Abstract

A light source has been built at the simulation chamber AIDA (Aerosol Interactions and Dynamics in the Atmosphere) 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, 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)) x 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, 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) x 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 photosensitizer the photosensitized degradation of organic acids (pinic and pinonic acid) was studied. Although, the light source only generates about 1/3 of the maximum solar irradiation at ground level 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.

Highlights

  • A light source has been built at the simulation chamber AIDA (Aerosol Interactions and Dynamics in the Atmosphere) at the 10 Karlsruhe Institute of Technology, simulating solar radiation at ground level

  • The light source only generates about 1/3 of the maximum solar irradiation at ground level 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 30 and with a large flexibility and control of the irradiation spectrum

  • The formation of the photolysis products acetaldehyde and formaldehyde show a distinct temperature dependence with yields decreasing with decreasing temperature

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Summary

Introduction

The interaction of light with the atmosphere is an important factor in understanding the physical and chemical processes influencing air quality and our climate. 60 Apart from field measurements, controlled experiments in cloud simulation chambers offer the possibility to understand the atmospheric processes. One of these simulations chambers is the AIDA-chamber (Aerosol Interactions and Dynamics in the Atmosphere) at the Institute of Meteorology and Climate Research at the Karlsruhe Institute of Technology (KIT). LEDs and fluorescent tubes have the advantages that they are easy to use, emit less heat and are relatively costefficient They are not emitting in the full spectral range of the photochemical relevant spectrum, which should start at around 300 nm depending on the atmospheric height (solar spectrum see supplementary information) and reach at least until 500 nm to include photochemical reactions of chromophores found in organic matter (Sharpless, 2014). The results of temperature and wavelength dependent photolysis experiments of 2,3-pentanedione, will be presented and a first insight will be given into the photochemical aging of a brown carbon proxy and photosensitized decomposition of atmospherically relevant organic acids

Instrumentation
Experimental procedures
Stability of the light source
Integral light intensity
Variation of temperature
Photolysis of 3,5-diacetyl-2,4,6-trimethyl-1,4-dihydropyridine (DTDP)
Depletion of pinic and pinonic acid by iron oxalate as photosensitizer
Findings
Conclusions
Full Text
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