Abstract
Abstract. Total hydroxyl radical (OH) reactivity measurements were conducted during the LANDEX intensive field campaign in a coniferous temperate forest located in the Landes area, southwestern France, during July 2017. In order to investigate inter-canopy and intra-canopy variability, measurements were performed inside (6 m) and above the canopy level (12 m), as well as at two different locations within the canopy, using a comparative reactivity method (CRM) and a laser photolysis–laser-induced fluorescence (LP-LIF) instrument. The two techniques were intercompared at the end of the campaign by performing measurements at the same location. Volatile organic compounds were also monitored at both levels with a proton transfer time-of-flight mass spectrometer and online gas chromatography instruments to evaluate their contribution to total OH reactivity, with monoterpenes being the main reactive species emitted in this forest dominated by Pinus pinaster Aiton. Total OH reactivity varied diurnally, following the trend of biogenic volatile organic compounds (BVOCs), the emissions and concentrations of which were dependent on meteorological parameters. Average OH reactivity was around 19.2 and 16.5 s−1 inside and above the canopy, respectively. The highest levels of total OH reactivity were observed during nights with a low turbulence (u*≤0.2 m s−1), leading to lower mixing of emitted species within the canopy and thus an important vertical stratification characterized by a strong concentration gradient. Comparing the measured and the calculated OH reactivity highlighted an average missing OH reactivity of 22 % and 33 % inside and above the canopy, respectively. A day–night variability was observed on missing OH reactivity at both heights. Investigations showed that during daytime, missing OH sinks could be due to primary emissions and secondary products linked to a temperature-enhanced photochemistry. Regarding nighttime missing OH reactivity, higher levels were seen for the stable and warm night of 4–5 July, showing that these conditions could have been favorable for the accumulation of long-lived species (primary and secondary species) during the transport of the air mass from nearby forests.
Highlights
The hydroxyl radical (OH) is considered the most important initiator of photochemical processes in the troposphere during daytime and the prevailing “detergent” from local to global scales
Total OH reactivity was measured at a different location inside the canopy from 13 to 19 July using the laser photolysis–laser-induced fluorescence (LP-LIF) instrument of the PC2A laboratory (UL-FAGE reactivity), which has already been used in several intercomparisons and field campaigns (Hansen et al, 2015; Fuchs et al, 2017)
The calculated OH reactivity inside and above the canopy (Fig. S3b and e) from (i) the use of the weighted OH reaction rate constant and the total concentration of monoterpenes measured by gas chromatographic (GC) and (ii) the use of individual species and their associated rate constants is in relatively good agreement as shown by the scatter plots
Summary
The hydroxyl radical (OH) is considered the most important initiator of photochemical processes in the troposphere during daytime and the prevailing “detergent” from local to global scales. A few intercomparisons were reported in the literature for urban and remote areas (Hansen et al, 2015; Zannoni, 2015; Sanchez et al, 2018) as well as in chamber experiments (Fuchs et al, 2017) aiming at reproducing the ambient conditions observed in various environments The latter, including a large number of OH reactivity instruments (FT-LIF, LP-LIF, CRM) for studies conducted in the SAPHIR atmospheric simulation chamber, allowed for a comparison of the performances of each technique. Total OH reactivity appeared to be impacted by several factors such as the forest type and the dominant emitted species, the seasonality, the canopy level, and specific atmospheric conditions (Hansen et al, 2014; Nölscher et al, 2013; Praplan et al, 2019; Sanchez et al, 2018; Zannoni et al, 2016) Among these biogenic hydrocarbons, monoterpenes represent a large class of C10H16 compounds, which are mainly emitted by conifers as well as broad-leaved trees. The deployment of two different instruments was a good opportunity to (i) compare measurements made with both methods in a real biogenic environment after the intercomparison experiment performed in the SAPHIR chamber and recent improvement of the CRM instrument, (ii) investigate the levels and diurnal variability of OH reactivity at two different heights, and (iii) investigate both the OH reactivity budget and the missing reactivity pattern using a large panel of concomitant trace gas measurements
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