Measuring Photolytic Mass Loss of Humic Substances with a Quartz Microbalance(QCM)

Secondary organic aerosol (SOA) is formed in the atmosphere through the oxidation of volatile organic compounds (VOCs) and represents a significant portion of global submicron-sized atmospheric organic aerosol. SOA plays a crucial role in multiple processes that impact climate and human health. Photodegradation of SOA under solar radiation is an important sink for atmospheric SOA and influences its evolution.[1] Therefore, a better understanding of the extent and rate of SOA material photodegradation is necessary to accurately represent its roles in climate models. Recent modeling studies have suggested that photolysis is especially important in the upper troposphere, as it can improve the agreement between model predictions and aircraft OA measurements.[2][3] In this work, We used standard terrestrial and aquatic humic substances as surrogates for atmospheric SOA. Mass loss kinetics of the humic substances’ photodegradation under one day of 254/305/405nm light exposure were investigated using a quartz crystal microbalance (QCM). For all wavelengths, the observed photolysis rate starts high but decays slowly as time goes on. This shows the potential problem with extrapolating the starting photolysis speed to a longer time frame. Notable acceleration of photodegradation in the presence of oxygen compared to N₂ flow was observed in all wavelengths. This acceleration can be explained through the photosensitization behavior of humic substances. The photolysis rate measured was scaled to atmospheric conditions and compared to the photolysis rates of lab-generated model SOA reported in the literature.[4][5]

Reference:

[1]Shrivastava, M., et al. (2017), Recent advances in understanding secondary organic aerosol: Implications for global climate forcing, Rev. Geophys., 55, 509– 559, doi:10.1002/2016RG000540.

[2]Hodzic, A., Kasibhatla, P. S., Jo, D. S., Cappa, C. D., Jimenez, J. L., Madronich, S., and Park, R. J.: Rethinking the global secondary organic aerosol (SOA) budget: stronger production, faster removal, shorter lifetime, Atmos. Chem. Phys., 16, 7917–7941, https://doi.org/10.5194/acp-16-7917-2016, 2016.

[3]Lou, S., Shrivastava, M., Easter, R. C., Yang, Y., Ma, P.-L., Wang, H., et al. (2020). New SOA treatments within the Energy Exascale Earth System Model (E3SM): Strong production and sinks govern atmospheric SOA distributions and radiative forcing. Journal of Advances in Modeling Earth Systems, 12, e2020MS002266, https://doi.org/10.1029/2020MS002266

[4]Baboomian, V. J., Gu, Y., & Nizkorodov, S. A. (2020). Photodegradation of secondary organic aerosols by long-term exposure to solar actinic radiation. ACS Earth and Space Chemistry, 4(7), 1078-1089, https://doi.org/10.1021/acsearthspacechem.0c00088

[5]Zawadowicz, M. A., Lee, B. H., Shrivastava, M., Zelenyuk, A., Zaveri, R. A., Flynn, C., ... & Shilling, J. E. (2020). Photolysis controls atmospheric budgets of biogenic secondary organic aerosol. Environmental Science & Technology, 54(7), 3861-3870. https://doi.org/10.1021/acs.est.9b07051