Abstract. 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 ∘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 NO2 photolysis frequency j(NO2) of
(1.58±0.21(1σ))×10-3 s−1 for temperatures
between 213 and 295 K. At constant temperature, the light intensity is
stable within ±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−1 of the mean gas
temperature, it is possible to perform experiments with aqueous droplets at
relative humidities up 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 (1.03±0.15)×10-4 s−1 independent of temperature in the range 213–298 K with a
quantum yield of 0.36±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∘ N, 8.404∘ 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.