Abstract
Abstract. State-of-the-art techniques allow for rapid measurements of total OH reactivity. Unknown sinks of OH and oxidation processes in the atmosphere have been attributed to what has been termed “missing” OH reactivity. Often overlooked are the differences in timescales over which the diverse measurement techniques operate. Volatile organic compounds (VOCs) acting as sinks of OH are often measured by gas chromatography (GC) methods which provide low-frequency measurements on a timescale of hours, while sampling times are generally only a few minutes. Here, the effect of the sampling time and thus the contribution of unmeasured VOC variability on OH reactivity is investigated. Measurements of VOC mixing ratios by proton transfer reaction time-of-flight mass spectrometry (PTR-ToF-MS) conducted during two field campaigns (ClearfLo and PARADE) in an urban and a semi-rural environment were used to calculate OH reactivity. VOCs were selected to represent variability for different compound classes. Data were averaged over different time intervals to simulate lower time resolutions and were then compared to the mean hourly OH reactivity. The results show deviations in the range of 1 to 25 %. The observed impact of VOC variability is found to be greater for the semi-rural site.The selected compounds were scaled by the contribution of their compound class to the total OH reactivity from VOCs based on concurrent gas chromatography measurements conducted during the ClearfLo campaign. Prior to being scaled, the variable signal of aromatic compounds results in larger deviations in OH reactivity for short sampling intervals compared to oxygenated VOCs (OVOCs). However, once scaled with their lower share during the ClearfLo campaign, this effect was reduced. No seasonal effect on the OH reactivity distribution across different VOCs was observed at the urban site.
Highlights
Atmospheric photochemistry produces a variety of radicals that exert a substantial influence on the ultimate composition of the atmosphere
The second was taken during the PARADE (PArticles and RAdicals: Diel observations of the impact of urban and biogenic Emissions, http://parade2011.mpich.de/) campaign in late summer 2011 at a semi-rural site located in the Taunus ridge, Germany
If only some Volatile organic compounds (VOCs) are taken into account for calculating the reactivity, this will be indicated, e.g. RPOTVRO,CCL,5is the OH reactivity calculated from the 5 min mean concentration of acetone, measured with the PTR-ToFMS during ClearfLo
Summary
Atmospheric photochemistry produces a variety of radicals that exert a substantial influence on the ultimate composition of the atmosphere. A similar approach is taken with the laser-induced pump and probe technique, whereby decay in OH is detected by timeresolved laser-induced fluorescence (Sadanaga et al, 2004) Another technique developed by Sinha et al (2008), called the comparative reactivity method (CRM), is based on the measurement of a single reactant (most often pyrrole) which first reacts with OH under clean air conditions and under competitive conditions with ambient air. Larger missing OH reactivity of up to 30 % was found for all other seasons in Tokyo by Yoshino et al (2006), presumably owing to secondary reaction products, including semi-volatile oxygenated compounds, from atmospheric oxidation of VOCs. A similar amount of missing OH reactivity was reported by Kovacs et al (2003) for urban measurements in Nashville. The sampling time was 10 min, while the analysis runtime was around 50 min, resulting in approximately one measurement per hour
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