Disregarding the Sun's (well characterized) ordinary cyclic and long term variation, all that changes with solar radiation (that you can calculate, anyway) is distance from the Sun and the Sun's apparently angle in the sky (which depends on date and time of day -- i.e. how much air the light must pass through).
The lower the Sun's apparent sky position, the more filtering the air gives. Air preferentially filters blue light (by scattering), but doesn't change the fundamental spectrum (number, position, and intensity of emission or absorption lines, for instance). You should be able to find information (for photographers, if no other source) for light intensity and color changes throughout the day -- I recall the correction for exposure by rule ("Sunny f/16" rule) being plus 1 stop before 10 AM or after 2 PM (standard time, correct for DST), 2 stops before 8 and after 4, three stops if the sun is within five degrees of the horizon, and a particular filter (or color sensitivity correction for some panchromatic black and white films) within an hour of sunrise or sunset, the so-called "golden hour". Add one additional stop if more than 45 degrees from the equator, another if within one month of the winter solstice.
Each "stop" corrects for a halving of the total light flux, so if you're down three stops, you have 1/8 the light level.