Nitrate loss associated with Federal Reference Method

Abstract:

The 24-h PM2.5 mass, measured by the Federal Reference Method

(FRM) designated by EPA, reports the bulk PM2.5 mass retained on a

single channel sampler with Teflon or Quartz filters. Semivolatile species

such as nitrate can be lost from the Teflon filter during sampling and

equilibration processes, and the FRM measurements may not reflect true

ambient concentrations of total PM2.5. Consequently, FRM PM2.5 can

be problematic when used for evaluating the fidelity of air quality models,

especially for domains where particulate nitrate is a dominant contributor.

In order to account for this discrepancy, a thermodynamic adjustment

method was developed by EPA to determine the retained nitrate

based upon the temperature (T) and relative humidity (RH). The nitrate

adjustment method was evaluated for the eastern US where summertime

is characterized by hot and humid weather and sulfate particles are the

dominant inorganic contributor. Potential differences exist for applications

to California due to its different PM2.5 composition and meteorology. In

this study we, (1) evaluate EPA's nitrate adjustment method for California

where particulate nitrate is a major contributor to total PM2.5 mass and (2)

apply this method to adjusting simulated PM2.5 by the Community Multiscale

Air Quality Model(CMAQ) over the California domain. The adjusted CMAQ

outputs are then compared to the observed total PM2.5. The positive

biases between CMAQ and observed PM2.5 can be largely explained by

evaporative loss of nitrate on filters.