Implications of Organic Mass to Carbon Ratios Increasing Over Time in the Rural United States

Authors

W.C. Malm, Colorado State University, Fort CollinsFollow
B.A. Schichtel, National Park Service, Air Resources Division
J.L. Hand, Colorado State University, Fort Collins
A.J. Prenni, National Park Service, Air Resources Division

ORCID IDs

https://orcid.org/0000-0001-8678-9820

https://orcid.org/0000-0002-5512-8082

https://orcid.org/0000-0002-4644-2459

https://orcid.org/0000-0002-0256-5166

Date of this Version

2-18-2020

Citation

States. Journal of Geophysical Research: Atmospheres, 125, e2019JD031480. https://doi.org/10.1029/2019JD031480

Comments

U.S. Government Works are not subject to copyright.

Abstract

The thermal evolution procedure used by most monitoring programs in the United States to determine carbonaceous aerosol concentrations is referred to as the thermal‐optical reflectance method, where an aerosol sample that has been collected on a quartz filter is heated and evolved carbon is characterized as either organic (OC) or light absorbing carbon (LAC). Evolved carbon assigned to OC is multiplied by a factor, Roc, to achieve an estimate of organic mass. Over the last 10–15 years, Roc, estimated through multiple linear regression analysis of data collected in the Interagency Monitoring of Protected Visual Environments (IMPROVE) program, has increased at about a rate of about 0.02 per year, reaching values above 2.0 in many regions of the United States. Analysis of evolved carbon concentration temporal trends suggests that thermal‐optical reflectance analysis, on the average, inaccurately bifurcates particulate carbon into the OC and LAC fractions with some LAC being inadvertently and wrongly assigned to the OC fraction. It is shown that misapportioned LAC assigned to OC is decreasing faster than true OC, resulting in a compensating increase in the Roc assigned to reported OC over time. A first‐order model is proposed to correct for the misapportioned carbon.