function [tau]=opt_no2(lambda,DU,temp)
% compute NO2 optical depth based on NO2 toal column in DU and temperature in C.
% Requaries NO2.dat file

if nargin<3
 temp=0;       % mean temperature of NO2 in C
end
load NO2

cros=data.coef(:,1)*temp.^2+data.coef(:,2)*temp+data.coef(:,3);
x=DU*2.69e16*cros;
tau=interp1(data.lambda,x,lambda);
