Hi, it seems that you are trying to calculate transmission map from air-light to perform some kind of haze reduction.
A function named imreducehaze() comes inbuilt starting from MATLAB 2017b. This function returns the Global atmospheric light value value and Transmission Map of an image. The following code might help.
A = imread('yourImage.jpg'); % Read your image
[J,T,L] = imreducehaze(A); % T = Transmission map, L = Global atmospheric light value