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

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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.


Average diurnal variation of measured Eabs at 405 and 781 nm.Error bars represent the 25th and 75th percentiles.
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f2: Average diurnal variation of measured Eabs at 405 and 781 nm.Error bars represent the 25th and 75th percentiles.

Mentions: Using an online instrument (PASS 3) ambient air volumes were directly assessed for total absorption coefficients in Mm−1 at 405 and 781 nm before and after passing through a thermal denuder (Supplementary Fig. S1). Denuded aerosols were heated to 300 °C presumably leaving BC as the only remaining species contributing to absorption (potential residual coating effects are discussed later)16. The ratio between atmospheric and denuded absorption, defined as the absorption enhancement (Eabs), provides an estimate of the contribution from lensing effects and externally-mixed BrC to total absorption. Average Eabs values at 405 and 781 nm are 1.21 ± 0.22 and 1.27 ± 0.14, respectively. Both lensing and externally-mixed BrC contribute to Eabs at 405 nm. At 781 nm lensing is the only contributing species as expected since BrC absorption is negligible at longer wavelengths17. Eabs_405 shows a strong diurnal variation while Eabs_781 demonstrates minimal variation (Fig. 2). Daytime Eabs_405 is lower than the corresponding Eabs_781, which is similar to reported trends observed by a recent study at 870 nm18. Also, ref.18 reports diurnal variations with lower daytime kBrC at 405 nm. These lower daytime Eabs_405 values have been attributed to the combined effects of low kBrC and possible photobleaching of BrC1920. These low kBrC values are due to less primary emissions and non-absorbing secondary organic aerosols formed via photochemical reactions during the daytime.


Refractive Index and Absorption Attribution of Highly Absorbing Brown Carbon Aerosols from an Urban Indian City-Kanpur
Average diurnal variation of measured Eabs at 405 and 781 nm.Error bars represent the 25th and 75th percentiles.
© Copyright Policy - open-access
Related In: Results  -  Collection

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

f2: Average diurnal variation of measured Eabs at 405 and 781 nm.Error bars represent the 25th and 75th percentiles.
Mentions: Using an online instrument (PASS 3) ambient air volumes were directly assessed for total absorption coefficients in Mm−1 at 405 and 781 nm before and after passing through a thermal denuder (Supplementary Fig. S1). Denuded aerosols were heated to 300 °C presumably leaving BC as the only remaining species contributing to absorption (potential residual coating effects are discussed later)16. The ratio between atmospheric and denuded absorption, defined as the absorption enhancement (Eabs), provides an estimate of the contribution from lensing effects and externally-mixed BrC to total absorption. Average Eabs values at 405 and 781 nm are 1.21 ± 0.22 and 1.27 ± 0.14, respectively. Both lensing and externally-mixed BrC contribute to Eabs at 405 nm. At 781 nm lensing is the only contributing species as expected since BrC absorption is negligible at longer wavelengths17. Eabs_405 shows a strong diurnal variation while Eabs_781 demonstrates minimal variation (Fig. 2). Daytime Eabs_405 is lower than the corresponding Eabs_781, which is similar to reported trends observed by a recent study at 870 nm18. Also, ref.18 reports diurnal variations with lower daytime kBrC at 405 nm. These lower daytime Eabs_405 values have been attributed to the combined effects of low kBrC and possible photobleaching of BrC1920. These low kBrC values are due to less primary emissions and non-absorbing secondary organic aerosols formed via photochemical reactions during the daytime.

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.