Going Faster: The concentrations of most tropospheric pollutants and trace gases are kept in check by their reactions with hydroxyl radicals (OH). OH is a short-lived, highly reactive species that is produced in the atmosphere by photochemical processes, and regenerated in the chain of chemical reactions that follows the oxidative destruction of those molecules. These regeneration mechanisms were thought to be fairly well understood, but now Hofzumahaus et al. (p. 1702 , published online 4 June) present evidence of a pathway not previously recognized. In a study of atmospheric composition in the Pearl River Delta, a highly polluted region of China, greatly elevated OH concentrations were observed without the correspondingly high levels of ozone expected from current models. Thus, OH concentrations may be augmented by a process that speeds the regeneration of OH without producing ozone.

A comprehensive field campaign was carried out in summer 2014 in Wangdu, located in the North China Plain. A month of continuous OH, HO₂ and RO₂ measurements was achieved. Observations of radicals by the laser-induced fluorescence (LIF) technique revealed daily maximum concentrations between (5–15) × 10⁶ cm⁻³, (3–14) × 10⁸ cm⁻³ and (3–15) × 10⁸ cm⁻³ for OH, HO₂ and RO₂, respectively. Measured OH reactivities (inverse OH lifetime) were 10 to 20 s⁻¹ during daytime. The chemical box model RACM 2, including the Leuven isoprene mechanism (LIM), was used to interpret the observed radical concentrations. As in previous field campaigns in China, modeled and measured OH concentrations agree for NO mixing ratios higher than 1 ppbv, but systematic discrepancies are observed in the afternoon for NO mixing ratios of less than 300 pptv (the model–measurement ratio is between 1.4 and 2 in this case). If additional OH recycling equivalent to 100 pptv NO is assumed, the model is capable of reproducing the observed OH, HO₂ and RO₂ concentrations for conditions of high volatile organic compound (VOC) and low NO_x concentrations. For HO₂, good agreement is found between modeled and observed concentrations during day and night. In the case of RO₂, the agreement between model calculations and measurements is good in the late afternoon when NO concentrations are below 0.3 ppbv. A significant model underprediction of RO₂ by a factor of 3 to 5 is found in the morning at NO concentrations higher than 1 ppbv, which can be explained by a missing RO₂ source of 2 ppbv h⁻¹. As a consequence, the model underpredicts the photochemical net ozone production by 20 ppbv per day, which is a significant portion of the daily integrated ozone production (110 ppbv) derived from the measured HO₂ and RO₂. The additional RO₂ production from the photolysis of ClNO₂ and missing reactivity can explain about 10 % and 20 % of the discrepancy, respectively. The underprediction of the photochemical ozone production at high NO_x found in this study is consistent with the results from other field campaigns in urban environments, which underlines the need for better understanding of the peroxy radical chemistry for high NO_x conditions.

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Ambient OH and HO₂ concentrations were measured by laser induced fluorescence (LIF) during the PRIDE-PRD2006 (Program of Regional Integrated Experiments of Air Quality over the Pearl River Delta, 2006) campaign at a rural site downwind of the megacity of Guangzhou in Southern China. The observed OH concentrations reached daily peak values of (15–26) × 10⁶ cm⁻³ which are among the highest values so far reported for urban and suburban areas. The observed OH shows a consistent high correlation with j(O¹D) over a broad range of NO_x conditions. The correlation cannot be reproduced by model simulations, indicating that OH stabilizing processes are missing in current models. The observed OH exhibited a weak dependence on NO_x in contrast to model predictions. While modelled and measured OH agree well at NO mixing ratios above 1 ppb, a continuously increasing underprediction of the observed OH is found towards lower NO concentrations, reaching a factor of 8 at 0.02 ppb NO. A dependence of the modelled-to-measured OH ratio on isoprene cannot be concluded from the PRD data. However, the magnitude of the ratio fits into the isoprene dependent trend that was reported from other campaigns in forested regions. Hofzumahaus et al. (2009) proposed an unknown OH recycling process without NO, in order to explain the high OH levels at PRD in the presence of high VOC reactivity and low NO. Taking a recently discovered interference in the LIF measurement of HO₂ into account, the need for an additional HO₂ → OH recycling process persists, but the required source strength may be up to 20% larger than previously determined. Recently postulated isoprene mechanisms by Lelieveld et al. (2008) and Peeters and Müller (2010) lead to significant enhancements of OH expected for PRD, but an underprediction of the observed OH by a factor of two remains at low NO (0.1–0.2 ppb). If the photolysis of hydroperoxy aldehydes from isoprene is as efficient as proposed by Peeters and Müller (2010), the corresponding OH formation at PRD would be more important than the primary OH production from ozone and HONO. While the new isoprene mechanisms need to be confirmed by laboratory experiments, there is probably need for other, so far unidentified chemical processes to explain entirely the high OH levels observed in Southern China.