Let's say the difference in intensity is entirely attributable to the greater distance in the atmosphere the morning and evening sunlight must travel to get to you. I think this is a very good assumption. A quick a dirty calculation says morning sunlight travels through 283 miles of air while noon sun travels through 10, if you're at the equator in midsummer. So now you can calculate the distance variation. Altitude makes no difference under the assumptions I've made.
Geographical location makes a big difference, of course.
Other factors that affect the relative intensity include clouds and mountains. Argonne National Laboratory, Division of Educational Programs, Harold Myron, Ph.D., Division Director.