Observation and modelling of OH and HO<sub>2</sub> concentrations in the Pearl River Delta 2006: a missing OH source in a VOC rich atmosphere

Abstract

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 <i>j</i>(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

Detection of HO&lt;sub&gt;2&lt;/sub&gt; by laser-induced fluorescence: calibration and interferences from RO&lt;sub&gt;2&lt;/sub&gt; radicals

“…The impact of interferences from RO 2 is highest, if the oxidation rate of pollutants such as alkenes including isoprene is high (high VOC and OH concentrations) but NO concentrations are small as found e.g. during the PRIDE-PRD2006 campaign (Lu et al, 2011). For this campaign, model calculations suggest that the measured HO 2 concentrations contain an interference by RO 2 of at least 30 % during daytime.…”

“…For example, a significant impact of the interference is expected for the recently published HO x measurements performed during the PRIDE-PRD2006 campaign in the Pearl River Delta, China . Model calculations suggest that the measured HO 2 concentrations contain an interference by RO 2 of at least 30 % during daytime, depending on the chemical mechanism used for the calculation of the RO 2 species (Lu et al, 2011). This also suggests that the unknown radical recycling, proposed by Hofzumahaus et al (2009) to explain the observed OH, must be even larger, if the HO 2 concentrations are smaller than assumed before (Lu et al, 2011).…”

“…An interference from RO 2 radicals would impact HO 2 concentration measurements in the past, when high loads of VOCs including isoprene were present such as during the field campaigns ECHO (Kleffmann et al, 2005;Dlugi et al, 2010) and PRIDE-PRD2006 Lu et al, 2011).…”

Isoprene in poplar emissions: effects on new particle formation and OH concentrations

“…Observations of atmospheric OH concentrations indicate that under low NO x conditions isoprene may be involved in a so far unidentified recycling mechanism that converts HO 2 into OH 1022 A. Kiendler-Scharr et al: Effects on new particle formation and OH concentrations without involving ozone formation (Tan et al, 2001;Thornton et al, 2002;Ren et al, 2008;Lelieveld et al, 2008;Hofzumahaus et al, 2009;Whalley et al, 2011;Lu et al, 2011). Contrary, suppression of new particle formation by isoprene was ascribed to its suppression of OH concentrations (Kiendler-Scharr et al, 2009b).…”

“…Air was continuously sampled from the chamber by gas expansion through a 0.6 mm nozzle into a low pressure (6 hPa) detection cell, in which OH was detected by pulsed laser-excited fluorescence at 308 nm. Unlike in previous instrument configurations (Holland et al, 2003;Lu et al, 2011), a long inlet tube (anodized aluminium) with a length of 22 cm and an internal diameter of 3.9 cm connected the nozzle to the fluorescence cell. This allowed in-situ measurement of OH in the reaction chamber at a distance of about 20 cm from the chamber bottom and the sidewall surfaces.…”