Relationship between the fine nitrate and ammonium annual surface concentrations

[yaoead96]

Hello,

I have an enquiry regarding the relationship between the fine nitrate and ammonium surface concentrations. All concentrations mentioned here are molar concentrations. Attached below please find the global map of the annual fine nitrate to ammonium ratio. I originally thought that ammonium consists of (NH4)2SO4 and NH4NO3 in the model and that fine nitrate is only associated with NH4NO3, thus the fine nitrate to ammonium ratio (NO3- / NH4+) represents the contribution of NH4NO3 to total NH4+. However, the global map shows that the fine nitrate to ammonium ratio is far larger than 1 (which means there is more fine nitrate than ammonium) in most marine areas of southern hemisphere (0°-60°S). The maximum ratio reaches to 19.6.

Therefore, I am wondering is there any potential marine source (e.g. sea salt) for fine nitrate in the EMEP MSC-W model as well? And is the model output of fine nitrate not only associated with NH4NO3?

The detailed model experiment information is listed below.
Model version: EMEP4.34 + WRF4.1.1
Domain: Global run excluding the North Pole belt with resolution of 1° x 1°.
Year: 2015
Emission inventory: ECLIPSE v6a
Aerosol Equilibrium scheme: EQSAM
Chemical mechanism: EmChem19

Please feel free to contact me should you need any further information.

Thank you very much for your help.

Yao (PhD student)
UKCEH & University of Edinburgh
UKCEH Supervisor: Dr Massimo Vieno

[mifads]

Hi Yao,
Hmm, odd. The model output of fine nitrate should indeed be just NH4NO3, so NO3- / NH4+ should indeed be less than one. I'll check how this looks in our runs.

Just a detail: the NH4 component isn't just associated with ammonium sulphate, (NH4)2SO4, but also with bisulphate (NH4HSO4) in both MARS and EQSAM. This doesn't affect your question though.

[mifads]

I just checked a global run I had handy, and get an NO3-/NH4- ratio which is less than one everywhere. The max value was 0.82. Two things to check:

1. Did you scale from ug compound to ug(N) before taking the ratio?
2. Are your high values something to do with very small NH4? (is is there some numerical issue in the script or data used?)

[yaoead96]

I just checked a global run I had handy, and get an NO3-/NH4- ratio which is less than one everywhere. The max value was 0.82. Two things to check:

1. Did you scale from ug compound to ug(N) before taking the ratio?
2. Are your high values something to do with very small NH4? (is is there some numerical issue in the script or data used?)

Many thanks for your checking.

For the first question, I used molar concentration to calculate the ratio, which should do the same thing as the ugN ratio.

For the second question, in places where NO3/NH4 ratio shows extremely high values (mainly in southern hemisphere), the NH4+ concentrations are indeed very low (e.g., the NH4+ concentration at 38S, 148W is 0.001595 ug/m3, while the fine NO3- concentration at the same grid is 0.021782 ug/m3). Is the NH4+ concentration as listed in above example too small?