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Accurate sampling and analysis of PM2.5 is essential to determine the compliance with promulgated ambient PM2.5 standards and identify the sources of PM2.5 pollutants.USA PM2.5 FRM (federal reference method) manual samplers or FEM (federal equivalent method) monitors are commonly used but there are still rooms to improve the accuracy.In several air monitoring stations in Taiwan,it was found PM2.5 concentrations are over-estimated by the VEREWA-F701 BAM (beta attenuation monitor) by as much as 58.4 %,and 28.4-29.8 % by the earlier version of the Met-One BAM-1020s (without FEM designation).In this talk,reasons why PM2.5 concentrations are overestimated by automatic beta attenuation monitors,including positive artifacts due to acid gas adsorption by the glass fiber filter tapes used in the monitors,aerosol water content and volatilization loss of inorganic semi-volatile species will be presented (Liu et al.,2013a).Then the size-selective inlets of current manual PM samplers,including impactors and cyclones,will be reviewed and problems associated with particle bounce,evaporation loss,ambient relative humidity and substrate coating conditions will be discussed.It was found that the BGI VSCC (very sharp cut cyclone) has a better solid particle loading performance than that of the WINS impactor and oil-coated Teflon substrates are capable of resolving solid particle loading problems in impactors (Tsai et al.,2012).The results of using the newly developed MFPPS (multi-filter PM10-PM2.5 sampler) in NCTU (Liu et al.,2011) to determine artifacts during filter sampling and conditioning of PM2.5 manual samplers will be presented.The evaluation of PM2.5 monitoring accuracy of the Model 1405-DF TEOM-FDMS,which is capable of correcting for positive and negative PM2.5 artifacts,will also be presented.For obtaining accurate size-classified aerosol samples,a newly developed 10-stage NCTU micro-orifice cascade impactor(NMCI,56nm to 10 m)which contains new nozzle plates with smooth nozzle shape made by the LIGA (Lithography,Electroplating,and Molding) process,will be introduced (Liu et al.,2013b).The laboratory and field comparison test results showed the NMCI outperforms the widely used micro-orifice uniform deposit impactor (MOUDI,MSP Model 110).