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

analysis. The ambient data is such that all of the ammonium data is from urban sites (STN), but

the sulfate and nitrate data is from both urban (STN) and rural (IMPROVE) sites. This leads to

an overestimation of ammonium concentration in rural areas when ammonium is directly

interpolated. Therefore, in the CAIR analysis, calculated DON, SO4 and NO3FRM were

interpolated to get quarterly average concentrations at each FRM site. The interpolated species

concentrations were then used to calculate NH4FRM using the following equation:

NH4FRM = DON * SO4 + 0.29*NO3FRM [5.4]

The indirect calculation of ammonium mass from interpolated fields tends to smooth out

the gradients in mass. This was deemed to be beneficial, due to the uncertainty in the


Particle Bound Water

Because ammoniated sulfate and ammonium nitrate are hygroscopic, the retained sulfate

and nitrate mass will include water. Particle bound water (PBW) can be estimated using an

equilibrium model. The CAIR analysis used the Aerosol Inorganic Model (AIM) (Clegg, 1998)

to calculate PBW. PBW was derived from quarterly average FRM concentrations of sulfate,

ammonium, and nitrate as describe above. Estimated hydronium ion, H+, needed to achieve

ionic balance was derived from the latter values. The model enables the distribution of water

and ions to be calculated between liquid, solid and vapor phases for specific temperature and

relative humidity conditions. Typical FRM filter equilibration conditions of 35% RH and 22 deg

C (295 deg K) temperature were used.



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