC-EC/BC, suggesting a common source of primary anthropogenic origin. %BrC and MACBrC,370 remain high during late night, with MAC reaching maximum just before sunrise, indicating highly absorbing BrC at night. Daytime photochemical processes bleach nighttime BrC, leading to reduced MACBrC,370. Evening peaks in MACBrC,370 coincide with OC concentration peaks, suggesting the transportation of brown carbon from low-land regions during low solar intensity.
This study is one of the very few studies that show the formation and transformation of atmospheric brown carbon using real-time observation over a high-altitude Himalayan site which indicates that solar radiation intensity and nighttime chemistry may play a complex role in the nature and absorbing properties of BrC aerosol.