5:15 PM - 6:30 PM
[AAS05-P12] Impact of nighttime aging on optical properties of toluene secondary organic aerosol
Keywords:Toluene, Secondary organic aerosol, Optical properties, Brown carbon
Toluene is known as an important precursor of anthropogenic secondary organic aerosol (SOA) in the troposphere. In the atmosphere, toluene is mainly oxidized by OH radical to generate gaseous oxidation products in addition to SOA. Whereas OH aging of the toluene-SOA will be negligible under dark conditions after sunset, a part of these oxidation products in gas-phase and/or SOA can be further oxidized with NO3 or O3 even in the nighttime. The SOA generated during OH oxidation of toluene in the presence of NOx is known to have significant light absorption at the ultraviolet and shorter visible wavelengths (i.e., known as brown carbon) [e.g., Nakayama et al. 2013] and possibly contributes to the radiation balance and photochemical reactions in the atmosphere. In this work, the impact of NO3 and O3 aging on wavelength-dependent optical properties of the toluene SOA has been studied.
The SOA was generated during OH oxidation of toluene in the absence of NOx in a 6 m3 stainless steel photochemical chamber with Teflon coating. As sources of OH, either of photolysis of H2O2 or ozonolysis of tetramethylethylene was used. After enough amounts of SOA were generated, N2O5 (precursor of NO3), O3, or NO2 was injected to the chamber under dark conditions. The NO2 injection experiment was conducted to confirm the possible effects of NO2 reactions, because both of NO3 and NO2 were generated during thermal decomposition of N2O5. Temporal variations of the optical properties of the SOA were measured by two photoacoustic spectrometers (absorption and scattering at 375, 405, 532, 781 nm) and a cavity ring-down spectrometer (extinction at 532 nm). Chemical properties and size distributions of SOA were measured by an Aerodyne aerosol mass spectrometer and a scanning mobility particle sizer, respectively. Gaseous reactants and products were monitored by a Fourier transform-infrared spectrometer and a proton transfer reaction time-of-flight mass spectrometer.
Mass concentrations of toluene-SOA were significantly enhanced and nitrocresols and nitrosalicylic alcohols were generated after NO3 aging, but no clear enhancement was observed after O3 aging [Ramasamy et al. 2019]. The toluene-SOA did not show light absorption properties at all wavelengths studied before NO3 or O3 aging. Significant light absorption was observed after NO3 aging at 375 and 405 nm, but neither O3 nor NO2 additions. Nitro-aromatic compounds such as nitrocresols and nitrosalicylic alcohols generated from cresols during NO3 aging are considered to be responsible for the observed light absorption. In the presentation, the atmospheric implications of the results will also be discussed.
References
Nakayama, T. et al., Wavelength and NOx dependent complex refractive index of SOAs generated from the photooxidation of toluene, Atmos. Chem. Phys., 13, 531-545, doi:10.5194/acp-13-531-2013 (2013).
Ramasamy, S. et al., Investigation of dark condition nitrate radical- and ozone-initiated aging of toluene secondary organic aerosol: Importance of nitrate radical reactions with phenolic products, Atmos. Environ., 219, 117049, doi:10.1016/j.atmosenv.2019.117049 (2019).
The SOA was generated during OH oxidation of toluene in the absence of NOx in a 6 m3 stainless steel photochemical chamber with Teflon coating. As sources of OH, either of photolysis of H2O2 or ozonolysis of tetramethylethylene was used. After enough amounts of SOA were generated, N2O5 (precursor of NO3), O3, or NO2 was injected to the chamber under dark conditions. The NO2 injection experiment was conducted to confirm the possible effects of NO2 reactions, because both of NO3 and NO2 were generated during thermal decomposition of N2O5. Temporal variations of the optical properties of the SOA were measured by two photoacoustic spectrometers (absorption and scattering at 375, 405, 532, 781 nm) and a cavity ring-down spectrometer (extinction at 532 nm). Chemical properties and size distributions of SOA were measured by an Aerodyne aerosol mass spectrometer and a scanning mobility particle sizer, respectively. Gaseous reactants and products were monitored by a Fourier transform-infrared spectrometer and a proton transfer reaction time-of-flight mass spectrometer.
Mass concentrations of toluene-SOA were significantly enhanced and nitrocresols and nitrosalicylic alcohols were generated after NO3 aging, but no clear enhancement was observed after O3 aging [Ramasamy et al. 2019]. The toluene-SOA did not show light absorption properties at all wavelengths studied before NO3 or O3 aging. Significant light absorption was observed after NO3 aging at 375 and 405 nm, but neither O3 nor NO2 additions. Nitro-aromatic compounds such as nitrocresols and nitrosalicylic alcohols generated from cresols during NO3 aging are considered to be responsible for the observed light absorption. In the presentation, the atmospheric implications of the results will also be discussed.
References
Nakayama, T. et al., Wavelength and NOx dependent complex refractive index of SOAs generated from the photooxidation of toluene, Atmos. Chem. Phys., 13, 531-545, doi:10.5194/acp-13-531-2013 (2013).
Ramasamy, S. et al., Investigation of dark condition nitrate radical- and ozone-initiated aging of toluene secondary organic aerosol: Importance of nitrate radical reactions with phenolic products, Atmos. Environ., 219, 117049, doi:10.1016/j.atmosenv.2019.117049 (2019).