Pergarnon SQO45-6535(96)00028-8 C he m o sp he re , Vol. 32, No. 4, pp. 739-758, 1996 Copyright 0 1996 Published by Ekvier Science Ltd Printed in Great Britain. All rights reserved 0045.6535/96 $15.00+0.00 THE DISTRII3UTION OF SOLAR RADIATION IN THE EARTH’S ATMOSPHERE: THK EFFECTS OF OZONE, AEROSOLS AND CLOUDS Yu Lu’ and M A K Klralil’ . . . ‘National Exposure Research Laboratory, MD-8OA, U.S.EPA, Research Triangle Park, NC 27711. USA *Dept. of Physics, Portland State University, P.O. Box 751. Portland, OR 97207-0751, USA (Received in USA 30 June 1995; accepted 15 September 1995) ABSTRACT We have developed a detailed model of solar radiation in the atmosphere as it is affected by atmospheric constituents, aerosols, clouds and the surface characteristics of the earth. Such a model is the foundation for studying global change and atmospheric chemistry under natural and disturbed conditions, The model includes radiative transfer processes for solar ultraviolet and visible wavelengths (290-700 mn) under different environmental conditions. It calculates the optical properties of aerosols and cloud droplets as well as the direct, diffuse, net, and actinic fluxes for different wavelengths, altitudes, and zenith angles at a relatively high computational speed and accuracy. It only takes about three and a half minutes to calculate all the optical properties and radiative fluxes in a cloudy air (including all the properties and fluxes in 100 sub-layers inside a cloud), and about 20 seconds in a clear sky and clean air condition at a SUN SPARCStation lo/50 (with single SPARC CPU running at about 50 MHz). We show that local environmental conditions, particularly in the lower atmosphere, can greatly alter the actinic flux throughout the atmosphere. This feature is especially apparent in the wavelengths with weak or no 0, absorption, as multiple scattering dominates the atmospheric radiative transfer. Compared to the actinic flux under clear sky and clean air conditions, for example, the actinic flux around 400 nm at zero zenith angle decreases by a factor of 5 at the earth’s surface while increasing by more than 100% at the top of the atmosphere when a one-km altostratus cloud is added to the middle troposphere. According to our calculations, the radiation field outside a cloud is mainly controlled by the total liquid water content of the cloud; however, the actinic flux inside a cloud is very sensitive to the macro structure of the cloud. Readers may acquire the computer model from the authors. 1. INTRODUCTION Visible and ultraviolet solar radiation plays an important role in atmospheric chemistry by initiating nearly all atmospheric chemical reactions. To correctly model and understand the atmospheric chemical interactions, it is essential to quantitatively characterize actinic fluxes’in the atmosphere. The radiation intensity in the atmosphere changes with time of day, longitude, latitude, and season, governed by the astronomical and geometrical relationships between the sun and the earth. The radiation intensity is also determined by the interactions of the solar radiation with various atmospheric scatterers and absorbers, and with the earth’s surface. These atmospheric constituents are unevenly distributed and their optical properties change with their physical properties and wavelength. All these factors complicate atmospheric radiative transfer, 739