Parametrising pNH3 over ammonium sulfate aerosols
As part of a paper I’m currently writing on interactions between the sulfur and reduced nitrogen cycles in the remote marine environment, I’m building a 1-d time-stepping model of ammonium release from the ocean and subsequent uptake into acid sulfate aerosol. In true Martin bodge style, I’m doing it in excel. The reason for this is that I’ve had to boot into Windows to run the group’s weekly backups to an NTFS external hard drive (Sudipta usually does this) and it’s copying at about 10k per second. Been going for nearly 24 hours now!! Anyway, back to the science…
In the terrestrial environment, Harrison and Kitto (1992) give a pseudo first-order rate constant for the reaction of ammonia with acid sulfate aersol as a function of aerosol acidity. This is great, frankly, as no-one else has managed anything similar. However, in the relatively acidic terrestrial environment (they made their measurements in Essex in the late eighties) complete neutralisation was never approached. The rate constant doesn’t inhibit reaction as complete neutrlaisation is approached, which seems rather unrealistic for the Marine environment. Certainly the Aerosol Inorganics model (Clegg et al, 1998), and that of Quinn et al, (1992) suggest that the aerosol and gas phases should be in equilibrium and that the partial pressure of ammonia (pNH3) over the aerosol should increase exponentially with ammonium to sulfate ratio, thus inhibiting further uptake of ammonia to the aerosol phase.
Great! But for my purposes I need to quantify pNH3 for a range of temperatures and relative humidities. I have done this using the excellent aerosol inorganics model written by Simon Clegg and co-workers. Version II of the model (H+ - NH4+ - SO42- - NO3- - H2O) was run in parametric mode, varying chemical composition and keeping relative humidity (r.h.) and temperature constant for each run. The formation of gas phase species was suppressed, so total aerosol composition stayed constant (except for the substitution of H+ for NH4+).This gave me curves of pNH3 vs NH4+:SO42- molar ratio for each Temperature and r.h.
Before I show you the results, let’s have a think about what we might expect (i.e. what I was expecting)...
- As temperature increases we would expect pNH3 to increase as volatilisation from the solid / aqueous phase is favoured. Another way to look at this is that at higher temperatures, it is harder to condense things from the gas phase.
- As relative humidity increases, I would expect to see a decrease in pNH3 as the aerosol phase becomes more dilute and thus the concentration of dissolved ammonia in the aerosol water goes down.
Well, I was partly right! Here are the results:
results from the AIM model
Sorry about the abysmal quality of the images, but I’m still stuck in Windows so had to use MS Paint of all things :-(
So, it turns out that relative humidity has the opposite effect to that expected. Why? To be honest, I haven’t quite figured that out. I’d welcome any suggestions.
However, I would like to be quietly smug about the predictive equations I’ve derived to represent these curves in my model (and for anyone else to use if you’d like to…)
They are as follows:
RH=60%
if r<1.1:
pNH3 = 5×10-5 x exp(0.132T + 4.4r)
else:
pNH3 = 0.0026 x exp(0.185T)
where r is the NH4+:SO42- molar ratio in aerosol and T is temperature of the system in °C.RH=75%
if r<1.58:
pNH3 = 1.6×10-4 x exp(0.124T + 4.6r)
else:
pNH3 = 0.106 x exp(0.18T)
RH=90%
pNH3 = 0.0003 x exp(0.25T + 4r)
Of course, I could further boil this down to one equation with a dependence on RH too but for two reasons I haven’t done this. 1) the relationship with r.h. is highly non-linear due to phase changes between solids and aqueous aerosol. ii) I’m lazy / busy.
However, these do the job pretty well (I’ll just run my model at these three humidities). Here’s the proof:
References
Harrison, R. M. and A.N. Kitto, 1992, Estimation of the Rate Constant for the Reaction of Acid Sulphate Aerosol with NH3 Gas from Atmospheric Measurements, Journal of Atmospheric Chemistry, 15, 133–143.
Clegg S. L., P. Brimblecombe and A.S. Wexler, 1998, A thermodynamic model of the system H+ – NH4+ – SO42— – NO3— – H2O at tropospheric temperatures, J. Phys. Chem. A 102, 2137–2154.
Quinn, P.K., W.E. Asher and R.J. Charlson, 1992, Equilibria of the Marine Multiphase Ammonia System, Journal of Atmospheric Chemistry, 14, 11–30.
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