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Discussion papers
https://doi.org/10.5194/acp-2018-1325
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/acp-2018-1325
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Submitted as: research article 17 May 2019

Submitted as: research article | 17 May 2019

Review status
This discussion paper is a preprint. It is a manuscript under review for the journal Atmospheric Chemistry and Physics (ACP).

Segregation in the Atmospheric Boundary Layer: The Case of OH – Isoprene

Ralph Dlugi1,2,3, Martina Berger1,2,3, Chinmay Mallik2, Anywhere Tsokankunku2,3, Michael Zelger1,2,3, Otávio C. Acevedo4, Efstratios Bourtsoukidis2, Andreas Hofzumahaus5, Jürgen Kesselmeier3, Gerhard Kramm6, Daniel Marno2, Monica Martinez2, Anke C. Nölscher2, Huug Ouwersloot2, Eva Y. Pfannerstill2, Franz Rohrer5, Sebastian Tauer2, Jonathan Williams2, Ana-Maria Yáñez-Serrano3, Meinrat O. Andreae3,7, Hartwig Harder2, and Matthias Sörgel2,3 Ralph Dlugi et al.
  • 1Arbeitsgruppe Atmosphärische Prozesse (AGAP), Munich, Germany
  • 2Atmospheric Chemistry Department, Max Planck Institute for Chemistry, P.O. Box 3060, 55020 Mainz, Germany
  • 3Biogeochemistry Department, Max Planck Institute for Chemistry, Mainz, Germany
  • 4Universidade Federal Santa Maria, Dept. Fisica, 97119900 Santa Maria, RS, Brazil
  • 5Institut für Energie-und Klimaforschung: Troposphäre, Forschungszentrum Jülich, Germany
  • 6Engineering Meteorology Consulting, Fairbanks, AK, USA
  • 7Scripps Institution of Oceanography, University of California San Diego, California, USA

Abstract. In the atmospheric boundary layer (ABL), incomplete mixing (i.e., segregation) results in reduced chemical reaction rates compared to those expected from mean values and rate constants derived under well mixed conditions. Recently, segregation has been suggested as a potential cause of discrepancies between modelled and measured OH radical concentrations, especially under high isoprene conditions. Therefore, the influence of segregation on the reaction of OH radicals with isoprene has been investigated by modelling studies and one ground-based and one aircraft campaign.

In this study, we measured isoprene and OH radicals with high time resolution in order to directly calculate the influence of segregation in a low-NOx and high-isoprene environment in the central Amazon basin. The calculated intensities of segregation (Is) at the Amazon Tall Tower Observatory (ATTO) above canopy top are in the range of values determined at a temperate deciduous forest (ECHO-campaign) in a high-NOx low-isoprene environment, but stay below 10 %. To establish a more general idea about the causes of segregation and their potential limits, further analysis was based on the budget equations of isoprene mixing ratios, the variance of mixing ratios, and the balance of the intensity of segregation itself. Furthermore, it was investigated if a relation of Is to the turbulent isoprene surface flux can be established theoretically and empirically, as proposed previously. A direct relation is not given and the amount of variance in Is explained by the isoprene flux will be higher the less the influence from other processes (e.g., vertical advection) is and will therefore be greater near the surface. Although ground based values of Is from ATTO and ECHO are in the same range, we could identify different dominating processes driving Is. For ECHO the normalized variance of isoprene had the largest contribution, whereas for ATTO the different transport terms expressed as a residual were dominating. To get a more general picture of Is and its potential limits in the ABL, we also compared these ground based measurements to ABL modelling studies and results from an aircraft campaign. The ground based measurements show the lowest values of the degree of inhomogenous mixing (< 20 %, mostly below 10 %). These values increase if the contribution of lower frequencies is added. Values integrated over the whole boundary layer (modelling studies) are in the range from 10 % to 30 % and aircraft measurements integrating over different landscapes are amongst the largest reported. This presents evidence that larger scale heterogeneities in land surface properties contribute substantially to Is.

Ralph Dlugi et al.
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Short summary
Incomplete mixing (segregation) results in reduced chemical reaction rates compared to those expected from mean values and rate constants. Segregation has been suggested to cause discrepancies between modelled and measured OH radical concentrations. In this work, we summarize the intensities of segregation for the reaction of OH and isoprene for different field and modelling studies and compare those to our results from measurements in a pristine environment.
Incomplete mixing (segregation) results in reduced chemical reaction rates compared to those...
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