% aou.m                                            by:  Edward T Peltzer, MBARI
%                                                  revised:  2007 Apr 26.
%
% CALCULATE OXYGEN 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 oxygen at STP obtained from NIST website on the
%          thermophysical properties of fluid systems:
%
%          http://webbook.nist.gov/chemistry/fluid/
%
%
% CALCULATE AOU BY DIFFERENCE:
%
%         AOU (umol/kg) = sat O2 (umol/kg) - obs o2 (umol/kg).
%
%
% Input:       S = Salinity (pss-78)
%              T = Potential Temp (deg C)
%              O2 = Meas'd Oxygen Conc (umol/kg)
%
% Output:      Apparant Oxygen Utilization (umol/kg).
%
%                        AOU = aou(S,T,O2).

function [AOU]=aou(S,T,O2)


% DEFINE CONSTANTS, ETC FOR SATURATION CALCULATION

%    The constants -177.7888, etc., are used for units of ml O2/kg.

  T1 = (T + 273.15) ./ 100;

  OSAT = -177.7888 + 255.5907 ./ T1 + 146.4813 .* log(T1) - 22.2040 .* T1;
  OSAT = OSAT + S .* (-0.037362 + T1 .* (0.016504 - 0.0020564 .* T1));
  OSAT = exp(OSAT);


% CONVERT FROM ML/KG TO UM/KG

  OSAT = OSAT * 1000 ./ 22.392;


% CALCULATE AOU

  AOU = OSAT - O2;