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Open AccessArticle

On Aerosol Liquid Water and Sulfate Associations: The Potential for Fine Particulate Matter Biases

1
Department of Chemistry, University of California, Irvine, CA 92617, USA
2
Department of Chemical, Biochemical, and Environmental Engineering, University of Maryland, Baltimore County, 1000 Hilltop Circle, Baltimore, MD 21250, USA
3
Environmental Science Division, Argonne National Laboratory, Argonne, IL 60439, USA
*
Author to whom correspondence should be addressed.
Atmosphere 2020, 11(2), 194; https://doi.org/10.3390/atmos11020194
Received: 16 January 2020 / Accepted: 5 February 2020 / Published: 12 February 2020
(This article belongs to the Special Issue Atmospheric Aqueous-Phase Chemistry)
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.
Keywords: PM2.5; air quality; aerosol liquid water PM2.5; air quality; aerosol liquid water
MDPI and ACS Style

Babila, J.E.; Carlton, A.G.; Hennigan, C.J.; Ghate, V.P. On Aerosol Liquid Water and Sulfate Associations: The Potential for Fine Particulate Matter Biases. Atmosphere 2020, 11, 194.

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