Ammonium Associated with Sulfates and Retained Nitrates

To determine the mass associated with nitrates, we first assume retained nitrate is

probably all ammonium nitrate. Thus the ammonium associated with nitrates can be derived

directly from the measured or predicted NO3FRM as

NH4NO3 = 0.29 * NO3FRM [5.3]

The difference between total FRM NH4 (amount associated with nitrates and sulfates),

termed NH4FRM, and the measured NH4, termed NH4STN,, is needed to determine the ammoniated

form of sulfates as described by equation 5.1. Recent measurement study by Collett (Collett,

2004) shows that NH4 may not be completely retained during collection on nylon filters

preceded by a nitric acid denuder. During sampling conditions associated with nitrate

volatilization, ammonium nitrate dissociates but the HNO3 downstream of the denuder is

recaptured on the basic nylon media and the result is reported as particle nitrate. On the other

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.



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