In the above characterization, RCFM equals reconstructed fine mass and all of the listed

chemical components reflect those retained during sampling and equilibration on the FRM’s

Teflon filter. Sulfate and nitrate mass include associated ammonium which may be different than

assumed ammonium sulfate and ammonium nitrate compounds. Also, sulfates, nitrates and

carbonaceous mass includes particle bound water associated with these hygroscopic aerosols. In

this characterization, other primary PM2.5 mass is intended to be a more general term that

includes fine soil, and oxides that result from other PM emissions. The following section

describes the recommended treatment for each of the components.

Retained Nitrate Mass

The first step in the procedure for identifying FRM mass components is to estimate the

retained nitrate mass on the FRM filters. The FRM does not capture all of the semi-volatile

components of the ambient air, such as ammonium nitrate. The retained amount of nitrate ion,

however, can be accurately estimated by a simple thermodynamic model that involves 24-hr

ambient nitrate speciation concentrations (as measured by a standard speciation sampler using a

nylon filter preceded by a HNO3 denuder) together with hourly ambient temperature and

humidity. Atmospheric nitrates are higher during the cooler months. Retention on the FRM is

also higher during the cooler months and essentially all the nitrates are lost during the summer.

The retention does not appear to depend on ambient NH3 or HNO3. More NO3 is retained at

low temperatures and high humidity which varies by sampling location and time of year.

Because nitrate retention varies by site and season, an ammonium nitrate equilibrium

model can be used to predict the amount of nitrates retained on the FRM Teflon filter. As used

by Hering (Hering, 1999; Zhang, 1992),

 

n601 - n602 - n603 - n604 - n605 - n606 - n607 - n608 - n609 - n610 - n611 - n612 - n613 - n614 - n615 - n616 - n617 - n618 - n619 - n620 - n621 - n622 - n623 - n624 - n625 - n626 - n627 - n628 - n629 - n630 - n631 - n632 - n633 - n634 - n635 - n636 - n637 - n638 - n639 - n640 - n641 - n642 - n643 - n644 - n645 - n646 - n647 - n648 - n649 - n650

 

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