Light absorption properties of brown carbon in the high Himalayas

Abstract

The light‐absorbing properties of water‐soluble brown carbon (WS‐BrC) and methanol‐soluble brown carbon (MeS‐BrC) were studied in PM10 aerosols collected at the “Nepal Climate Observatory‐Pyramid” (NCO‐P) station (5079 m above sea level) during the period 2013–2014. The light absorption coefficients of WS‐BrC and MeS‐BrC were the highest during the premonsoon season and the lowest during monsoon. MeS‐BrC absorbs about 2 times higher at 365 nm and about 3 times more at 550 nm compared to WS‐BrC. The mass absorption cross section (MAC) of WS‐BrC measured at 365 nm is similar to that observed previously at South Asian low‐altitude sites. Fractional solar radiation absorption by BrC compared to BC considering the full solar spectrum showed that WS‐BrC absorbs 4 ± 1% and MeS‐BrC absorbs 9 ± 2% compared to BC at NCO‐P. Such ratios become 8 ± 1% (for WS‐BrC respect to BC) and 17 ± 5% (for MeS‐BrC respect to BC) when accounting for correction factors proposed by previous studies to convert absorption coefficients in bulk solutions into light absorption by accumulation mode aerosol particles. These results confirm the importance of BrC in contributing to light‐absorbing aerosols in this region of the world. However, the BrC absorption at 550 nm appears small compared to that of BC (1–5%, or 3–9% with conversion factors), and it is lower compared to global model estimates constrained by Aerosol Robotic Network observations. Finally, our study provides no clear evidence of a change in the fractional contribution of BrC with respect to BC to light absorption in the middle troposphere respect to the Indo‐Gangetic plain boundary layer.

Regional Differences of Chemical Composition and Optical Properties of Aerosols in the Tibetan Plateau

“…Values of the mass absorption efficient at 365 nm (MAE 365 ), calculated from the ratio of the absorption coefficient at 365 nm and mass concentration of WSOC, recorded for the samples from these two sites, were 0.81 for QOMS and 0.48 for WLG, respectively. The MAE 365 of the QOMS samples in our study is similar to that reported previously (0.66-0.72 m 2 g −1 ) for the same area of study-the Himalayas (Kirillova et al, 2016). These MAE 365 values are significantly lower than those characteristic for fresh and/or less aged aerosol in urban areas of Beijing during winter (1.26), but close to the values reported during summer (0.52) (Du et al, 2014).…”

“…The absorption coefficient of WSOC at QOMS was significantly higher than that of WLG although the concentration of WSOC at QOMS was lower than that at WLG. The MAE 365 of the QOMS samples in our study is similar to that reported previously (0.66-0.72 m 2 g −1 ) for the same area of study-the Himalayas (Kirillova et al, 2016). The absorption variation with respect to wavelength (AAE; absorption Ångström exponent) was determined from the slope of a linear regression fit of base 10 absorption coefficient versus wavelength within the range of 300 to 400 nm.…”

“…AAE was slightly higher for QOMS than WLG (6.83 vs. 5.96), suggesting potential difference in the chemical composition of BrC. Similar results (AAE:~5.0) were also observed in Himalayas during pre-monsoon (Kirillova et al, 2016) and the southeastern TP (AAE: 6.9) (Zhu et al, 2018). Values of the mass absorption efficient at 365 nm (MAE 365 ), calculated from the ratio of the absorption coefficient at 365 nm and mass concentration of WSOC, recorded for the samples from these two sites, were 0.81 for QOMS and 0.48 for WLG, respectively.…”

High Contribution of Secondary Brown Carbon to Aerosol Light Absorption in the Southeastern Margin of Tibetan Plateau

“…Current BrC studies on the TP have mainly focused on the sources and optical properties of total BrC or extracted water‐/methanol‐soluble BrC and humic‐like substances (e.g., Kirillova et al, ; Li, Chen, et al, ; Li, Yan, et al, ; Wu et al, ; Zhu et al, , ). Here a statistical approach that we call the minimum R‐squared (MRS) method was developed to separate light absorption by secondary BrC versus primary BrC.…”

Year‐round optical properties and source characterization of Arctic organic carbon aerosols on the North Slope Alaska

“…Fall was also the most variable season, with a 50% RSD, while MAE 365 values in the winter and spring were more consistent. In order to compare BC light absorption coefficient ( b abs ) at 678 nm ( λ of laser in Sunset OCEC carbon analyzer) with light absorption by BrC at 365 nm, the light absorption coefficient of BC at 365 was calculated using the power law wavelength dependence of b abs , using AAE BC = 1 [ Kirillova et al ., ]. Annually, WSOC BrC absorbs on average 3.4% relative to BC, ranging from 2.1% to 5.0% in the winter and summer respectively.…”