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Refractive Index and Absorption Attribution of Highly Absorbing Brown Carbon Aerosols from an Urban Indian City-Kanpur

View Article: PubMed Central - PubMed

ABSTRACT

Atmospheric aerosols influence Earth’s radiative balance, having both warming and cooling effects. Though many aerosols reflect radiation, carbonaceous aerosols such as black carbon and certain organic carbon species known as brown carbon have the potential to warm the atmosphere by absorbing light. Black carbon absorbs light over the entire solar spectrum whereas brown carbon absorbs near-UV wavelengths and, to a lesser extent, visible light. In developing countries, such as India, where combustion sources are prolific, the influence of brown carbon on absorption may be significant. In order to better characterize brown carbon, we present experimental and modeled absorption properties of submicron aerosols measured in an urban Indian city (Kanpur). Brown carbon here is found to be fivefold more absorbing at 365 nm wavelength compared to previous studies. Results suggest ~30% of total absorption in Kanpur is attributed to brown carbon, with primary organic aerosols contributing more than secondary organics. We report the spectral brown carbon refractive indices along with an experimentally constrained estimate of the influence of aerosol mixing state on absorption. We conclude that brown carbon in Kanpur is highly absorbing in nature and that the mixing state plays an important role in light absorption from volatile species.

No MeSH data available.


Van Krevelen diagram with Eabs_405 in the color scale.Plot in inset shows the diurnal variation of O:C and H:C.
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f3: Van Krevelen diagram with Eabs_405 in the color scale.Plot in inset shows the diurnal variation of O:C and H:C.

Mentions: Photochemical reactions during daytime are known to convert primary organics to semi-volatile oxygenated organics (SV-OOA) and then further oxidize them to low volatile oxygenated organics (LV-OOA)21. Analysis of high-resolution organic aerosol mass spectra from an Aerosol Mass Spectrometer (AMS) provides atomic oxygen to carbon (O:C) and hydrogen to carbon (H:C) ratios, which provides information about the degree of oxygenation. Photochemical reactions that occur with organic particulates in the atmosphere transform the compounds with low O:C values (hydrocarbon-like aerosols, HOA) to high O:C compounds (LV-OOA). Figure 3 shows a Van Krevelen diagram22 of color-coded Eabs_405 values at various O:C and H:C ratios; diurnal variations of O:C and H:C are also plotted. H:C ratios are lowest during daytime hours when O:C ratios are elevated. Higher Eabs_405 values are associated with lower O:C and higher H:C values indicating semi to moderately volatile aerosols. LV-OOA compounds with high O:C values are contributing less to Eabs_405. This result differs from a lab study10 that reported LV-OOA to be highly absorbing in nature. In this study, Eabs_405 was high during nighttime hours when direct emissions did not experience photochemical aging, thereby maintaining high H:C and low O:C ratios.


Refractive Index and Absorption Attribution of Highly Absorbing Brown Carbon Aerosols from an Urban Indian City-Kanpur
Van Krevelen diagram with Eabs_405 in the color scale.Plot in inset shows the diurnal variation of O:C and H:C.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC5121896&req=5

f3: Van Krevelen diagram with Eabs_405 in the color scale.Plot in inset shows the diurnal variation of O:C and H:C.
Mentions: Photochemical reactions during daytime are known to convert primary organics to semi-volatile oxygenated organics (SV-OOA) and then further oxidize them to low volatile oxygenated organics (LV-OOA)21. Analysis of high-resolution organic aerosol mass spectra from an Aerosol Mass Spectrometer (AMS) provides atomic oxygen to carbon (O:C) and hydrogen to carbon (H:C) ratios, which provides information about the degree of oxygenation. Photochemical reactions that occur with organic particulates in the atmosphere transform the compounds with low O:C values (hydrocarbon-like aerosols, HOA) to high O:C compounds (LV-OOA). Figure 3 shows a Van Krevelen diagram22 of color-coded Eabs_405 values at various O:C and H:C ratios; diurnal variations of O:C and H:C are also plotted. H:C ratios are lowest during daytime hours when O:C ratios are elevated. Higher Eabs_405 values are associated with lower O:C and higher H:C values indicating semi to moderately volatile aerosols. LV-OOA compounds with high O:C values are contributing less to Eabs_405. This result differs from a lab study10 that reported LV-OOA to be highly absorbing in nature. In this study, Eabs_405 was high during nighttime hours when direct emissions did not experience photochemical aging, thereby maintaining high H:C and low O:C ratios.

View Article: PubMed Central - PubMed

ABSTRACT

Atmospheric aerosols influence Earth’s radiative balance, having both warming and cooling effects. Though many aerosols reflect radiation, carbonaceous aerosols such as black carbon and certain organic carbon species known as brown carbon have the potential to warm the atmosphere by absorbing light. Black carbon absorbs light over the entire solar spectrum whereas brown carbon absorbs near-UV wavelengths and, to a lesser extent, visible light. In developing countries, such as India, where combustion sources are prolific, the influence of brown carbon on absorption may be significant. In order to better characterize brown carbon, we present experimental and modeled absorption properties of submicron aerosols measured in an urban Indian city (Kanpur). Brown carbon here is found to be fivefold more absorbing at 365 nm wavelength compared to previous studies. Results suggest ~30% of total absorption in Kanpur is attributed to brown carbon, with primary organic aerosols contributing more than secondary organics. We report the spectral brown carbon refractive indices along with an experimentally constrained estimate of the influence of aerosol mixing state on absorption. We conclude that brown carbon in Kanpur is highly absorbing in nature and that the mixing state plays an important role in light absorption from volatile species.

No MeSH data available.