关键词: 大气细粒子, 棕碳, 光吸收系数, 吸收埃指数, 质量吸收效率

Abstract: Objective　Changes in anthropogenic emission patterns significantly influence the spatiotemporal distribution and physicochemical properties of carbonaceous aerosols. Emission source compositions vary considerably across different regions, with rural areas exhibiting particularly complex emission characteristics that are strongly influenced by seasonal activities, and as coal or biomass is often used as the main heating and cooking fuel in rural areas of northern China during winter, the emitted carbon aerosols in rural areas of northern China in winter, especially during the Spring Festival, cannot be ignored. However, the dynamic response of brown carbon (BrC) light absorption characteristics before, during, and after holidays in rural areas remains inadequately understood, especially under the influence of distinct human activities such as residential solid fuel combustion and firework displays. This study aims to quantitatively evaluate the light absorption properties of BrC extracted from atmospheric PM2.5 samples collected in rural Shandong during the 2022 Spring Festival holiday period by multiple solvents. Specifically, methanol-soluble organic carbon (MSOC) and water-soluble organic carbon (WSOC) were extracted to represent the total extractable fractions and water-soluble fractions of BrC, respectively. The study tries to address a critical knowledge gap regarding rural BrC optical characteristics and their temporal variations under changing emission scenarios, providing essential baseline data for regional climate assessment and targeted pollution control. Methods　From January 27 to February 10, 2022, PM2.5 samples were collected continuously for 15 days in a typical rural village in Shandong Province, China, covering pre-holiday, 7-day official holiday (Spring Festival holiday), and post-holiday periods. Sampling was conducted using a portable MiniVol air sampler at a flow rate of 5 L/min, and the quartz fiber filters used in the sampler were pre-baked at 500 °C for 3 hours to remove organic impurities. Each sample was collected for more than 24 hours at a height of 1.2 m above ground. For each sampling filter membrane, a small piece of 0.495 cm2 was taken and then the original organic carbon (OC) content was measured using a DRI-2015 thermal/optical carbon analyzer. Each filter membrane was extracted with 10 mL of chromatographic-grade methanol and 10 mL of ultrapure water (18.25 MΩ·cm) via ultrasonication for 1 hour. The extracts were filtered through 0.22 μm PTFE (methanol) and hydrophilic nylon (water) syringe filters, respectively. The residual OC on the freeze-dried filter membranes was re-measured, and the mass concentrations of MSOC and WSOC were determined by the difference between the OC on the filter membrane before extraction and the residual OC on the filter membrane after different dissolution extractions. UV-visible absorption spectra (190–900 nm) were recorded using a TU-1900 dual-beam spectrophotometer with regular baseline, dark current, and solvent blank corrections. Light absorption coefficients (Aλ), absorption Ångström exponents (Å₃₀₀-₅₀₀), and mass absorption efficiencies (MAE) at 365 nm were calculated based on the Beer-Lambert law. Results and Discussion　The absorption spectra of extracts extracted with different solvents showed distinct characteristics: MSOC exhibited a strong primary peak at 202 nm (2988 Mm−1) and a weak shoulder peak at 280 nm (295 Mm−1), while WSOC only showed a single peak at 198 nm (987 Mm−1). The slight redshift of MSOC's maximum absorption peak relative to WSOC was attributed to the change of chromophore conformation caused by solvent polarity differences. The average AMSOC,365 and AWSOC,365 were 48.58 Mm−1 and 31.76 Mm−1, respectively, with values during the Spring Festival (56.51 Mm−1 and 35.83 Mm−1) significantly higher than pre- and post-holiday periods. This enhancement reflects the increased emissions from residential coal/biomass combustion (including a documented corn cob burning event) and firework displays, and these values exceed measurements in most urban areas in China and South Asia. The averaged Å₃₀₀-₅₀₀ was 5.18 for MSOC and 4.47 for WSOC, showing remarkable stability throughout the sampling period, indicating that the consistent emission source types were dominated by residential solid fuel combustion. Notably, MSOC exhibited slightly higher Ångström exponents than WSOC, suggesting that methanol extracted additional highly conjugated aromatic or nitrogen-containing chromophores. The averaged EMSOC,365 and EWSOC,365 were 3.16 m2/g and 2.52 m2/g, respectively, with a significant enhancement during the holiday period, attributed to both higher emission intensities and photochemical aging effects such as glyoxal-ammonium sulfate reaction and ozone-induced chromophore formation. Conclusion　This study demonstrates that human activities during the Spring Festival substantially enhance BrC light absorption in rural areas of Shandong, with residential solid fuel combustion and firework displays being the predominant driving factors. The stable Ångström exponents confirm consistent emission source characteristics despite varying intensities. Compared to WSOC, MSOC consistently shows superior light absorption capacity, underscoring the necessity of using methanol for comprehensive BrC extraction in rural environments. These findings provide crucial scientific data for accurately assessing the radiative effects of carbonaceous aerosols in rural China, and highlight the need to incorporate specific holiday emission sources in rural areas into climate models and environmental policies. Future research should focus on molecular-level characterization of BrC chromophores and their aging mechanisms to refine regional climate impact assessments.

Key words: PM2.5, brown carbon, light absorption coefficient, absorption ?ngstr?m exponent, mass absorption efficiency 662