References

ACP

Atmospheric Chemistry and Physics

ACP

1680-7324

Copernicus GmbH

Göttingen, Germany

10.5194/acp-11-7223-2011

Quantifying the magnitude of a missing hydroxyl radical source in a tropical rainforest

Whalley

L. K.

¹ ² Edwards

P. M.

¹ Furneaux

K. L.

¹ Goddard

¹ Ingham

¹ ² Evans

M. J.

³ Stone

¹ Hopkins

J. R.

⁴ ⁵ Jones

C. E.

⁴ ⁵ Karunaharan

⁶ Lee

J. D.

⁴ ⁵ Lewis

A. C.

⁴ ⁵ Monks

P. S.

⁶ Moller

S. J.

⁴ Heard

D. E.

¹ ²

School of Chemistry, University of Leeds, Woodhouse Lane, Leeds, LS2 9JT, UK

National Centre for Atmospheric Science, University of Leeds, Leeds, LS2 9JT, UK

School of Earth and Environment, University of Leeds, Woodhouse Lane, Leeds, LS2 9JT, UK

Chemistry Department, University of York, Heslington, YO10 5DD, UK

National Centre for Atmospheric Science, University of York, Heslington, YO10 5DD, UK

Department of Chemistry, University of Leicester, University Road, Leicester, LE1 7RH, UK

22 07 2011

11 14 7223 7233

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The lifetime of methane is controlled to a very large extent by the abundance of the OH radical. The tropics are a key region for methane removal, with oxidation in the lower tropical troposphere dominating the global methane removal budget (Bloss et al., 2005). In tropical forested environments where biogenic VOC emissions are high and NOx concentrations are low, OH concentrations are assumed to be low due to rapid reactions with sink species such as isoprene. New, simultaneous measurements of OH concentrations and OH reactivity, k'OH, in a Borneo rainforest are reported and show much higher OH than predicted, with mean peak concentrations of ~2.5×106 molecule cm−3 (10 min average) observed around solar noon. Whilst j(O1D) and humidity were high, low O3 concentrations limited the OH production from O3 photolysis. Measured OH reactivity was very high, peaking at a diurnal average of 29.1±8.5 s−1, corresponding to an OH lifetime of only 34 ms. To maintain the observed OH concentration given the measured OH reactivity requires a rate of OH production approximately 10 times greater than calculated using all measured OH sources. A test of our current understanding of the chemistry within a tropical rainforest was made using a detailed zero-dimensional model to compare with measurements. The model over-predicted the observed HO2 concentrations and significantly under-predicted OH concentrations. Inclusion of an additional OH source formed as a recycled product of OH initiated isoprene oxidation improved the modelled OH agreement but only served to worsen the HO2 model/measurement agreement. To replicate levels of both OH and HO2, a process that recycles HO2 to OH is required; equivalent to the OH recycling effect of 0.74 ppbv of NO. This recycling step increases OH concentrations by 88 % at noon and has wide implications, leading to much higher predicted OH over tropical forests, with a concomitant reduction in the CH4 lifetime and increase in the rate of VOC degradation.

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