Strong Primary Contribution to Brown Carbon Light Absorption in Tibet and Urban Areas: Insights based on in situ measurements

Abstract. To investigate optical properties, sources, and radiative effects of brown carbon (BrC), we conducted synchronous field campaigns in the Qinghai–Tibet Plateau (Yangbajing) and urban Guangzhou in July 2022, using multi-wavelength Aethalometer (AE33) and aerosol mass spectrometer (AMS) measurements. Total aerosol and BrC light absorption coefficients at 370 nm (Abs_total: 1.6 ± 1.6 M m⁻¹; BrC: 0.2 ± 0.3 M m⁻¹) in Tibet were an order of magnitude lower than Guangzhou, attributed to extremely low aerosol/organic aerosol (OA) mass concentrations. However, BrC fractions in Abs_total (15 % vs. 21 % at 370 nm) correlated with primary OA (POA) ratios, highlighting anthropogenic emission impacts even in this clean background. Diurnal variations (morning/evening peaks) of source-specific BrC absorption were regulated by local emissions (e.g., biomass burning, traffic) and regional secondary formation. Source apportionment (PMF/MLR) revealed primary sources (biomass burning OA, hydrocarbon-like OA) dominated BrC absorption (>75 %). Vehicle hydrocarbon-like OA (HOA) MAC (2.08 m² g⁻¹ in Tibet; 2.57 m² g⁻¹ in Guangzhou) was comparable to biomass burning OA (1.11–2.54/1.91 m² g⁻¹), indicating high fossil fuel BrC absorption. Integrated "simple forcing efficiency" (370–660 nm) showed primary emissions contributed >98 % of total radiative forcing at both sites. This study advances understanding of BrC dynamics and sources in diverse environments, underscores primary sources’ critical role in BrC absorption, and emphasizes the need for source-specific OA optical parameterization.