A light source has been built at the AIDA (Aerosol Interactions and Dynamics in the Atmosphere) simulation chamber at the Karlsruhe Institute of Technology, simulating solar radiation at ground level. Instead of full spectra light sources, it uses a combination of LEDs with a narrow emission spectrum, resulting in a combined spectrum similar to the solar spectrum between 300 and 530 nm. The use of LEDs leads to an energy-efficient, robust and versatile illumination concept. The light source can be used over a wide temperature range down to -90 .sup." C and is adjustable in intensity and spectral width as well as easily adjustable to new technological developments or scientific needs. Characterization of the illumination conditions shows a vertical intensity gradient in the chamber. The integral intensity corresponds to a NO.sub.2 photolysis frequency j(NO.sub.2) of (1.58Â±0.21(1Ï))x10-3 s.sup.-1 for temperatures between 213 and 295 K. At constant temperature, the light intensity is stable within Â±1 %. While the emissions of the different LEDs change with temperature, they can be adjusted, and thus it is possible to adapt the spectrum for different temperatures. Although the illumination of the simulation chamber leads to an increase of 0.7 K h.sup.-1 of the mean gas temperature, it is possible to perform experiments with aqueous droplets at relative humidities up to [less than or equal to]95 % and also above water or ice saturation with corresponding clouds. Additionally, temperature- and wavelength-dependent photolysis experiments with 2,3-pentanedione have been conducted. The photolysis of 2,3-pentanedione occurs mainly between 400 and 460 nm, resulting in a mean photolysis frequency of (1.03Â±0.15)x10-4 s.sup.-1 independent of temperature in the range 213-298 K with a quantum yield of 0.36Â±0.04. In contrast, the yield of the two main photolysis products, acetaldehyde and formaldehyde, decreases with temperature. Furthermore, the light source was applied to study the photochemistry of aerosol particles. For the atmospheric brown carbon proxy compound 3,5-diacetyl-2,4,6-trimethyl-1,4-dihydropyridine, photochemical reaction products were identified. In aerosol particles containing iron oxalate as a photosensitizer, the photosensitized degradation of organic acids (pinic and pinonic acid) was studied. Although the light source only generates about one-third of the maximum solar irradiation at ground level at Karlsruhe (49.007.sup." N, 8.404.sup." E; 12:00 UTC+2) on a clear summer day with a substantial intensity gradient throughout the simulation chamber, it could be shown that this type of light source allows reproducible experiments over a wide range of simulated atmospheric conditions and with a large flexibility and control of the irradiation spectrum.