% n2sat.m                                          by:  Edward T Peltzer, MBARI
%                                                  revised:  2007 Apr 25.
%
% CALCULATE NITROGEN CONCENTRATION AT SATURATION
%
% Source:  The solubility of nitrogen, oxygen and argon in water and
%          seawater - Weiss (1970) Deep Sea Research V17(4): 721-735.
%
% Molar volume of nitrogen at STP obtained from NIST website on the
%          thermophysical properties of fluid systems:
%
%          http://webbook.nist.gov/chemistry/fluid/
%
%
% Input:       S = Salinity (0/00)
%              T = Temp (deg C)
%
% Output:      Nitrogen saturation at one atmosphere (umol/kg).
%
%                        N2 = n2sat(S,T).

function [N2] = n2sat(S,T)


% DEFINE CONSTANTS, ETC FOR SATURATION CALCULATION

%    The constants -177.0212, etc., are used for units of ml N2/kg.

  T1 = (T + 273.15) ./ 100;

  NSAT = -177.0212 + 254.6078 ./ T1 + 146.3611 .* log(T1) - 22.0933 .* T1;
  NSAT = NSAT + S .* (-0.054052 + T1 .* (0.027266 - 0.0038430 .* T1));
  NSAT = exp(NSAT);


% CONVERT FROM ML/KG TO UM/KG

  N2 = NSAT * 1000 ./ 22.404;