Abstract
In humid locations of the Eastern U.S., sulfate is a surrogate for aerosol liquid water (ALW), a poorly measured particle constituent. Regional and seasonal variation in ALW–sulfate relationships offers a potential explanation to reconcile epidemiology and toxicology studies regarding particulate sulfur and health endpoints. ALW facilitates transfer of polar species from the gas phase to the particle phase and affects particle pH and metal oxidation state. Though abundant and a potential indicator of adverse health endpoints, ALW is largely removed in most particulate matter measurement techniques, including in routine particulate matter (PM2.5) networks that use federal reference method (FRM) monitors, which are used in epidemiology studies. We find that in 2004, a typical year in the available record, ambient ALW mass is removed during sampling and filter equilibration to standard laboratory conditions at most (94%) sites, up to 85% of the ambient water mass. The removal of ALW can induce the evaporation of other semi-volatile compounds present in PM2.5, such as ammonium nitrate and numerous organics. This produces an artifact in the PM mass measurements that is, importantly, not uniform in space or time. This suggests that PM2.5 epidemiology studies that exclude ALW are biased. This work provides a plausible explanation to resolve multi-decade discrepancies regarding ambient sulfate and health impacts in some epidemiological and toxicological studies.
Highlights
More than one million life years are lost in the U.S due to fine particulate matter (PM2.5 ) exposure during a single year. [1] Studies link PM2.5 mass with a variety of adverse health consequences [2].Sulfate, a major PM2.5 constituent, is often positively correlated with increased risk in many epidemiology studies [3,4], including the Harvard six cities study [5], but not all [6]
The pattern observed for 2004 persists throughout the entire IMPROVE record analyzed in this work (Video S1, Video S2), though sulfate concentrations and absolute values of aerosol liquid water (ALW) mass concentrations are decreasing over past decades, consistent with findings in the Southeast U.S [40]
The patterns in ALW and ALW:PM2.5 ratios estimated here are independently consistent with remote optical measurements that demonstrate higher relative humidity (RH)-dependent extinction per unit of PM2.5 dry mass in the Eastern U.S [37]
Summary
More than one million life years are lost in the U.S due to fine particulate matter (PM2.5 ) exposure during a single year. [1] Studies link PM2.5 mass with a variety of adverse health consequences [2]. PM2.5 mass concentration measurements using federal associated with or formed in ALW, and lost during filter sampling and/or equilibration remains a equivalent methods (FEM) evaporate ALW from ambient particles prior to collection [15]. Organic species vary greatly in their effects on ALW [46,47], and detailed chemical characterization of the organic fraction of PM2.5 is not available Neglect of these species introduces uncertainty, and while absolute mass concentrations are uncertain, we are confident that overall trends are qualitatively robust in space and time [37,40] Laboratory conditions for estimated ALW are 21.5 ◦ C and 35% RH for all sites on all measurement days. These are the central values established for equilibration of FRM PM2.5 filters, and we calculate laboratory ALW estimates for the central values, as an idealized case [31]
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