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Production and Measurement of Organic Particulate Matter in the Harvard Environmental Chamber
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Tropospheric aerosol as a reactive intermediate.

Agustín J Colussi1, Shinichi Enami2, Akihiro Yabushita3

  • 1Ronald and Maxine Linde Center for Global Environmental Science, California Institute of Technology, Pasadena, California 91125, USA. ajcoluss@caltech.edu

Faraday Discussions
|March 8, 2014
PubMed
Summary

Secondary organic aerosol (SOA) is a reactive intermediate, not an end product. Carboxylate anions catalyze reactions that generate nitrous acid (HONO) and hydroxyl radicals (*OH), influencing atmospheric chemistry.

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Area of Science:

  • Atmospheric Chemistry
  • Environmental Science

Background:

  • Secondary organic aerosol (SOA) has been traditionally viewed as an end product in tropospheric chemistry.
  • Existing models do not fully account for the dynamic role of SOA in atmospheric reactions.
  • The daytime sources of nitrous acid (HONO) and the kinetics of nitrogen oxides (NOx) decay require further mechanistic understanding.

Purpose of the Study:

  • To present evidence that secondary organic aerosol (SOA) functions as a key reactive intermediate in tropospheric chemistry.
  • To investigate the role of carboxylate anions in catalyzing the disproportionation of nitrogen dioxide (NO2) in the presence of water.
  • To re-evaluate daytime HONO sources and NOx decay kinetics in urban environments based on new mechanistic insights.

Main Methods:

  • Laboratory experiments demonstrating the catalysis of NO2 disproportionation by carboxylate anions.
  • Quantum chemical calculations to support the proposed reaction mechanisms.
  • Analysis of existing field data on HONO concentrations and NOx decay rates in relation to SOA loadings.

Main Results:

  • Laboratory results confirm that carboxylate anions catalyze the reaction: 2NO2 + H2O = HONO + NO(9-) + H+ (R1).
  • Daytime HONO generation via R1 is linked to sunlight and anion production from pollutant oxidation.
  • Reaction R1 can compete with NO2 photolysis (R2) and directly generates *OH radicals, influencing atmospheric chemistry and buffering *OH concentrations.

Conclusions:

  • Secondary organic aerosol (SOA) should be considered a reactive intermediate due to its role in catalyzing important atmospheric reactions.
  • The carboxylate-catalyzed reaction (R1) provides a significant daytime source of HONO and *OH radicals, impacting air quality.
  • This mechanism helps explain observed HONO levels and NOx decay rates, particularly under varying sunlight conditions.