Gas-solid Phase Distribution Of Aerosol Organic Molecular Tracers And Their Impact On Source Apportionment

Organic aerosol is an important component of atmospheric particulate matter,of which the chemical composition and atmospheric processes are closely related to its health and climate effects.The characterization of molecular composition and gas/particle partitioning behavior are key to understanding the source and formation of organic aerosols.However,most studies only use the particulate concentration data of organic molecular markers(OMMs)for source apportionment based on receptor model,and do not consider the influence of gas/particle partitioning process.The gas/particle partitioning of non-(or weak)polar components(such as polycyclic aromatic hydrocarbons)have been intensively investigately,while the gas/particle partitioning characteristics and mechanism of polar components are still unclear.In this study,two identical medium volume samplers equipped with two quartz fiber filters(QFFs),polyurethane foam(PUF)and XAD-4 resin were used to collect particulate and gaseous samples of OMMs with different polarity.From September 2018 to September 2019,102 groups of filter and adsorbent samples were collected,and three methods were used to adjust sampling error of particulate organics and separate gaseous and particulate OMMs concentrations.According to the absorptive gas/particle partitioning theory,the measured and simulated values of gas/particle partitioning coefficients of each target OMM were calculated and compared,so as to evalute the applicability of the absorptive partitioning model.Finally,positive matrix factorization(PMF)was performed based on total(PMF_total)and particle-phase OMMs data,respectively,to elucidate the influence of gas/particle partitioning on source apportionment.The main results are as follows:The distribution of non-polar OMMs in different sampling media highly depends on their vapor pressures.Low molecular weight OMMs(Log p_L^o,*>-6)mainly exist in gas phase,while high molecular weight ones(Log p_L^o,*<-10)primarily exist in the particle phase.The total concentrations of n-alkanes and PAHs were dominated by light n-alkanes with carbon numbers of 12–17(n-C12–n-C17,25.77±15.55–43.09±16.32 ng m^-3)and naphthalene(91.98±56.80 ng m^-3),respectively.Polar OMMs were mainly distributed into the particle phase(>60%),and their particulate fractions did not vary consistently with vapor pressures.Glycerol(73.52±37.95 ng m^-3)and levoglucosan(64.79±71.02 ng m^-3)had the highest average concentrations among all target polar OMMs.The measurement-based gas/particle partitioning coefficients(K^m_p,OM,m^-3(?)g^-1)of non-polar tracers were in good agreement with theoretical simulation(K^t_p,OM,m^-3(?)g^-1).Particle-phase concentrations of low molecular weight non-polar OMMs(Log p_L^o,*>-7)were overestimated due to gaseous adorption,and their K^m_p,OM values were higher than corrsponding K^t_p,OM.While the K^m_p,OMvalues of high molecular weight non-polar OMMs(Log p_L^o,*<-9)were generally lower than K^t_p,OM,which is likely ascribed to the underestimation of the evaporation loss from particles.Regarding polar OMMs,the difference between K^m_p,OM and K^t_p,OM varied by 1–3 orders of magnitude,and no correlation was observed(r=-0.55–0.44),indicating that the gas/particle partitioning of polar OMMs might not follow the equilibrium absorptive partitioning theory.Most of the target polar OMMs in this study are highly water soluble and are likely partitioned in to aersol liquid water under high relative humidity,which might also be influenced by the“salting-in”effect.Because the total and particle-phase concentrations of non-polar OMMs had distinct temporal variations,they was utilized as input data for PMF analysis(PMF_total and PMF_particle)and four factors were resolved.The contribution time series of low molecular weight OMMs factor in the two scenarios are least correlated(r=0.11),its contribution to OC and EC in PMF_particle solution was larger than that in PMF_total solution by 48.87%.When polar OMMs data were involved,four additional factors were identified,and the secondary organic aerosol factor contributed 23.37% of OC.