Potential underestimation of ambient brown carbon absorption based on the methanol extraction method and its impacts on source analysis

The methanol extraction method was widely applied to isolate organic carbon (OC) from ambient aerosols, followed by measurements of brown carbon (BrC) absorption. However, undissolved OC fractions will lead to underestimated BrC absorption. In this work, water, methanol (MeOH), MeOH / dichloromethane (MeOH /DCM, 1 : 1, v/v), MeOH /DCM (1 :2, v/v), tetrahydrofuran (THF), and N,N-dimethylformamide (DMF) were tested for extraction efficiencies of ambient OC, and the light absorption of individual solvent extracts was determined. Among the five solvents and solvent mixtures, DMF dissolved the highest fractions of ambient OC (up to similar to 95 %), followed by MeOH and MeOH / DCM mixtures (<90 %), and the DMF extracts had significantly (p <0.05) higher light absorption than other solvent extracts. This is because the OC fractions evaporating at higher temperatures (>280 degrees) are less soluble in MeOH (similar to 80 %) than in DMF (similar to 90 %) and contain stronger light-absorbing chromophores. Moreover, the light absorption of DMF and MeOH extracts of collocated aerosol samples in Nanjing showed consistent temporal variations in winter when biomass burning dominated BrC absorption, while the average light absorption of DMF extracts was more than 2 times greater than the MeOH extracts in late spring and summer. The average light absorption coefficient at 365 nm of DMF extracts was 30.7 % higher (p <0.01) than that of MeOH extracts. Source apportionment results indicated that the MeOH solubility of BrC associated with biomass burning, lubricating oil combustion, and coal combustion is similar to their DMF solubility. The BrC linked with unburned fossil fuels and polymerization processes of aerosol organics was less soluble in MeOH than in DMF, which was likely the main reason for the large difference in time series between MeOH and DMF extract absorption. These results highlight the importance of testing different solvents to investigate the structures and light absorption of BrC, particularly for the low-volatility fraction potentially originating from non-combustion sources.

Automated summary

The study compared the efficiency of different solvents in extracting organic carbon from ambient aerosols and found that N,N-dimethylformamide (DMF) had the highest dissolution rate, leading to significantly higher light absorption compared to other solvents. The time series analysis showed that DMF extracts had greater light absorption, especially in late spring and summer. Testing different solvents is important for investigating the structure and light absorption of brown carbon (BrC), particularly for the low-volatility fraction potentially originating from non-combustion sources.