Properties Of Polycyclic Aromatic Hydrocarbons(PAHs) In Atmospheric Particulates Using Online Aerosol Mass Spectrometer

Carbonaceous aerosols are one of the most important components of atmospheric particulate matter,causing significant effects on atmospheric visibility,solar radiation and atmospheric photochemistry.Carbonaceous aerosols include black carbon aerosols（BC）with strong light absorption and organic aerosols（OA）with light-scattering effects.OA also contains some light-absorbing substances,which are defined as brown carbon（Br C）.The absorbing BC can be coated by secondary substances during the aging after being emitted into the atmosphere,causing absorption enhancement(E_abs),which is referred as“lensing effect”.The presence of Br C in the coatings will further enhance the absorption of BC.Polycyclic aromatic hydrocarbons（PAHs）are a special kind of Br C with low volatility and strong absorption.During the aging of BC,PAHs in the coatings affect the mixing state and absorption of BC,causing the quantification of E_abs difficult.Therefore,it is necessary to the volatility of coatings and absorption enhancement of BC when coated by PAHs.The oxidation of PAHs in the atmosphere determines its lifetime,toxicity and long-distance transportation,and then indirectly causes human health issues,whose photochemical products are PAH derivatives,which are more toxic.Solid residential fuel combustion is an important source of particulate PAHs.However,different combustion conditions,such as fuel type and burning stage,affect the primary emission and the consequent evolution of PAHs.Some field observation experiments found that PAHs were stable and difficult to be oxidized,which could be transported distantly.Therefore,it is necessary to analyze the evolution of PAHs emitted by solid fuel combustion.In this study,a High-Resolution Time-of-Flight Aerosol Mass Spectrometer（HR-To F-AMS）was placed to analyze PAHs in both field observation experiments and laboratory experiments.A catalytic stripper（CS）was used to achieve effective evaporation of coatings on BC,to analyze the absorption and volatility of carbonaceous aerosols.The chemical composition,source appointment and the absorption of PAHs were also studied in the field observation experiments.In the laboratory experiments,a novel chamber was designed to simulate the evolution of PAHs in residential solid fuel combustion,and the effect of mixing state to the oxidation of PAHs.In the field observation experiments,OA was divided by Positive Matrix Factorization（PMF）,which were HOA,O-HOA,LO-OOA and MO-OOA for the AM.In the CS,O-HOA and MO-OOA were identified by PMF.HOA represented traffic emissions,which was highly related to the primary emissions of PAHs.The oxidized OA（LO-OOA and MO-OOA）were related to the aged OA.Inorganic components have the highest volatility,while OA has the lowest volatility for the non-refractory particles.The mass fraction remaining（MFR）of N-containing fragments in the HR-To F-AMS proved the existence of organic nitrates（ON）in the refractory components,and that the high level of oxidation corresponds to the low volatility of particles.The MFR of O-containing fragments in the HR-To F-AMS showed the important contribution of low-volatile PAHs to the coatings of BC.The absorption of BC was enhanced by 70%totally by coatings,with the E_abs of 30%due to the lensing effect.The refractory Br C was composed of ON and PAHs with high molecular weight by the MFR analysis of all components and fragments.In the laboratory experiments,the results showed that smoldering experiments produced more PAHs than flaming experiments,and that the chemical composition of PAHs in the primary emissions was determined by fuel type.The oxidation of PAHs occurred under solar radiation,proving the importance of photochemical reaction to the removal of PAHs in the atmosphere.PAHs with high molecular weight decreased in flaming experiments and maintained unmodified in smoldering experiments during the evolution,indicating the suppression of photochemical oxidation of PAHs in smoldering experiments.The increasing of PAHs with low molecular weight existed in all light experiments,which was caused by the fragmentation of PAH derivatives in HR-To F-AMS and the production of secondary organic aerosols（SOA）.OA was mostly internally mixed with BC in the flaming experiments,causing the exposure of PAHs to the ambient oxidants such as O₃,and leading to the photochemical oxidation of PAHs.While in smoldering experiments,large fraction of OA was externally mixed with BC,with BC and primary organic aerosols（POA）both well coated by SOA.In smoldering experiments,particulate PAHs were shielded by SOA and resistant to the photochemical oxidation,which prolonging PAHs’atmospheric lifetime.The images of Transmission Electron Microscope（TEM）further confirmed the theory.Therefore,the effects of the burning phase and the mixing state to the emissions and the consequent evolution of PAHs from solid fuel combustion need to be considered in the future research.In this study,field observation experiments were conducted combined with laboratory experiments.The contribution of PAHs-containing coatings to the absorption enhancement of BC was analyzed in the field observation experiments,providing basic insights for the impact of PAHs to the absorption enhancement of BC and further solar radiation.The oxidation of PAHs,which was influenced by different burning phase,real-world solar radiation and the mixing state of aerosols,was studied in the laboratory experiments,with great importance of fully understanding the oxidation process of PAHs in the atmosphere.