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),
delta NO3 (ug/m3)= 745.7/TR* 1/24* E (Ki
where delta NO3 is the amount of volatilized nitrate mass, TR is the reference temperature for the
sampled air volume in degrees Kelvin and Ki is the dissociation constant for ammonium nitrate
evaluated at the ambient temperature for hour i. The nitrate loss can be predicted for each day,
based on hourly temperature and relative humidity data. The SMAT analysis for CAIR used
National Weather Service temperature and relative humidity data for the closest station. Other
sources of meteorological data may also be appropriate. Further details on the nitrate loss
calculations can be found in (Frank, 2006).
Ammonium Associated with Sulfates and Retained Nitrates
To determine the mass associated with nitrates, we first assume
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