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Atmospheric Chemistry and Physics An interactive open-access journal of the European Geosciences Union
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https://doi.org/10.5194/acp-2020-800
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/acp-2020-800
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

  21 Aug 2020

21 Aug 2020

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This preprint is currently under review for the journal ACP.

Measurement Report: Sulfuric Acid Nucleation and Experimental Conditions in a Photolytic Flow Reactor

David Roy Hanson, Seakh Menheer, Michael Wentzel, and Joan Kunz David Roy Hanson et al.
  • Chemistry Department, Augsburg University, Minneapolis, MN, USA

Abstract. Nucleation involving sulfuric acid and water has been studied in a photolytic flow reactor over a time period of several years. Results show that the system – flow reactor, gas supplies and lines, flow meters, valves, H2SO4 photo-oxidant sources – has a baseline stability that yields nucleation information such as cluster free energies. The baseline nucleation rate is punctuated by temporary bursts that in many instances are linked to cylinder changes, delineating this source of potential contaminants. Diagnostics were performed to better understand the system include growth studies to assess H2SO4 levels, chemiluminescent NO and NOx detection to assess the HONO source, and deployment of a second particle detector to assess the nanoparticle detection system. The growth studies show trends consistent with calculated H2SO4 levels and also provide an anchor for H2SO4 concentrations. The chemiluminescent detector revealed that small amounts of NO are present in the HONO source, ∼10 % of HONO. The second, condensation-type particle counter indicates that the nanoparticle sizing system has a bias at low sulfuric acid levels. Modeling studies yield nucleation rates as a function of sulfuric acid concentration that probably represent upper limits to nucleation in the binary system, H2SO4-H2O, as contaminants might act to enhance nucleation rates or ion-mediated nucleation may contribute. Nonetheless, the experimental nucleation rates are some of the lowest reported so far in experiments where sulfuric acid is photolytically-generated. Results from experiments with varying water content and with ammonia addition are also described. The energetics of clusters in this three component system reveal a challenging interplay between the components (e.g. previously reported ion-mediated and homogeneous rates are unintentionally similar) and experiments indicate water plays a significant role in nucleation involving sulfuric acid and ammonia.

David Roy Hanson et al.

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David Roy Hanson et al.

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Latest update: 29 Sep 2020
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Short summary
We report experimental measurements of particle formation in a flow reactor that extend the results from this experiment to a total of more than 270 runs over a time period of ~3 years. This has allowed us to detect a general increase in the cleanliness of the system and improve our knowledge of its chemistry. In-house simulations allowed us to construct phenomenological free energies of molecular clusters of sulfuric acid and ammonia that are appropriate for application to the atmosphere.
We report experimental measurements of particle formation in a flow reactor that extend the...
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