Errors in coarse particulate matter mass concentrations and spatiotemporal characteristics when using subtraction estimation methods

摘要

In studies of coarse particulate matter (PM_(10-2.5)), mass concentrations are often estimated through the subtraction of PM_(2.5) from collocated PM_(10) tapered element oscillating microbalance (TEOM) measurements. Though all field instruments have yet to be updated, the Filter Dynamic Measurement System (FDMS) was introduced to account for the loss of semivolatile material from heated TEOM filters. To assess errors in PM_(10-2.5) estimation when using the possible combinations of PM_(10) and PM_(2.5) TEOM units with and without FDMS, data from three monitoring sites of the Colorado Coarse Rural-Urban Sources and Health (CCRUSH) study were used to simulate four possible subtraction methods for estimating PM_(10-2.5) mass concentrations. Assuming all mass is accounted for using collocated TEOMs with FDMS, the three other subtraction methods were assessed for biases in absolute mass concentration, temporal variability, spatial correlation, and homogeneity. Results show collocated units without FDMS closely estimate actual PM_(10-2.5) mass and spatial characteristics due to the very low semivolatile PM_(10-2.5) concentrations in Colorado. Estimation using either a PM_(2.5 )or PM_(10) monitor without FDMS introduced absolute biases of 2.4 μg/m~3 (25%) to -2.3 μg/m~3 (-24%), respectively. Such errors are directly related to the unmeasured semivolatile mass and alter measures of spatiotemporal variability and homogeneity, all of which have implications for the regulatory and epidemiology communities concerned about PM_(10-2.5). Two monitoring sites operated by the state of Colorado were considered for inclusion in the CCRUSH acute health effects study, but concentrations were biased due to sampling with an FDMS-equipped PM_(2.5) TEOM and PM_(10) TEOM not corrected for semivolatile mass loss. A regression-based model was developed for removing the error in these measurements by estimating the semivolatile concentration of PM_(2.5) from total PM_(2.5) concentrations. By estimating nonvolatile PM_(2.5) concentrations from this relationship, PM_(10-2.5) was calculated as the difference between nonvolatile PM_(10) and PM_(2.5) concentrations.