Atmospheric OH reactivities in the Pearl River Delta – China in summer 2006: measurement and model results

Abstract

Abstract. Total atmospheric OH reactivities (k OH ) have been measured as reciprocal OH lifetimes by a newly developed instrument at a rural site in the densely populated Pearl River Delta (PRD) in Southern China in summer 2006. The deployed technique, LP-LIF, uses laser flash photolysis (LP) for artificial OH generation and laser-induced fluorescence (LIF) to measure the time-dependent OH decay in samples of ambient air. The reactivities observed at PRD covered a range from 10 s −1 to 120 s −1 , indicating a large load of chemical reactants. On average, k OH exhibited a pronounced diurnal profile with a mean maximum value of 50 s −1 at daybreak and a mean minimum value of 20 s −1 at noon. The comparison of reactivities calculated from measured trace gases with measured k OH reveals a missing reactivity of about a factor of 2 at day and night. The reactivity explained by measured trace gases was dominated by anthropogenic pollutants (e.g., CO, NO x , light alkenes and aromatic hydrocarbons) at night, while it was strongly influenced by local, biogenic emissions of isoprene during the day. Box model calculations initialized by measured parameters reproduce the observed OH reactivity well and suggest that the missing reactivity is contributed by unmeasured, secondary chemistry products (mainly aldehydes and ketones) that were photochemically formed by hydrocarbon oxidation. Overall, k OH was dominated by organic compounds, which had a maximum contribution of 85% in the afternoon. The paper demonstrates the usefulness of direct reactivity measurements, emphasizes the Correspondence to: A. Hofzumahaus (a.hofzumahaus@fz-juelich.de) need for direct measurements of oxygenated organic compounds in atmospheric chemistry studies, and discusses uncertainties of the modelling of OVOC reactivities.

Impact of isoprene and HONO chemistry on ozone and OVOC formation in a semirural South Korean forest

“…Isoprene accounts for the highest OH reactivity especially during the daytime. This analysis is consistent with reports from other suburban observations from East Asian megacities such as Beijing (Ran et al, 2011) and the Pearl River Delta region (Lou et al, 2010) in China and the Kinki region in Japan (Bao et al, 2010).…”

“…In general, at low to intermediate NO levels (∼ 100 pptv or lower), the yields of C5-hydroxy-carbonyl compounds become higher. These new findings in the isoprene oxidation process are also closely related to recent findings in unexpectedly high OH concentrations (Hofzumahaus et al, 2009;Lelieveld et al, 2008) and substantial missing OH sinks also known as unexpectedly high OH reactivity in high isoprene environments (Di Carlo et al, 2004;Edwards et al, 2013;Lou et al, 2010).…”

How the OH reactivity affects the ozone production efficiency: case studies in Beijing and Heshan, China

“…Compared to the 2006 measurements in Backgarden, a semirural site in the PRD region, the modeled glyoxal was twice as high as around 0.8 ppbV . This difference possibly resulted from higher levels of precursors in 2014 measurements, where the measured reactivity was about 50 % higher than the results in Backgarden in 2006 (Lou et al, 2010).…”

“…Besides primary emitted species, unknown secondary species were not negligible. Yoshino et al (2006) found a good correlation between missing reactivity and measured oxygenated VOCs (OVOCs) in three seasons, except for winter, assuming that the unmeasured OVOCs could be major contributors of missing reactivity; in one case, the OVOCs could increase reactivity by over 50 % (Lou et al, 2010). The observation-based model (OBM) was widely used to evaluate the measured reactivity (Lee et al, 2010;Lou et al, 2010;Whalley et al, 2016), confirming the important contribution from OVOCs and undetected intermediate compounds.…”

“…b For sources from different studies, the calculated reactivity was presented within an uncertainty range as a percentage reduction or s −1 reduction. c Measured species have been used for the calculated reactivity (following Lou et al, 2010): S indicates inorganic compounds (CO, NOx , SO 2 , etc.) plus hydrocarbons (including isoprene); F indicates formaldehyde; O indicates OVOCs other than formaldehyde; B indicates BVOCs other than isoprene.…”