Evolution Of Biomass Burning Emissions Under Quasi-real Atmospheric Conditions

Biomass burning(BB)is the largest source of carbonaceous aerosol emissions in the world,which has important impacts on human health,atmospheric environment and climate effects.BB emissions include primary organic aerosol(POA),volatile organic compounds(VOCs)and inorganic gases,etc.Some semi-volatility and intermediate volatility organic compounds(SVOCs/IVOCs)are more easily oxidized to produce low volatile organic compounds,which can condense into particulate to form secondary organic aerosol(SOA).Numerous field observations and laboratory studies have consistently proved that BB emissions can rapidly undergo secondary conversion under solar radiation.However,the research results are still controversial,including the difference between the net growth and oxidation state of SOA.BB is also an important source of light-absorbing carbonaceous aerosols,including black carbon(BC)and light-absorbing organic aerosol(brown carbon,BrC),which have efficient absorption of visible spectrum.However,they have different absorption efficiency in the visible spectrum,which makes the light absorption characteristics of atmospheric particles more complex.Moreover,the particle size distribution of each component and the mixing state between components will have a significant impact on the absorption effect.Few studies have been able to separate the internal and external mixed organic aerosol(OA)and BC,which is the key to the radiation effect of light-absorbing carbonaceous aerosols.Based on this,our novel-designed single plume chamber was used to focus on the emissions of BB.Our chamber system can isolate single plume from a certain combustion period,and can be used to explore the evolution of the single plume.During aging experiments,the chamber was directly exposed to natural solar radiation to simulate the light reaction during the day and simulated dark reaction at night.The realtime ambient air without particles was introduced into the chamber to provide oxidants.The chemical and optical evolution of BB plumes were simulated under a series of atmosphere related conditions.The main results are as follows:(1)Bruning stage was one of the key factors affecting the composition of BB emissions.Flaming phase had a large amount of BC and OA,while during smoldering phase,only OA was dominant.The emissions in different bruning phases were significantly different,which had an important impact on its subsequent evolution in the atmosphere.In the whole aging process,there was a competitive relationship between oxidation-driven condensation and dilution-driven evaporation.During evolution,the OA from smoldering had more and faster SOA formation and higher oxidation state than that from flaming through photochemical reaction.The reason was the impact of POA,BC,VOCs and NOx in the initial emission.In the dark reaction,flaming and smoldering plumes both had weak SOA mass formation and the rate of SOA formation was equal to the evaporation rate of OA.Note that,even without the net growth of SOA,the chemical composition of the plume had absolutely changed,and the oxygen and nitrogen components in OA had increased.(2)The emission characteristics of VOCs in different burning phases were studied through BB experiments,and the emission factors of VOCs based on CO concentration in different combustion phases were quantified.Smoldering plume had more VOCs,which was by a factor of 4 than that of flaming.Among them,carbonyl compounds were the most abundant and had the highest concentration.The following were a series of furans and aromatic hydrocarbons(including oxygenated aromatic hydrocarbons).VOCs and IVOCs were divided through the organic volatile molecular corridors model.We found that wood combustion can emit a certain amount of IVOCs(account for about 5%of total VOCs).The results of the evolution of VOCs from BB showed that a series of furans decreased rapidly by 50%in the process of light reaction,which were important SOA precursors.The decreasing concentration and change rate of furans from smoldering were higher than those from flaming.Other major SOA precursors included aromatic hydrocarbons(benzene and toluene),carbonyls(acrolein and 2,4cyclopentadiene-l-one)and phenols(phenol and cresol compounds),etc.We also found that carboxylic acids were the major gas-phase oxidation products.The concentration of carboxylic acids from flaming and smoldering increased by a factor of 2 during aging,which were consistent with the trends of SOA formation.During the dark reaction,only 40%of the gas-phase oxidation products were produced due to the slow oxidation rate.By linking the oxidation reaction of VOCs with the formation of SOA,we found that the initial emission and type of VOCs were the key factors affecting the formation of SOA,and the interaction between NOx and VOCs was the main reason for the difference of OA oxidation state.(3)In the optical aging experiments,we found that the coating thickness of BC particles increased with the increase of the proportion of OA emission.From flamig to mixed combustion to smoldering,the volume ratio between coating and rBC increased by an order of magnitude.By observing the light absorption characteristics of the plume,it was found that the initial values and evolution of absorption Angstrom exponent(AAE)were significant different in flaming and smoldering.The range of AAE before and after the aging of flaming emissions was 1.1-1.6,and the range of smoldering emissions was 2.8-6.0.Based on the measured absorption value and the microphysical information of single particle BC aerosol and combined with Mie theory,the absorption model was established.The effect of BrC on the absorption enhancement of BB aerosols was confirmed by the comparison between measured and simulated results.A new model parameter was introduced to calculate the imaginary part of the refractive index(kOA)of OA to represent the absorptivity of OA.The real-time evolution of kOA was obtained by the coincidence between the mass absorption cross section(MAC)calculated by simulated kOA and the measured MAC.It was found that BrC emitted from flaming had stronger absorptivity than smoldering,which had light absorption at 781 nm.During aging,the formation of SOA and photobleaching processes together caused the absorption decay of BrC,with a half-life of 1-3 hours.Based on the realtime kcoating parameters and the core-shell model,the MACBC was calculated about BC particles,which was coated by light absorbing coating.The light absorption of BC particles was enhanced by 25%during evolutionThese results above reveal the chemical and optical characteristics of emissions in different burning stages,through a series of BB experiments.The evolution of aerosols,VOCs and optical properties of light absorbing carbonaceous aerosols from BB plume were studied through the aging experiment via single plume evolution chamber under real atmospheric conditions.This work focused on the differences between the two burning phases and the different chemical and optical oxidation mechanisms with or without solar radiation.These results provide the evolution parameters and measured basis based on different burning phases for the model simulation of BB about emission and transmission,and provide strong support for the future study of high-resolution evolution characteristics,driving factors and prediction and estimation of rural biomass burning emissions in China.