Oxidation photochemistry in the Southern Atlantic boundary layer: Unexpected deviations of photochemical steady state

Ozone (O[subscript 3]) is a photochemical oxidant, an air pollutant and a greenhouse gas. As the main precursor of the hydroxyl radical (OH) it strongly affects the oxidation power of the atmosphere. The remote marine boundary layer (MBL) is considered an important region in terms of chemical O[subscript 3] loss; however surface-based atmospheric observations are sparse and the photochemical processes are not well understood. To investigate the photochemistry under the clean background conditions of the Southern Atlantic Ocean, ship measurements of NO, NO[subscript 2], O[subscript 3], J[subscript NO2], J(O[superscript 1]D), HO[subscript 2], OH, RO[subscript x] and a range of meteorological parameters were carried out. The concentrations of NO and NO[subscript 2] measured on board the French research vessel Marion-Dufresne (28° S–57° S, 46° W–34° E) in March 2007, are among the lowest yet observed. The data is evaluated for consistency with photochemical steady state (PSS) conditions, and the calculations indicate substantial deviations from PSS (Φ>1). The deviations observed under low NO[subscript x] conditions (5–25 pptv) demonstrate a remarkable upward tendency in the Leighton ratio (used to characterize PSS) with increasing NO[subscript x] mixing ratio and J[subscript NO2] intensity. It is a paradigm in atmospheric chemistry that OH largely controls the oxidation efficiency of the atmosphere. However, evidence is growing that for unpolluted low-NO[subscript x] (NO + NO[subscript 2]) conditions the atmospheric oxidant budget is poorly understood. Nevertheless, for the very cleanest conditions, typical for the remote marine boundary layer, good model agreement with measured OH and HO[subscript 2] radicals has been interpreted as accurate understanding of baseline photochemistry. Here we show that such agreement can be deceptive and that a yet unidentified oxidant is needed to explain the photochemical conditions observed at 40°–60° S over the Atlantic Ocean

Isoprene oxidation mechanisms: Measurements and modelling of OH and HO[subscript 2] over a South-East Asian tropical rainforest during the OP3 field campaign

Forests are the dominant source of volatile organic compounds into the atmosphere, with isoprene being the most significant species. The oxidation chemistry of these compounds is a significant driver of local, regional and global atmospheric composition. Observations made over Borneo during the OP3 project in 2008, together with an observationally constrained box model are used to assess our understanding of this oxidation chemistry. In line with previous work in tropical forests, we find that the standard model based on MCM chemistry significantly underestimates the observed OH concentrations. Geometric mean observed to modelled ratios of OH and HO[subscript 2] in airmasses impacted with isoprene are 5.32[subscript −4.43,superscript +3.68] and 1.18[subscript −0.30,superscript +0.30] respectively, with 68 % of the observations being within the specified variation. We implement a variety of mechanistic changes into the model, including epoxide formation and unimolecular decomposition of isoprene peroxy radicals, and assess their impact on the model success. We conclude that none of the current suggestions can simultaneously remove the bias from both OH and HO[subscript 2] simulations and believe that detailed laboratory studies are now needed to resolve this issue

HO[subscript x] observations over West Africa during AMMA: impact of isoprene and NO[subscript x]

Aircraft OH and HO[subscript 2] measurements made over West Africa during the AMMA field campaign in summer 2006 have been investigated using a box model constrained to observations of long-lived species and physical parameters. "Good" agreement was found for HO[subscript 2] (modelled to observed gradient of 1.23 ± 0.11). However, the model significantly overpredicts OH concentrations. The reasons for this are not clear, but may reflect instrumental instabilities affecting the OH measurements. Within the model, HO[subscript x] concentrations in West Africa are controlled by relatively simple photochemistry, with production dominated by ozone photolysis and reaction of O([superscript 1]D) with water vapour, and loss processes dominated by HO[subscript 2] + HO[subscript 2] and HO[subscript 2] + RO[subscript 2]. Isoprene chemistry was found to influence forested regions. In contrast to several recent field studies in very low NO[subscript x] and high isoprene environments, we do not observe any dependence of model success for HO[subscript 2] on isoprene and attribute this to efficient recycling of HO[subscript x] through RO[subscript 2] + NO reactions under the moderate NO[subscript x] concentrations (5–300 ppt NO in the boundary layer, median 76 ppt) encountered during AMMA. This suggests that some of the problems with understanding the impact of isoprene on atmospheric composition may be limited to the extreme low range of NO[subscript x] concentrations