Effects of combustion temperature on the optical properties of brown carbon from biomass burning

Biomass burning has been known as one of main sources of Brown Carbon (BrC) in atmosphere. In this study, by controlling the combustion temperature at 250°C, 350°C, and 450°C, the methanol soluble organic carbon (MSOC) and methanol insoluble carbon (MISC) from pine wood burning was collected by impinger. UV-Vis, excitation emission matrix (EEM), TEM and FTIR spectra were applied to investigate the properties of BrC collected. For MSOC at 250°C and 350°C, all the spectral profiles of UV-Vis absorption and excitation emission matrix are almost the same, while the EEM of MSOC at 450°C are different from that of the other two. For MISC fluorescence was observed only in the case of 450°C. In the FTIR spectra, with the temperature increasing the peaks associated to the oxygen-contained functions was weakened, indicating the formation of the fluorophores with larger conjugated system, especially aromatic hydrocarbons. Our results show that biomass combustion at low temperature produces more oxygen-riched BrC, which possesses relatively lower light absorption, while at high temperature produces more aromatics hydrocarbons with relatively strong light absorption. The results of this work are helpful to trace the source of brown carbon and optimize biomass energy utilization.

A new method for extraction of methanol-soluble brown carbon: Implications for investigation of its light absorption ability

As an important component of organic carbon (OC), brown carbon (BrC) plays a significant role in radiative forcing in the atmosphere. Water-insoluble OC (WIOC) generally has higher light absorption ability than water-soluble OC (WSOC). The mass absorption cross-section (MAC) of WIOC is normally investigated by dissolving OC in methanol. However, all the current methods have shortcomings due to neglecting the methanol insoluble particulate carbon that is detached from the filter and suspended in methanol extracts, which results in MAC uncertainties of the methanol-soluble BrC and its climate warming estimation. In this study, by investigating typical biomass combustion sourced aerosols from the Tibetan Plateau and ambient aerosols from rural and urban areas in China, we evaluated the light absorption of extractable OC fraction for the existing methods. Moreover, a new method was developed to overcome the methanol insoluble particulate carbon detachment problem to achieve more reliable MAC values. We found that OC can be dissolved in methanol in a short time (e.g., 1 h) and ultrasonic treatment and long-term soaking do not significantly increase the extractable OC fraction. Additionally, we proved that methanol insoluble particulate carbon detachment in methanol does exist in previous methods, causing overestimation of the BrC mass extracted by methanol and thus the underestimation of MAC values. We therefore recommend the newly developed extraction method in this study to be utilized in future related studies to quantitatively obtain the light absorption property of methanol-soluble BrC.