To what extent can aerosol water explain the discrepancy between model calculated and gravimetric PM10 and PM2.5?

[摘要] Inter-comparisons of European air quality models show that regionaltransport models, including the EMEP (Co-operative Programme for monitoringand evaluation of the long-range transmission of air pollutants in Europe)aerosol model, tend to underestimate the observed concentrations ofPM₁₀ and PM_2.5. Obviously, an accurate representation of theindividual aerosol constituents is a prerequisite for adequate calculationof PM concentrations. On the other hand, available measurements on thechemical characterization of ambient particles reveal that full chemical PMmass closure is rarely achieved. The fraction unaccounted for by chemicalanalysis can comprise as much as 30-40% of gravimetric PM₁₀ orPM_2.5 mass. The unaccounted PM mass can partly be due to non-C atoms inorganic aerosols and/or due to sampling and measurement artefacts. Moreover,a part of the unaccounted PM mass is likely to consist of water associatedwith particles. Thus, the gravimetrically measured particle mass does notnecessarily represent dry PM₁₀ and PM_2.5 mass. This is thought tobe one of the reasons for models under-prediction of observed PM, ifcalculated dry PM₁₀ and PM_2.5 concentrations are compared withmeasurements. The EMEP aerosol model has been used to study to what extentparticle-bound water can explain the chemically unidentified PM mass infilter-based particle samples. Water content of PM_2.5 and PM₁₀ hasbeen estimated with the model for temperature 20°C and relative humidity50%, which are conditions required for equilibration of dust-loadedfilters according to the Reference method recommended by the EuropeanCommittee for Standardization (CEN). Model calculations for Europe showthat, depending on particle composition, particle-bound water constitutes20-35% of the annual mean PM₁₀ and PM_2.5 concentrations, whichis consistent with existing experimental estimates. At two Austrian sites,in Vienna and Streithofen, where daily measurements of PM_2.5 mass andchemical composition are available, calculated PM_2.5 water content isfound to be about 75-80% of the undetermined PM_2.5 mass and there iscorrelation between them. Furthermore, accounting for aerosol water hasimproved the agreement between modelled and measured daily PM_2.5concentrations, whilst model calculated dry PM_2.5 concentrations appearto agree quite well with the total identified PM_2.5 mass. Noinformation on the composition of PM measured at EMEP sites is presentlyavailable. Given that PM₁₀ and PM_2.5 concentrations are measuredat EMEP stations with gravimetric methods they are likely to contain water.We show that the levels of modelled PM₁₀ and PM_2.5 concentrationswith aerosol water included agree with measurements better than dry PMconcentrations. As expected, the spatial correlation has not changedsignificantly, whereas the temporal correlation of daily PM₁₀ andPM_2.5 with monitoring data has slightly improved at most of the EMEPsites. Our results suggest that aerosol water should be accounted for inmodelled PM₁₀ and PM_2.5 when compared with filter-basedgravimetric measurements.

[学科分类] 大气科学