Characterization of Urban Grime Photochemistry as Sink or Source for Air Pollutants (GrimePaSS)
Final Report Abstract
Improving urban air quality is a major concern that is increasing interest in understanding the key chemical processes and pathways of air pollutants such as the nitrogen oxides (NOx), ozone (O3), and various volatile organic compounds (VOCs). In addition to gas-phase chemistry, heterogeneous and multiphase reactions in the interaction of gases with urban surfaces (e.g., windows, building walls, roofs, sidewalks, roads, etc.) are processes that could affect the behavior of air pollutants. It is known that when surfaces are exposed to the atmosphere for a certain period of time, a so-called urban grime film ("urban grime") of inorganic and organic components is formed. However, knowledge about the interaction of this new environmental compartment with air pollutants under solar irradiation potentially impacting urban air quality is still limited. The present project therefore combined real urban grime sampling on glass surfaces, collected at a European urban background site in Leipzig, Germany, with photochemical flow reactor (PCFR) and aerosol chamber experiments to characterize its photoreactive behavior. PCFR-experiments using clean compressed air are performed under variation of the sampling conditions (e.g., shaded vs. non-shaded surfaces) and the experimental conditions. The results clearly prove the formation of NO2 and HONO with formation rates of (2.6±0.6)×10^9 molecules cm-2 s-1 and (8±1)×10^9 molecules cm-2 s-1 for shaded glass beads at 70% relative humidity (RH) and 21 W m-2 (range 300-400 nm) light intensity. Also, a release of organics under dark conditions and a clear proof of photochemically induced formation of short chain organics is shown. All observed formation rates are depending on the relative humidity, light intensity and sampling type. Furthermore, experiments under addition of NO, NO2 and O3 are performed under variation of experimental conditions to study trace gas uptake and possible product releases to the gas phase. Uptake coefficients of γ(NO) ≤ 1×10^-7, γ(NO2) ≤ 2×10^-7 and γ(O3) = 1.0×10^-6 were determined for 70% RH and 21 W m-2 (range 300-400 nm) light intensity. Experiments in the atmospheric simulation chamber ACD-C at TROPOS confirmed the UV-light induced formation of reaction products as observed during the small scale PCFR-experiments also under more realistic chamber conditions (S/V = 2 m-1). An urban grime box model using the detailed CAPRAM framework was applied for a Leipzig urban background scenario. The model results demonstrate for the first time that the measured formation of HONO under clean air experimental conditions results in an overall increase of the OH radical concentration of max. 6% for the dedicated Leipzig urban air quality situation in June 2019. The model shows that the impact is strongly dependent on the overall NOx emission scheme of the respective area, highlighting the (regional / local) variability of the source capacity of such surface films in urban areas.
Publications
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Characterization of Urban Grime Photochemistry as Sink or Source for Air Pollutants (GrimePaSS) AGU Fall Meeting 2019, 09-13 December 2019, San Francisco, CA
Falk Mothes & Hartmut Herrmann
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Laboratory- and field studies on chemical processes in the gas-, aqueous- and particle phase + on surfaces to improve the understanding of the tropospheric multiphase system Science For Future: All Starts With Basic Science, 08-11 September 2019, UCAS Yanqi Lake Campus, Beijing, China
Falk Mothes
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Urban grime photochemistry as a source for air pollutants and its potential impact on air quality Air quality in megacities, 17–20 November 2020, Online, Faraday Discussion
Falk Mothes & Hartmut Herrmann
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Urban grime photochemistry and its interaction with the air pollutants NO and NO2 11th International Aerosol Conference (IAC), 4th-9th September 2022, Athens
Falk Mothes & Hartmut Herrmann
