Hydroxyl radicals in the tropical troposphere over the Suriname rainforest: comparison of measurements with the box model MECCA

Abstract

As a major source region of the hydroxyl radical OH, the Tropics largely control the oxidation capacity of the atmosphere on a global scale. However, emissions of hydrocarbons from the tropical rainforest that react rapidly with OH can potentially deplete the amount of OH and thereby reduce the oxidation capacity. The airborne GABRIEL field campaign in equatorial South America (Suriname) in October 2005 investigated the influence of the tropical rainforest on the HO<sub>x</sub> budget (HO<sub>x</sub> = OH + HO<sub>2</sub>). The first observations of OH and HO<sub>2</sub> over a tropical rainforest are compared to steady state concentrations calculated with the atmospheric chemistry box model MECCA. The important precursors and sinks for HO<sub>x</sub> chemistry, measured during the campaign, are used as constraining parameters for the simulation of OH and HO<sub>2</sub>. Significant underestimations of HO<sub>x</sub> are found by the model over land during the afternoon, with mean ratios of observation to model of 12.2 ± 3.5 and 4.1 ± 1.4 for OH and HO<sub>2</sub>, respectively. The discrepancy between measurements and simulation results is correlated to the abundance of isoprene. While for low isoprene mixing ratios (above ocean or at altitudes >3 km), observation and simulation agree fairly well, for mixing ratios >200 pptV (<3 km over the rainforest) the model tends to underestimate the HO<sub>x</sub> observations as a function of isoprene. <br></br> Box model simulations have been performed with the condensed chemical mechanism of MECCA and with the detailed isoprene reaction scheme of MCM, resulting in similar results for HO<sub>x</sub> concentrations. Simulations with constrained HO<sub>2</sub> concentrations show that the conversion from HO<sub>2</sub> to OH in the model is too low. However, by neglecting the isoprene chemistry in the model, observations and simulations agree much better. An OH source similar to the strength of the OH sink via isoprene chemistry is needed in the model to resolve the discrepancy. A possible explanation is that the oxidation of isoprene by OH not only dominates the removal of OH but also produces it in a similar amount. Several additional reactions which directly produce OH have been implemented into the box model, suggesting that upper limits in producing OH are still not able to reproduce the observations (improvement by factors of ≈2.4 and ≈2 for OH and HO<sub>2</sub>, respectively). We determine that OH has to be recycled to 94% instead of the simulated 38% to match the observations, which is most likely to happen in the isoprene degradation process, otherwise additional sources are required

Biomass burning emissions disturbances on the isoprene oxidation in a tropical forest

“…Our results suggest that in the fresh plumes, the vertical transport predominates with the oxidative capacity reaching its maximum at 1,000 m. In the flight tracks classified as BG, we observed the widest variation in the average OH concentration using the new sequential reaction model (Figure 9, on bottom), especially in upper levels (0.5 -1  10 6 molecules cm -3 ), although reported a lower confidence level in this presented an increase near to CL (~1,000 m), in agreement with previous studies (Karl et al, 2007;Kuhn et al, 2007;Langford et al, 2005;Mauldin et al, 1997). Several studies investigated the uncertainties in the isoprene oxidation mechanism, and most of them focus on OH concentration levels through observational and modeling studies (de Gouw et al, 2006;Kubistin et al, 2010;Kuhn et al, 2007;Lelieveld et al, 2008;Lu et al, 2012;Whalley et al, 2014;Yokelson et al, 2007). Under a high isoprene and low-NO atmospheric regime, there is a controversial discussion about the impact on the oxidative capacity in forest sites.…”

OH reactivity in a South East Asian tropical rainforest during the Oxidant and Particle Photochemical Processes (OP3) project

“…Previous modelling studies of forested environments with high biogenic VOC emissions have significantly underestimated the observed OH concentrations (Tan et al, 2001;Kubistin et al, 2010;Hofzumahaus et al, 2009;Whalley et al, 2011;Stone et al, 2011). A simulation with unconstrained OH for OP3 exhibits a similar result (not shown but consistent with Whalley et al, 2011).…”

LES study of the impact of moist thermals on the oxidative capacity of the atmosphere in southern West Africa

“…This reaction imply that isoprene have no net impact on OH concentrations. This conclusion is drawn also by Kubistin et al (2010) 10 who found better agreement between simulated and measured HO x concentrations by ignoring the isoprene chemistry. As the chemistry of isoprene is not well understood, the results obtained in our work are subject to these uncertainties.…”