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

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 j(O1D) over a broad range of NOx 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 NOx 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.

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.…”

Tropospheric aerosol as a reactive intermediate

“…[22][23][24] Even more troubling is that, contrary to expectations, noon-time _OH concentrations (in Birmingham) during winter were found to be only a factor of 2 smaller than in summer, despite a factor of 15 reduction in J(O 1 D) over the same period. 25 Given the key role of _OH, this represents a major deciency in our understanding of tropospheric chemistry.…”

“…detected at a pristine site yearlong, 21 and episodically at a polluted one, 22 is not well reproduced by standard chemical models. [22][23][24] Even more troubling is that, contrary to expectations, noon-time _OH concentrations (in Birmingham) during winter were found to be only a factor of 2 smaller than in summer, despite a factor of 15 reduction in J(O 1 D) over the same period.…”

Atmospheric photochemistry of aromatic hydrocarbons: OH budgets during SAPHIR chamber experiments

“…Based on a steady-state approach for OH, Hofzumahaus et al (2009) compared the known production and destruction rates of OH by taking measured concentrations of OH, HO 2 , NO and the total OH reactivity of ambient air. It was concluded that during a field campaign in the Pearl River Delta in southern China, the conventional OH recycling rate by HO 2 + NO was too small by a factor of three to explain the measured OH under low-NO conditions Lu et al, 2012).…”

“…In contrast, RO 2 has to be converted to HO 2 first which requires at least one additional O 2 reaction following the RO 2 + NO reaction. This RO 2 -to-HO 2 conversion is accomplished in a pre-reactor (Fuchs et al, 2008 Lu et al, 2012). was then reduced to 0.17 by means of technical changes of the experimental setup .…”