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

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.

Seasonal Contribution of Isoprene-Derived Organosulfates to Total Water-Soluble Fine Particulate Organic Sulfur in the United States

“…While daily OM concentrations were also available, IMPROVE calculates OM from OC by applying a constant multiplier (OM/OC ratio, f OC ) of 1.8 by default . In reality, f OC is dependent on the form of OC, and therefore varies seasonally and spatially, with lowest values typically observed at urban sites in winter and highest values in rural/suburban sites in summer. − Recently, Hand et al performed multiple linear regression analysis using the daily IMPROVE site data for the contiguous U.S. and reported seasonal regression median OC coefficients between 2011 and 2016 for the four study regions (i.e., Northwest [NW], Northeast [NE], Southwest [SW], and Southeast [SE]) separated at 40° N and −100° W. The reported median OC coefficients during 2011–2016 for NW, NE, SW, and SE regions are 1.60, 1.79, 1.38, and 1.75, respectively, in winter and 2.01, 2.13, 1.98, and 2.21, respectively, in summer. Following the same region definition, we applied the season- and region-specific median OC coefficients reported by Hand et al as f OC to the daily samples and recalculated OM as f OC × OC.…”

Multiphase Atmospheric Chemistry in Liquid Water: Impacts and Controllability of Organic Aerosol

“…Organic aerosol collected at IMPROVE sites often contains highly oxidized functional groups, such as alcohols, carboxylic acids, and organosulfur moieties that contribute noncarbonaceous organic mass. The ratio of functionalized organic matter-to-organic carbon (OM/OC) measured at surface sites across the U.S. in the IMPROVE network is lowest in urban locations in winter and highest in rural areas in summer . OM/OC ratios are increasing across the CONUS at rates that vary by location, and aqSOA generally has higher oxygen-to-carbon (O/C) ratios .…”

“…The ratio of functionalized organic matter-to-organic carbon (OM/OC) measured at surface sites across the U.S. in the IMPROVE network is lowest in urban locations in winter and highest in rural areas in summer . OM/OC ratios are increasing across the CONUS at rates that vary by location, and aqSOA generally has higher oxygen-to-carbon (O/C) ratios . While reduction in mass concentrations of TOC are noted for the CONUS, in particular in the east, ,,, organic aerosol mass concentrations do not decline at the same rate.…”

Brownness of Organic Aerosol over the United States: Evidence for Seasonal Biomass Burning and Photobleaching Effects

“…OC and EC are defined as carbon evolved before and after the split point for the 635 nm reflectance, respectively, while TC is the sum of OC and EC. The TOA OC–EC split is based on three assumptions on OC: (1) OC absorptivity is negligible at ∼635 nm; (2) OC is either evolved or pyrolyzed (i.e., cannot remain intact) prior to the introduction of oxygen; and (3) pyrolyzed OC is equivalent to EC with respect to the reflectance or transmittance responses. , Deviations from these assumptions in some samples may result in EC overestimates or underestimates. − …”