Long-term total OH reactivity measurements in a boreal forest

Abstract

<p><strong>Abstract.</strong> Total hydroxyl radical (OH) reactivity measurements were conducted at the second Station for Measuring Ecosystem-Atmosphere Relations (SMEAR II), a boreal forest site located in Hyytiälä, Finland, from April to July 2016. The measured values were compared with OH reactivity calculated from a combination of data from the routine trace gas measurements (station mast) as well as online and offline analysis with gas chromatography coupled to mass spectrometry (GC-MS) and offline liquid chromatography. Up to 104 compounds, mostly Volatile Organic Compounds (VOCs) and oxidised VOCs, but also inorganic compounds, were included in the analysis, even though the data availability for each compound varied with time. The averaged experimental total OH reactivity increased from April to June (from 5.3 to 11.3 s<sup>−1</sup>) and decreased in July (8.8 s<sup>−1</sup>) due to different environmental conditions during the measurement days. In general, the total OH reactivity increased in late-afternoon and is high at night. It decreases in the morning and is low during the day, following the pattern of mixing ratios due to change of the boundary layer height. The missing reactivity fraction (defined as the different between measured and calculated OH reactivity) was found to be high. Several reasons that can explain the missing reactivity are discussed in detail such as (1) missing measurements due to technical issues, (2) not measuring oxidation compounds of detected biogenic VOCs, (3) missing important reactive compounds or classes of compounds with the available measurements. In order to test the second hypothesis, a one-dimensional chemical transport model (SOSAA) has been used to estimate the amount of unmeasured oxidation products and their expected contribution to the reactivity for three different short periods in April, May, and July. However, only a small fraction (< 9 %) of the missing reactivity can be explained by modelled secondary compounds (mostly oxidised VOCs). These findings indicate that compounds measured but not included in the model as well as unmeasured primary emissions contribute the missing reactivity. In the future, non-hydrocarbon compounds from other sources than trees (e.g. soil) should be included in OH reactivity studies.</p>

OH reactivity from different tree species: Investigating the missing reactivity in a boreal forest

“…The experimental total OH reactivity, R exp , can be measured with the Comparative Reactivity Method (CRM, Sinha et al,115 2008; Michoud et al, 2015). The specific instrument used for this study is described in Praplan et al (2017Praplan et al ( , 2019 and the measurement principle briefly explained in the following section together with the application of the method to measure the OH Reactivity of Emissions (OHRE).…”

“…Originally, Sinha the observed response of pyrrole in the reactor, despite alterations to take into account secondary OH chemistry. In the present study we use the experimental results derived in Praplan et al (2019) based on α-pinene calibrations, which show that the measured OH reactivity (R eqn ) is roughly half the expected reactivity, so that the exact relationship between the reactivity in the reactor (R CRM ) and R eqn is the following:…”

“…Therefore we apply a correction described in detail in (Praplan et al, 2019) for this deviation from pseudo first-order kinetics, based on experimental reactivity calibrations with α-pinene (see section 2.5.3). The only difference here compared to Praplan et al (2019) who applied the correction factor to ambient measurements is that the background reactivity of the empty enclosure (R eqn,blank ) is also taken into account. R eqn,blank was determined between 28 September and 4 October and is 2.6 ± 3.0 s −1…”

“…However, corrections due to the presence of ozone (O 3 ) and nitrogen oxides (NO x ) described elsewhere (e.g. Michoud et al, 2015;Fuchs et al, 2017;Praplan et al, 2017Praplan et al, , 2019 are not required in the present study due to the use of zero air through the dynamic branch enclosure. Only the correction due to the difference in relative humidity (RH) in the reactor between C 2 and C 3 levels and the correction due to the deviation 155 from pseudo-first-order kinetics need to be taken into account.…”

Review of the manuscript titled "Contributions to OH reactivity from unexplored volatile organic compounds measured by PTR-ToF-MS -- A case study in a suburban forest of the Seoul Metropolitan Area during KORUS-AQ 2016", by Sanchez et al.

“…-p.3, ll.61-66: Are the authors aware of the more recent publications by Nölscher et al (2012) and by Praplan et al (2019) for additional OH reactivity measurements in Hyytiälä? Their findings were similar to the ones of Sinha et al (2010) in terms of missing reactivity.…”