PHOTOSOA (Photosensitization: A novel pathway to SOA generation and property change in tropospheric particles)
Tropospheric aerosol particles have often been depicted in models in a simplistic way considering them as non-volatile and chemically inert. Such assumptions were recently been challenged in frontline research, according to which volatile organic compounds (VOCs) and secondary organic aerosols (SOA) form a system that evolves in the atmosphere by chemical and dynamical processing. However, processes leading to the formation of SOA are poorly understood in atmospheric science. Models based on available parametrization of laboratory studies are still underestimating SOA and are not conform with the measured SOA concentrations from field studies. This discrepancy indicates that there are still sources and processes of SOA formation missing. In order to close this gap, further studies have been carried out showing that gaseous glyoxal has a higher affinity to the particulate phase than previously assumed and could significantly contribute to the SOA mass through multiphase chemistry. Such a sink in the condensed phase could explain an important part of the missing SOA mass in models, often referred to as aqSOA. However, observations imply that these conventional aqSOA sources cannot fully explain the missing SOA and there are still large uncertainties in the understanding of SOA formation in the troposphere. Furthermore, multiphase processes have shown to produce light absorbing compounds in the particle phase. The formation of such light absorbing species could induce new photochemical processes, addressed as photosensitization, within the aerosol particles and/or at the gas-particle interface. The photosensitization is known from other scientific fields such as the wastewater treatment and the surface water chemistry of lakes and rivers. Organic molecules absorb light in the range of solar radiation leading to an excited state and can undergo photo-induced charge or energy transfer to another molecule. Such organic molecules are aromatics, substituted carbonyls and/or nitrogen containing compounds, which are all current in tropospheric aerosols. Therefore, while aquatic photochemistry has recognized several of these processes that accelerate degradation of dissolved organic matter, only little is known about such processes in/on atmospheric particles. Therefore, within PHOTOSOA it is suggested to study photosensitization in the troposphere as this process may play a significant role in SOA formation and aging. The photosensitized processes can initiate new chemical pathways so far unconsidered affecting the atmospheric chemical composition and can thus contribute to close the current SOA underestimation. The aim of PHOTOSOA is to reduce such uncertainties in the SOA formation by models. Therefore, basic laboratory studies on the chemistry of triplet state of relevant photosensitizers under different experimental conditions are needed to evaluate the impact and importance of these photosensitized processes on SOA formation in the atmospheric particulate-phase chemistry.
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Teich M., van Pinxteren D., Kecorius S., Wang Z., Herrmann H. (2016) First Quantification of Imidazoles in Ambient Aerosol Particles: Potential Photosensitizers, Brown Carbon Constituents, and Hazardous Components. Environ Sci Technol 50 (3), 1166-1173, doi: DOI 10.1021/acs.est.5b05474.
van Pinxteren M., Herrmann H. (2013) Glyoxal and methylglyoxal in Atlantic seawater and marine aerosol particles: method development and first application during the Polarstern cruise ANT XXVII/4. Atmos Chem Phys 13 (23), 11791-11802, doi: 10.5194/acp-13-11791-2013.