function tau=rayleighAERONET(lambda,p)
% compute the Rayleigh optical depth based Bodhaine et al., 1999 JAOT 
% See Eq. 30 
% This formula is recommeded by AERONET
% input: lambda in nm
%         p preshure in hPA
lambda=lambda/1000;
p0=1013.25;
term1=1.0455996-341.29061./lambda.^2-0.90230850*lambda.^2;
term2=1+0.0027059889./lambda.^2-85.968563*lambda.^2;
tau=p/p0*0.0021520*term1./term2;