Study Of The Light Absorption Properties And Molecular Compositions Of Typical Biomass Burning Organic Aerosol

Organic aerosol emitted from biomass burning seriously reduces air quality,damages the atmospheric environment,endangers human health,and even threats to human survival.Organic aerosol includes primary organic aerosol(POA)and secondary organic aerosol(SOA),which can change the global radiation balance by absorbing solar radiation.The light absorption properties of organic aerosols depend on their molecular compositions.Therefore,it is of great significance to study the light absorption properties and molecular compositions of biomass burning organic aerosol for assessing the effect of biomass burning on atmospheric radiation.However,the light absorption properties and molecular compositions in the atmospheric processes are dynamics,resulting in the light absorption properties,the molecular compositions and the relationships between the two are still unclear.Given the uncertainty of biomass emissions and their evolutions,the light absorption properties and molecular compositions of typical biomass burning organic aerosol were studied.The specific research works are as follows:(1)Study of the light absorption properties and molecular compositions of primary organic aerosolPOA is produced by combustion and pyrolysis of fir wood in a tube furnace under different heating atmospheres and temperatures.Dissolved organic carbon was obtained by extracting POA with methanol and pure water.The absorption spectra,functional groups,and molecular composition of the dissolved organic carbon were measured and analyzed.The results found that dissolved organic carbon under nitrogen conditions had higher light absorption properties than dissolved organic carbon under air conditions and that methanol-soluble organic carbon had a higher light absorption capacity than water-soluble organic carbon.These results are attributed to the higher molecular weight,unsaturation,and modified aromaticity of methanol-soluble organic carbon compared to water-soluble organic carbon;and dissolved organic carbon under nitrogen has a higher molecular weight,unsaturation,and modified aromaticity than that under air.By the correlation between the mass absorption efficiency values at 365 nm and the molecular properties,the positive and negative correlation between the absorption properties of dissolved organic carbon and its molecular properties was established,which is helpful to understand the essential reasons for the changes in the absorption properties of POA at the molecular level.(2)Study of the formation of secondary organic aerosol in an oxidation flow reactorBased on the self-designed and developed oxidation flow reactor,the experimental study on the SOA formation by oxidation of α-pinene was conducted.The results showed that this reactor is capable of independently controlling temperature,humidity,and oxidant concentration.For particles larger than 50 nm,the reactor has very small wall losses.In the O3 oxidation experiments,SOA yields were found to be dependent on O3 concentration,precursor concentration,and SOA mass concentration.In the OH radical oxidation experiments,SOA yields were dependent on relative humidity,precursor concentration,SOA mass concentration,OH radical concentration,and the presence of acidic seed aerosol.In addition,the oxidation performance of this reactor was validated by comparative analysis with previous results.These results can provide technical support and theoretical guidance for the continued use of this reactor to simulate the aging of biomass burning emissions in the future.(3)Experimental study on the effect of aging conditions on smoke evolutionBy carrying out the experiment of smoldering of fir,poplar,and pine and the oxidation of smoke,the differences in gas emission,particle size distribution,optical properties,and molecular compositions in smoke under the conditions of OH radical oxidation,O3 oxidation,and photolytic oxidation were analyzed.The aging conditions were found to have a much greater influence on the evolution of smoke than the fuel type,with OH radical oxidation having the greatest influence on the evolution of smoke.The results found that the photobleaching effect of photolytic oxidation on smoke particles was lower than that of O3 oxidation and lower than that of OH radical oxidation.The carbon oxidation state of organic matter demonstrates that OH radicals are the most powerful oxidants,followed by O3 oxidation and photolytic oxidation.These results can provide a reference for the controlling of air pollution caused by smoke aging from biomass burning emissions.(4)Experimental study on the effect of photooxidation on the evolution of smoke particlesBy carrying out the experiment of burning of rice straw and the photooxidation of smoke particles,the evolution of particle size distribution of smoke particles and the evolution of light absorption properties and molecular compositions of polar and nonpolar organic fractions in smoke particles were analyzed.After photooxidation,the particle size distribution of smoke particles differs from that of smoke photooxidation.The absorption spectra of the polar and non-polar components are significantly different.After photooxidation,although both polar and non-polar fractions undergo photobleaching,the decrease in the absorption properties of the non-polar fraction is always higher than the decrease in the absorption properties of the polar fraction.At the same time,the non-polar fraction has higher light-absorbing and radiative properties than the polar fraction.The potential brown carbon molecular composition in the polar fraction suggests that the change in molecular properties supports the decrease in light absorption properties.These results can provide basic data for understanding the evolution of polar and non-polar organic fractions in biomass burning smoke particles.