Quantification of PM2.5 organic carbon sampling artifacts in US networks

摘要

Field blanks (bQF) and backup filters (quartz-fiber behind quartz-fiberfilter; QBQ) have been adopted by US long-term air quality monitoringnetworks to estimate PM organic carbon (OC) sampling artifacts. Thisstudy documents bQF and QBQ carbon levels for the: 1) Interagency Monitoring ofProtected Visual Environments (IMPROVE); 2) Speciation Trends Network (STN;part of the Chemical Speciation Network [CSN]); and 3) Southeastern AerosolResearch and Characterization (SEARCH) networks and examines thesimilarities/differences associated with network-specific samplingprotocols. A higher IMPROVE sample volume and smaller filter deposit arearesults in PM areal density (μg/cm on filter) 3–11 timesthose of STN/CSN and SEARCH samples for the same ambient PMconcentrations, thus reducing the relative contribution of samplingartifacts from passive OC adsorption. A relatively short (1–15 min)passive exposure period of STN/CSN and SEARCH bQF OC (0.8–1 μg/cm)underestimates positive and negative OC artifacts resultingfrom passive adsorption or evaporation of semi-volatile organic compounds onquartz-fiber filters. This is supported by low STN/CSN and SEARCH bQF levelsand lack of temporal or spatial variability among the sites within thenetworks. With a much longer period, ~7 days of ambient passiveexposure, average IMPROVE bQF and QBQ OC are comparable (2.4±0.5 and3.1±0.8 μg/cm, respectively) and more than twice levelsfound in the STN/CSN and SEARCH networks. Sampling artifacts in STN/CSN wereestimated from collocated IMPROVE samples based on linear regression. At sixof the eight collocated sites in this study, STN/CSN bQFs underestimated OCartifacts by 11–34%. Using a preceding organic denuder in the SEARCHnetwork minimized passive adsorption on QBQ, but OC on QBQ may not beattributed entirely to the negative sampling artifact (e.g., evaporated orvolatilized OC from the front filter deposits after sample collection).