Abstract
Abstract. Field blanks (bQF) and backup filters (quartz-fiber behind quartz-fiber filter; QBQ) have been adopted by US long-term air quality monitoring networks to estimate PM2.5 organic carbon (OC) sampling artifacts. This study documents bQF and QBQ carbon levels for the: 1) Interagency Monitoring of Protected Visual Environments (IMPROVE); 2) Speciation Trends Network (STN; part of the Chemical Speciation Network [CSN]); and 3) Southeastern Aerosol Research and Characterization (SEARCH) networks and examines the similarities/differences associated with network-specific sampling protocols. A higher IMPROVE sample volume and smaller filter deposit area results in PM2.5 areal density (μg/cm2 on filter) 3–11 times those of STN/CSN and SEARCH samples for the same ambient PM2.5 concentrations, thus reducing the relative contribution of sampling artifacts from passive OC adsorption. A relatively short (1–15 min) passive exposure period of STN/CSN and SEARCH bQF OC (0.8–1 μg/cm2) underestimates positive and negative OC artifacts resulting from passive adsorption or evaporation of semi-volatile organic compounds on quartz-fiber filters. This is supported by low STN/CSN and SEARCH bQF levels and lack of temporal or spatial variability among the sites within the networks. With a much longer period, ~7 days of ambient passive exposure, average IMPROVE bQF and QBQ OC are comparable (2.4±0.5 and 3.1±0.8 μg/cm2, respectively) and more than twice levels found in the STN/CSN and SEARCH networks. Sampling artifacts in STN/CSN were estimated from collocated IMPROVE samples based on linear regression. At six of the eight collocated sites in this study, STN/CSN bQFs underestimated OC artifacts by 11–34%. Using a preceding organic denuder in the SEARCH network minimized passive adsorption on QBQ, but OC on QBQ may not be attributed entirely to the negative sampling artifact (e.g., evaporated or volatilized OC from the front filter deposits after sample collection).
Highlights
PM2.5 and PM10 sampling onto quartzfiber filters is accompanied by positive and negative artifacts
This study examines the methods and results of organic carbon (OC) artifact assessment in these networks by: 1) documenting procedures to acquire blank and backup filters; 2) comparing laboratory blank, bQF, trip blank, and quartzfiber behind quartz-fiber filters (QBQ) filter OC levels for the period from 1 January 2005 to 31 December 2006; and 3) assessing blank OC levels from eight collocated Interagency Monitoring of Protected Visual Environments (IMPROVE) and Speciation Trends Network (STN)/CSN sites using the SANDWICH method (Frank, 2006)
Average bQF levels for individual sampling sites and the number of bQF acquired for IMPROVE and STN/CSN are available as supplemental information (Tables S1–S4, http://www.atmos-chem-phys.net/10/5223/ 2010/acp-10-5223-2010-supplement.pdf) and in more detailed reports (Chow et al, 2008b; Watson et al, 2008)
Summary
PM2.5 and PM10 (particulate matter with aerodynamic diameters
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