function y=no2a(u)
% Oxidation of NO to NO2.
% Atkins and Jones, second edition, pp. 720-721. 
% This function computes the quotient of 
% the rate u4' and the overall third order 
% rate law u2*u1^2.  This quotient should 
% be equal to k=2*k11*k21/k12.

global k11 k12 k21 k22 

r21=k21.*u(:,2).*u(:,3);
r22=k22.*u(:,4).^2;

y=(2*r21-2*r22)./(u(:,2).*u(:,1).^2);
