Light absorption by water-soluble organic carbon in atmospheric fine particles in the central Tibetan Plateau

Abstract

Brown carbon (BrC) has recently received much attention because of its light absorption features. The chemical compositions, optical properties, and sources of fine aerosol at a high-elevation mountain observatory (4730 m a.s.l.) in the central Tibetan Plateau were measured between 31 May and 1 July 2015. A low flow-rate sampler was used to collect 24-h average fine particulate matter (PM) filter samples. Water-soluble ions, organic carbon (OC), elemental carbon, water-soluble organic carbon (WSOC), and light absorption by water-soluble BrC were determined for 26 filter samples. The mean (± 1σ) OC and WSOC concentrations were 0.76 ± 0.43 and 0.39 ± 0.15 μgC/m, respectively, and the mean WSOC/OC mass ratio was 0.59 ± 0.22. The OC and WSOC concentrations were relatively higher (0.59-1.80 and 0.33-0.83 μgC/m, respectively) during the pre-monsoon period (2-13 June) and were relatively lower (0.27-0.77 and 0.12-0.50 μgC/m, respectively) during the monsoon period (14 June to 1 July), probably because of wet scavenging of aerosols during long-range transport and the presence of cleaner marine air masses during the monsoon period. The absorption spectra of PM water extracts smoothly increase from visible range to ultraviolet range. The absorption Ångström exponent, which describes the wavelength dependence of water-soluble BrC, was 2.74-10.61 (mean 6.19 ± 1.70), and its value was similar in the pre-monsoon period (6.57 ± 0.56) to that in the monsoon period (5.91 ± 2.14). The water-soluble BrC mass absorption efficiency, 0.38 ± 0.16 m/(g C), was much lower than those observed in most urban areas but similar to those in other remote sites. Absorption coefficient at 365 nm, typically used as a proxy for water-soluble BrC, correlated well with the WSOC concentration (R = 0.57), K concentration (R = 0.75), and organic aerosol biomass burning markers characterized by an Aerodyne aerosol mass spectrometer (CHO + CHO, R = 0.60). It can be inferred that biomass burning was an important source of water-soluble BrC in the study area combined with air mass back trajectory analysis using the NOAA HYSPLIT as well as MODIS data of fire dots and aerosol optical depths. The water-soluble BrC to BC light absorption (at 365 nm) coefficient ratios were 9-27%.

Characteristics of airborne water-soluble organic carbon (WSOC) at a background site of the North China Plain

“…And this result was corresponding to the higher AAE in summer than in winter, because the larger AAE value indicated the lower absorptive ability. The MAE 365 at YRDNNR was similar to the values observed in Beijing (winter: 1.26 m 2 g −1 ; summer: 0.51 m 2 g −1 , in the North China Plain) (Du et al, 2014b) and central Tibetan Plateau (summer: 0.38 m 2 g −1 , background site in China) (Zhang et al, 2017b), but higher than that at a remote site in Indian Ocean (winter: 0.45 m 2 g −1 ) (Srinivas and Sarin, 2013), indicating the light absorbability of WSOC at YRDNNR was much stronger, and WSOC would have the greater influence on the radiation forcing in this region with the same mass concentration.…”

“…OC may be divided into water-soluble organic carbon (WSOC) and water-insoluble organic carbon (WISOC). WSOC is a critical constituent, accounting for approximately 20-80% of OC (Du et al, 2014a;Kirillova et al, 2013;Zhang et al, 2017b). WSOC exerts significant effects on radiative forcing and climate change because some components of WSOC, e.g., water-soluble brown carbon (BrC) and humic-like substances (HULIS), have light absorption properties (Baduel et al, 2010;Duarte et al, 2005;Teich et al, 2016).…”

Unexpected High Absorption of Atmospheric Aerosols Over a Western Tibetan Plateau Site in Summer

“…Ca 2+ might been originated from wind‐blown dust due to a high positive correlation between Ca 2+ and wind speed (Table 1). The K + concentration at Shiquanhe station was much higher than those in the central (0.01 ± 0.01 μg m −3 ), southeast (0.04 ± 0.03 μg m −3 ), and northeast TP (0.01 μg m −3 ) (Xu et al., 2015; Zhao et al., 2013; Zhang, Xu, et al., 2017), indicating more biomass burning aerosols were present in the western TP than in other regions in summer. Negative correlations were found between secondary inorganic species (i.e., NH 4 + , SO 4 2− , and NO 3 ‐ ) and relative humidity, inferring the removal of these species by wet deposition at Shiquanhe station.…”

Characterization of the light-absorbing properties, chromophore composition and sources of brown carbon aerosol in Xi'an, northwestern China

“…Condensed-phase reactions and aqueousphase reactions have also been found to be important formation pathways for secondary BrC in ambient air (Gilardoni et al, 2016). In addition, atmospheric aging processes can lead to either enhancement or bleaching of the BrC absorption (Lambe et al, 2013;Lee et al, 2014;Zhong and Jang, 2014), further challenging the characterization of BrC.…”