Perchlorate is of increasing interest due to its widespread occurrence at trace levels in groundwater, surface water, soil, vegetation, and crops. Exposure to perchlorate affects human growth and development. Local contamination can occur from the release of man-made perchlorate, however there is clear evidence that most environmental perchlorate is of atmospheric origin and one of its main sources is photochemical production in the stratosphere.
Yet, very little information exists on perchlorate’s atmospheric distribution, its formation pathways in the atmosphere, and the influence of human activities on its abundance.
Some of our findings:
Chan et al. (2023). Stratospheric Gas-Phase Production Alone Cannot Explain Observations of Atmospheric Perchlorate on Earth. Geophysical Research Letters. In this study, we use a global three-dimensional chemical transport model to simulate perchlorate’s gas-phase photochemical production, atmospheric transport, and deposition on Earth’s surface. Model predictions are compared to newly compiled observations of atmospheric concentrations, deposition flux, and oxygen isotopic composition of perchlorate.

We find that the modeled gas-phase production of perchlorate is consistent with reported stratospheric observations. Nevertheless, we show that this mechanism alone cannot explain the high levels of perchlorate observed at many near-surface sites (aerosol concentrations >0.1 ng m−3 and deposition fluxes >10 g km−2 yr−1) or the low 17O-excess observed in perchlorate sampled from pristine environments (<+18.4‰). We discuss four hypotheses to explain the model-observation discrepancies: (a) uncertainty in assumed gas-phase kinetics, (b) multiphase chemical production of perchlorate, (c) combustion sources of non-synthetic perchlorate, and (d) perchlorate re-emission from Earth’s surface.

Jaeglé, Chan, Kim, Campuzano-Jost, & Jimenez (2021). Global distribution and origin of atmospheric perchlorate: Novel insights from the ATom aircraft campaign. AGU presentation. Perchlorate observations collected by an Aerosol Mass Spectrometer (AMS) on NASA’s DC-8 aircraft as part of ATom (2016-2018) provide the first near-global distribution of atmospheric perchlorate. We are combining these new observations with the GEOS-Chem global chemical transport model to elucidate the origin of perchlorate in the lowermost stratosphere and troposphere. The ATom observations show enhanced perchlorate in the lowermost stratosphere, with values between 1 and 10 ng/sm3. The highest concentrations are observed at high latitudes during summer.

We infer a perchlorate stratospheric flux of 2.4 ± 1.6 Gg yr-1 based on perchlorate-ozone correlations in the lowermost stratosphere, with a much stronger flux in the southern hemisphere than in the northern hemisphere. In addition, perchlorate levels of ~0.5-2 ng/sm3 are measured in the lower troposphere, often associated with elevated tracers of combustion. A GEOS-Chem simulation including perchlorate production via the gas-phase reaction of OH with ClO3 can broadly reproduce the lower stratospheric enhancements, but does not capture the seasonality of the observations nor the lower tropospheric enhancement. We will use the GEOS-Chem model to test other stratospheric chemical production mechanisms as well as the potential contribution of tropospheric primary and secondary sources to the global distribution of atmospheric perchlorate. This is an on-going collaboration with Prof. Jimenez’ group. Stay tuned for more upcoming results!

People: Yuk Chun Chan, Lyatt Jaeglé and our colleagues at CU Boulder, Jose Jimenez, Pedro Campuzano Jost, Dongwook Kim

Funding: NASA, Atmospheric Composition Campaign Data Analysis and Modeling (ACCDAM) program.

Publications:

  • Chan, Y.-C., Jaeglé, L., Campuzano-Jost, P., Catling, D. C., Cole-Dai, J., Furdui, V. I., Jackson, W. A., Jimenez, J. L., Kim, D., Wedum, A. E., Alexander, B., 2023. Stratospheric Gas-Phase Production Alone Cannot Explain Observations of Atmospheric Perchlorate on Earth, Geophys. Res. Lett., 50, e2023GL102745. https://doi.org/10.1029/2023GL102745
  • Jaeglé, L., Yung, Y.L., Toon, G.C., Sen, B., Blavier, J.F., 1996. Balloon observations of organic and inorganic chlorine in the stratosphere: The role of HClO4 production on sulfate aerosols. Geophys. Res. Lett. 23, 1749–1752. https://doi.org/10.1029/96GL01543

Presentations:

  • Jaeglé, L., Chan, Y.C., Kim, D., Campuzano-Jost, P., and Jimenez, J.L., Global distribution and origin of atmospheric perchlorate: Novel insights from the ATom aircraft campaign, Abstract A23D-01, Oral presentation, AGU Fall Meeting, 13-17 December 2021, New Orleans.
  • Kim, D., et al., Tropospheric and stratospheric submicron aerosol chemical composition over the remote northern Pacific ocean during the TI3GER campaign, American Geophysical Union Fall Meeting, 12-16 December 2022, Chicago.
  • Chan, Y.C., et al., Modeling the atmospheric production of perchlorate, 10th international GEOS-Chem Meeting (IGC10), Washington University, June 7-10, 2022.
  • Campuzano-Jost, P., et al., Sources and Global Distribution of Atmospheric Perchlorate Aerosol: Insights from the Reanalysis of Multiple NASA Airborne Datasets, 103rd Annual AMS Meeting, January 8-12, 2023. Oral Presentation. Contributed.