and pressure. They are usually determined by laboratory

experiments for specific photolytic species. Actinic flux

measures the spectral radiance integrated over all solid

angles per unit area on the spherical receiving surface

(compare this with irradiance, which is the radiance falling

on a horizontal surface). Thus, the actinic flux can be called

spherical spectral irradiance. The actinic flux changes with

time of day, longitude, latitude, altitude, and season, and is

governed by the astronomical and geometric relationships

between the sun and the earth. It is affected by the earth's

surface albedo as well as by atmospheric scattering and

absorption.

The current approach for setting photolysis rates in CMAQ

follows the approach used in RADM (Chang et al. [10]).

The photolysis rates are estimated in two processing stages:

first, a table of clear-sky photolysis rates is calculated for

specified heights, latitudes, and hours from local noon; and

then photolysis rates are interpolated from the table within

the CCTM based on grid cell location and the model time,

and are corrected for cloud cover. This approach is

computationally efficient and has been shown by Madronich

[68] to give clear-sky photolysis rates within the uncertainty

of the surface-based measurements.

Calculation of clear-sky photolysis rate table

A preprocessor (JPROC) calculates clear-sky photolysis

rates for 6 latitudinal bands (at every 10 degrees for 10°- 60°

N), 7 altitudes (0 km, 1 km, 2 km, 3 km, 4 km, 5 km, and 10

km), and ±9 hours from local noon (0–8 h). The delta-

Eddington two-stream radiative transfer model (Joseph et al.

[69], Toon et al. [48]) is used for computing the actinic flux.

The two-stream approximations are limited in application to

cases where the scatter is not highly anisotropic. In

computing the actinic flux, a description of the

extraterrestrial radiation, aerosol, ozone absorption, oxygen

absorption in the Schumann-Runge Bands, Rayleigh

scattering (WMO [70]) and surface albedo are provided to

the radiation model. Users can specify the extraterrestrial

radiation; however, these data should be resolved at

wavelengths that capture the features important to the

photolysis reactions of interest. A modified WMO

extraterrestrial radiation data distribution (Chang et al. [71])

is used as input to JPROC, which has a variable wavelength

resolution ranging from 1 to 10 nm.

n1051 - n1052 - n1053 - n1054 - n1055 - n1056 - n1057 - n1058 - n1059 - n1060 - n1061 - n1062 - n1063 - n1064 - n1065 - n1066 - n1067 - n1068 - n1069 - n1070 - n1071 - n1072 - n1073 - n1074 - n1075 - n1076 - n1077 - n1078 - n1079 - n1080 - n1081 - n1082 - n1083 - n1084 - n1085 - n1086 - n1087 - n1088 - n1089 - n1090 - n1091 - n1092 - n1093 - n1094 - n1095 - n1096 - n1097 - n1098 - n1099 - n1100

 

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