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

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 (PM₂.₅) 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/m³, 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/m³, respectively) during the pre-monsoon period (2–13 June) and were relatively lower (0.27–0.77 and 0.12–0.50 μgC/m³, 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 PM₂.₅ 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 m²/(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 ² = 0.57), K⁺ concentration (R ² = 0.75), and organic aerosol biomass burning markers characterized by an Aerodyne aerosol mass spectrometer (C₂H₄O₂⁺ + C₃H₅O₂⁺, R ² = 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%.