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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.


(a) Diurnal variation of fractional absorption contribution from BC, lensing, and BrC at 405 nm. Figure 4b shows the same for BC and lensing at 781 nm. Error bars indicates the 25th and 75th percentiles.
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f4: (a) Diurnal variation of fractional absorption contribution from BC, lensing, and BrC at 405 nm. Figure 4b shows the same for BC and lensing at 781 nm. Error bars indicates the 25th and 75th percentiles.

Mentions: Direct atmospheric and denuded absorption measurements indicate that BC’s percent contribution to total absorption at 405 nm and 781 nm is 71.5% and 67%, respectively. At 405 nm, remaining 28.5% is contributed by volatile species in the form of lensing and externally-mixed BrC. Mie calculations estimated the contributions from lensing and externally-mixed BrC to be 16.5% and 12%, respectively. Since BrC is a component of the coating, it influences the particle’s net refractive index. Assuming BrC in shell to be completely scattering (kBrC = 0) reduces the lensing contribution from 16.5% to 12%. Figure 4a shows the estimated diurnal variation of fractional absorption contribution from BC, lensing, and BrC at 405 nm. During the daytime, aerosol mass concentrations decrease due to low emissions and an increased boundary layer depth. These low daytime mass concentrations, along with decreased absorbing capacities reduce the percent contribution from BrC to total absorption during the daytime. Absorptive contributions from lensing are nearly constant, indicating large-scale internal mixing. Contribution to absorption from BC alone is lower during night hours than in the daytime. This is due to the enhanced absorbing capacity of primary BrC during nighttime, which contributes to more to total absorption effectively compared to daytime. Figure 4b shows the diurnal variation of fractional absorption contribution from BC, and lensing at 781 nm. Since BrC is assumed to be not absorbing at 781 nm, volatile absorption at 781 nm only includes the lensing effect. Low aerosol mass concentrations during daytime hours show the lowest CF ratio (1.38) while nighttime hours show the highest CF ratio (1.6). Low relative humidity and high temperature during the day favor the partitioning of organics from particle phase to gas phase. This could also contribute to the lower CF values observed during the day (Supplementary Fig. S2b). The decrease in CF value was not reflected in the Eabs_781 diurnal trend due to the increase in the number of coated BC particles. Therefore, Eabs_781 shows a constant diurnal trend when compared to Eabs_405.


Refractive Index and Absorption Attribution of Highly Absorbing Brown Carbon Aerosols from an Urban Indian City-Kanpur
(a) Diurnal variation of fractional absorption contribution from BC, lensing, and BrC at 405 nm. Figure 4b shows the same for BC and lensing at 781 nm. Error bars indicates the 25th and 75th percentiles.
© Copyright Policy - open-access
Related In: Results  -  Collection

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

f4: (a) Diurnal variation of fractional absorption contribution from BC, lensing, and BrC at 405 nm. Figure 4b shows the same for BC and lensing at 781 nm. Error bars indicates the 25th and 75th percentiles.
Mentions: Direct atmospheric and denuded absorption measurements indicate that BC’s percent contribution to total absorption at 405 nm and 781 nm is 71.5% and 67%, respectively. At 405 nm, remaining 28.5% is contributed by volatile species in the form of lensing and externally-mixed BrC. Mie calculations estimated the contributions from lensing and externally-mixed BrC to be 16.5% and 12%, respectively. Since BrC is a component of the coating, it influences the particle’s net refractive index. Assuming BrC in shell to be completely scattering (kBrC = 0) reduces the lensing contribution from 16.5% to 12%. Figure 4a shows the estimated diurnal variation of fractional absorption contribution from BC, lensing, and BrC at 405 nm. During the daytime, aerosol mass concentrations decrease due to low emissions and an increased boundary layer depth. These low daytime mass concentrations, along with decreased absorbing capacities reduce the percent contribution from BrC to total absorption during the daytime. Absorptive contributions from lensing are nearly constant, indicating large-scale internal mixing. Contribution to absorption from BC alone is lower during night hours than in the daytime. This is due to the enhanced absorbing capacity of primary BrC during nighttime, which contributes to more to total absorption effectively compared to daytime. Figure 4b shows the diurnal variation of fractional absorption contribution from BC, and lensing at 781 nm. Since BrC is assumed to be not absorbing at 781 nm, volatile absorption at 781 nm only includes the lensing effect. Low aerosol mass concentrations during daytime hours show the lowest CF ratio (1.38) while nighttime hours show the highest CF ratio (1.6). Low relative humidity and high temperature during the day favor the partitioning of organics from particle phase to gas phase. This could also contribute to the lower CF values observed during the day (Supplementary Fig. S2b). The decrease in CF value was not reflected in the Eabs_781 diurnal trend due to the increase in the number of coated BC particles. Therefore, Eabs_781 shows a constant diurnal trend when compared to Eabs_405.

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.