Solar Radiation

As you should know from previous experience or from earlier courses (particulary OS411B: Essential Atmospheric Sciences, the sun is the major driving force for atmospheric events, including the formation of secondary pollutants such as ozone. At its most basic, we need to account for various components of sunlight, such as: reflected radiation direct radiation ground refelected radiation diffuse radiation absorbed radiation scattered radiation The graphic at the right (click on image to see larger) shows each of these six types of radiation. Each type has its own influence on the behavior of the atmosphere, and hence, air quality.

(Click on image to see full-sized)

(Click on image to see full-sized)

Most air quality models need information about the location being studied. For example, in one of the EPA's tropospheric ozone models (EKMA/OZIP), the model requires the value of the solar zenith angle (the angle between the direction of the sun and the zenith (directly overhead, shown in the graphic at left). This value is then used to determine the rate constant for many of the 100+ chemical reactions that are used to describe the chemistry of the atmosphere. Many models, including EKMA/OZIP, determine the solar zenith angle automatically. The analyst typically only needs to determine the latitude and longitude for the area being studied. Given the location, date, and other data about the area being studied, we can calculate many important and useful datapoints regarding solar radiation.

For example, the table below shows the results of a calculation for Durham, NC (35.98N, 78.91W) on November 28, 2000 at 3 PM Eastern Standard Time. The amount of solar radiation reported is 1405 W (watts, a unit of power) per square mile. This is roughly 3% more than the solar constant (1367 W/square meters). The solar constant is a global average of solar radiation, including the dark parts of the Earth, so the fact that the radiation is slightly more than the solar constant here during daytime suggests we're in late autumn, afternoon conditions with less sunlight than some parts of the daylit Earth are receiving.

To do this calculation, we used an online solar calculator.

	Inputs Year 2000 Month 11 Day 28 Hour 15 Minute 0 Latitude 35 Longitude -78 Timezone -5 Pressure 1013 Temperature 10 Aspect 180 Solar Constant 1367		Outputs Declination (degrees) -21.4633 Solar Zenith Angle (No refraction) 70.9488 Solar Zenith Angle (With refraction) 70.9020 Julian Day 51877.3333 Equation of Time (minutes) 11.7954 Hour Angle (degrees) 44.0388 Extraterrestrial Global Horizontal Solar Irradiance (W/sq m) 458.92 Extraterrestrial Direct Normal Solar Irradiance (W/sq m) 1405.96 Daily Global ETR (W/sq m) 4793.9 Daily Direct Normal ETR (W/sq m) 13762.4 Earth Radius Vector 1.0285 Sunrise (hour) 7.1698 Sunset (hour) 16.9584