Nighttime observation and chemistry of HO<sub>x</sub> in the Pearl River Delta and Beijing in summer 2006

Abstract

Abstract. Nighttime HO x chemistry was investigated in two ground-based field campaigns (PRIDE-PRD2006 and CAREBEIJING2006) in summer 2006 in China by comparison of measured and modeled concentration data of OH and HO 2 . The measurement sites were located in a rural environment in the Pearl River Delta (PRD) under urban influence and in a suburban area close to Beijing, respectively. In both locations, significant nighttime concentrations of radicals were observed under conditions with high total OH reactivities of about 40-50 s −1 in PRD and 25 s −1 near Beijing. For OH, the nocturnal concentrations were within the range of (0.5-3) × 10 6 cm −3 , implying a significant nighttime oxidation rate of pollutants on the order of several ppb per hour. The measured nighttime concentration of HO 2 was about (0.2-5) ×10 8 cm −3 , containing a significant, modelestimated contribution from RO 2 as an interference. A chemical box model based on an established chemical mechanism is capable of reproducing the measured nighttime values of the measured peroxy radicals and k OH , but underestimates in both field campaigns the observed OH by about 1 order of magnitude. Sensitivity studies with the box model demonstrate that the OH discrepancy between measured and modeled nighttime OH can be resolved, if an additional RO x production process (about 1 ppb h −1 ) and additional recycling (RO 2 → HO 2 → OH) with an efficiency equivalent to 1 ppb NO is assumed. The additional recycling mechanism was also needed to reproduce the OH observations at the same locations during daytime for conditions with NO mixing ratios below 1 ppb. This could be an indication that the same missing process operates at day and night. In principle, the required primary RO x source can be explained by ozonolysis of terpenoids, which react faster with ozone than with OH in the nighttime atmosphere. However, the amount of these highly reactive biogenic volatile organic compounds (VOCs) would require a strong local source, for which there is no direct evidence. A more likely explanation for an additional RO x source is the vertical downward transport of radical reservoir species in the stable nocturnal boundary layer. Using a simplified one-dimensional two-box model, it can be shown that ground-based NO emissions could generate a large vertical gradient causing a downward flux of peroxy acetic nitrate (PAN) and peroxymethacryloyl nitrate (MPAN). The downward transport and the following thermal decomposition of these compounds can produce up to 0.3 ppb h −1 radicals in the atmospheric layer near the ground. Although this rate is not sufficient to explain the complete OH discrepancy, it indicates the potentially important role of vertical transport in the lower nighttime atmosphere.

Response to review

“…The OH reactivities measured at the Wangdu site in the North China Plain show diurnal profiles that are comparable to those reported for other polluted environments all over the world (see review by Yang et al, 2016). The total reactivities lie within the range of values observed during summertime at other locations that were mainly influenced by anthropogenic emissions, like Nashville (Kovacs et al, 2003), New York and Houston (Mao et al, 2010) in the US, Tokyo in Japan (Chatani et al, 2009), Beijing in China, Seoul in South Korea (Kim et al, 2016), and London (Whalley et al, 2016) in Great Britain. Also, the shapes of the diurnal profiles were similar, with peak values between 15 and 50 s −1 in the early morning and minimum values in the afternoon.…”

Development of a portable cavity-enhanced absorption spectrometer for the measurement of ambient NO<sub>3</sub> and N<sub>2</sub>O<sub>5</sub>: experimental setup, lab characterizations, and field applications in a polluted urban environment

“…Since significant night-time OH concentrations (up to 1 × 10 6 cm −3 ) were also found for these environments (e.g. Lu et al, 2014;Tan et al, 2017), the contribution of NO 3 -N 2 O 5 and HO x chemistry toward the night-time oxidation capacity in Beijing is worthy of future exploration.…”

Impact of pollution controls in Beijing on atmospheric oxygenated volatile organic compounds (OVOCs) during the 2008 Olympic Games: observation and modeling implications

“…As NMHCs play significant roles on the formation of secondary OVOCs, emission ratios (ERs) of NMHCs before and during the full control were compared in this section to determine whether the emission restrictions also altered the relative emission of OVOC precursors (i.e., source pattern of NMHCs), in addition to reducing their total emission amounts. According to previous studies on NMHC source apportionment in Beijing (Liu et al, 2005;Song et al, 2007;Lu et al, 2007;Wang et al, 2010a), vehicle exhaust is the largest contributor to ambient NMHCs with 40-58 %, followed by solvent usage and painting processes (18-30 %), gasoline evaporation (7-13 %) and chemical plants (3-15 %).…”