Optical characteristics and source apportionment of brown carbon in winter PM_2.5 over Yulin in Northern China

In this study, daily PM_2.5 samples were collected at an urban site in Yulin of Northern China during a winter season. Eight carbon fractions, 13 kinds of polycyclic aromatic hydrocarbons (PAHs), and nine water-soluble ions in PM_2.5 were measured. The light-absorption characteristics of brown carbon (BrC) both in water and in methanol extracts were evaluated and quantified. The total quantified PAHs exhibited high concentrations (228.4 ± 52.6 ng m⁻³), contributing 0.2% of the PM_2.5 mass. High indeno[1,2,3-cd]pyrene/(indeno[1,2,3-cd]pyrene + benzo[ghi]-perylene) ratio but low NO₃ ⁻/SO₄ ²⁻ ratio revealed the important contribution of coal combustion to PM_2.5. The absorption coefficient (b_abs) for methanol extracts measured at 365 nm averaged 27.5 ± 12.0 Mm⁻¹. Light absorption by methanol extracts exhibited strong wavelength dependence, with an average absorption Ångström exponent of 5.2 in the 330–400 nm range. The mass absorption cross section (for methanol extracts averaged 1.4 ± 0.4 m² g⁻¹ by normalizing b_abs measured at 365 nm to organic carbon mass. A relatively strong positive relationship between b_{abs, methanol} and benzo[a]pyrene as well as with six carbon fractions indicated the important contribution of coal burning to BrC. Source apportionment based on the positive matrix factorization receptor model and multiple linear regression showed that residential coal combustion accounted for 37.4% of b_{abs365,methanol}. The estimated relative radiative forcing by methanol-soluble organic carbon relative to elemental carbon was 36.9% at 300–400 nm.