Light absorption of brown carbon aerosol in the PRD region of China

Abstract

Abstract. The strong spectral dependence of light absorption of brown carbon (BrC) aerosol is regarded to influence aerosol's radiative forcing significantly. The Absorption Angstrom Exponent (AAE) method has been widely used in previous studies to attribute light absorption of BrC at shorter wavelengths for ambient aerosols, with a theoretical assumption that the AAE of "pure" black carbon (BC) aerosol equals to 1.0. In this study, the AAE method was applied to both urban and rural environments in the Pearl River Delta (PRD) region of China, with an improvement of constraining the realistic AAE of "pure" BC through statistical analysis of on-line measurement data. A threewavelength photo-acoustic soot spectrometer (PASS-3) and aerosol mass spectrometers (AMS) were used to explore the relationship between the measured AAE and the relative abundance of organic aerosol to BC. The regression and extrapolation analysis revealed that more realistic AAE values for "pure" BC aerosol (AAE BC ) were 0.86, 0.82, and 1.02 between 405 and 781 nm, and 0.70, 0.71, and 0.86 between 532 and 781 nm, in the campaigns of urban winter , urban fall , and rural fall , respectively. Roadway tunnel experiments were conducted and the results further confirmed the representativeness of the obtained AAE BC values for the urban environment. Finally, the average light absorption contributions of BrC (± relative uncertainties) at 405 nm were quantified to be 11.7 % (±5 %), 6.3 % (±4 %), and 12.1 % (±7 %) in the campaigns of urban winter , urban fall , and rural fall , respectively, and those at 532 nm were 10.0 % (±2 %), 4.1 % (±3 %), and 5.5 % (±5 %), respectively. The relatively higher BrC absorption contribution at 405 nm in the rural fall campaign could be reasonably attributed to the biomass burning events nearby, which was then directly supported by the biomass burning simulation experiments performed in this study. This paper indicates that the BrC contribution to total aerosol light absorption at shorter wavelengths is not negligible in the highly urbanized and industrialized PRD region.

Relative importance of black carbon, brown carbon, and absorption enhancement from clear coatings in biomass burning emissions

“…Our estimation of BrC contribution to total biomass burning aerosol absorption shows large variations, but it is also clear that, independent of methodology, the contribution of BrC to total absorption is large and often of similar or greater magnitude than the contribution from BC. Most ambient studies show relatively low (< 20 %) BrC contribution to absorption at 405 nm (Yuan et al, 2016;Cappa et al, 2012), but these studies were not focused on biomass burning emissions and had significantly less biomass burning influence. Lack et al (2012a) estimated that BrC contributed 27 ± 15 % of absorption at 405 nm during a wild fire in Boulder, Colorado, based on thermal denuder measurements.…”

“…Laboratory measurements of absorption enhancement in biomass burning aerosols show significant variation with values greater than 2 observed at 405 nm in some samples and values ranging from 1.2 to 1.5 at 532 and 781 nm (McMeeking et al, 2014). There are a number of studies that estimate the absorption due to BrC in the blue and ultraviolet wavelengths (Yuan et al, 2016;Washenfelder et al, 2015;Guo et al, 2014;Lack et al, 2012a;Cappa et al, 2012;Bahadur et al, 2012;Flowers et al, 2010;Wang et al, 2016;Stockwell et al, 2016). Results from these studies show large variations in the estimated percentage of absorption attributable to BrC.…”

“…Various studies show large variations in the percentage of absorption due to BrC (Yuan et al, 2016;Washenfelder et al, 2015;Guo et al, 2014;Lack et al, 2012a;Cappa et al, 2012;Bahadur et al, 2012;Flowers et al, 2010) at blue wavelengths. While some of the variation is certainly due to variations in the ambient aerosol, some could potentially be the result of various approaches used to estimate the contribution of BrC to total light absorption in previous studies.…”

“…Others have used the difference in absorption enhancement from thermally denuding the aerosol at different wavelengths to determine the contribution from evaporated BrC (Guo et al, 2014;Cappa et al, 2012), with the assumption that the lensing effect depends weakly on wavelength (Lack and Cappa, 2010). Another method that is simple and widely applied assumes an absorption Ångström exponent (AAE) value for BC (typically 1) and defines absorption above the predicted BC absorption at low wavelengths to be BrC, while sometimes also accounting for lensing (Fialho et al, 2005;Favez et al, 2009;Bahadur et al, 2012;Cazorla et al, 2013;Yuan et al, 2016). In this study, we estimate the percentage of absorption due to BC, lensing, and BrC following methodologies based on assuming an AAE and based on thermally denuding the aerosol, thereby providing a range that covers variations caused by experimental approach, of potential contributions to absorption in biomass burning emissions from BrC, lensing and BC.…”

Light absorption of brown carbon in eastern China based on 3-year multi-wavelength aerosol optical property observations at the SORPES station and an improved Absorption Ångstrom exponent segregation method

“…However, studies concerning BrC in China, especially YRD region, are still limited up to now. Yuan et al (2016) reported light absorption contributions of BrC in the Pearl River Delta (PRD) region to be 6.3% to 12.1% at…”