hand, the NH4+ volatilizes to gaseous NH3 and apparently passes thru the filter. At several FRM

study sites, the STN NH4 which is adjusted for evaporated NH4NO3 tends to more closely

correspond to the measured NH4 from the FRM Teflon filter. However, for other sites, the

measured STN NH4 appears to agree with FRM NH4.

In the CAIR analysis, 50% of the ammonium associated with volatilized nitrate was

assumed to also volatilize. But more recent information suggests that using measured

ammonium (assuming none is volatilized) may be a better assumption. We recommend using

measured ammonium, but further analysis of this issue is warranted.

There are several ways to estimate ammonium mass for use in SMAT. The most direct

way is to use measured ammonium concentrations from the STN network (IMPROVE does not

measure ammonium ion).

A second, more indirect method is to calculate the ammonium associated with sulfate and

the degree of neutralization of sulfate (DON), and then use the resulting information to calculate

ammonium mass. Due to uncertainties associated with the ammonium measurements and the

lack of ammonium measurements in rural areas, this indirect method was used for the CAIR


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