Ozone stress as a driving force of sesquiterpene emissions: a suggested parameterization

Abstract

Sesquiterpenes (C<sub>15</sub>H<sub>24</sub>) are semi-volatile organic compounds emitted by vegetation and are of interest in atmospheric research because they influence the oxidative capacity of the atmosphere and contribute to the formation of secondary organic aerosols. However, little is known about their emission pattern and no established parameterization is available for global emission models. The aim of this study is to investigate a Central European spruce forest and its emission response to meteorological and environmental parameters, looking for a parameterization that incorporates heat and oxidative stress as the main driving forces of the induced emissions. Therefore, a healthy ca. 80 yr old Norway spruce (<i>Picea abies</i>) tree was selected and a dynamical vegetation enclosure technique was applied from April to November 2011. The emissions clearly responded to temperature changes with small variations in the β-factor along the year (β<sub>spring</sub>=0.09 ± 0.01, β<sub>summer</sub>=0.12 ± 0.02, β<sub>autumn</sub>=0.11 ± 0.02). However, daily calculated values revealed a vast amount of variability in temperature dependencies ((0.02 ± 0.002)< β<(0.27 ± 0.04)) with no distinct seasonality. <br><br> By separating the complete dataset in 10 different ozone regimes, we found that in moderately or less polluted atmospheric conditions the main driving force of sesquiterpene emissions is the temperature, but when ambient ozone mixing ratios exceed a~critical threshold of (36.6 ± 3.9) ppb<sub>v</sub>, the emissions become primarily correlated with ozone. Considering the complete dataset, cross correlation analysis resulted in highest correlation with ambient ozone mixing ratios (CC<sub>O</sub><sub>3</sub>=0.63 ± 0.01; CC<sub>T</sub>=0.47 ± 0.02 at <i>t</i>=0 h for temperature) with a time shift 2–4 h prior to the emissions. An only temperature dependent algorithm was found to substantially underestimate the induced emissions (20 % of the measured; <i>R</i><sup>2</sup>=0.31). However, the addition of an ozone dependent term improved substantially the fitting between measured and modeled emissions (81 % of the measured; <i>R</i><sup>2</sup>=0.63), providing confidence about the reliability of the suggested parameterization for the spruce forest site investigated

Seasonal measurements of total OH reactivity fluxes, total ozone loss rates and missing emissions from Norway spruce in 2011

“…Interestingly, highest total OH reactivity fluxes as well as peak ozone loss rates inside the cuvette coincided with the climatological, seasonal maximum of ambient ozone. It also has been found, that sesquiterpene emissions were triggered not only by temperature but also by atmospheric ozone levels (Bourtsoukidis et al, 2012).…”

“…In order to determine the nature and rate of direct emissions from Norway spruce, a dynamic branch enclosure system (Ruuskanen et al, 2005;Bourtsoukidis et al, 2012) was mounted on a branch 5 m above ground (Fig. 1).…”

“…Optimization of the instrument resulted in sustained high sensitivity. More details of the PTR-MS configuration are presented in Bourtsoukidis et al (2012). Amongst other VOCs (see Table 1) the sum of monoterpenes (m/z 137) and the sum of sesquiterpenes (m/z 205) were monitored and calibrated using gas standards (L4763, Ionimed analytic GmbH, Austria).…”

“…While the system was closed ambient air was drawn into the cuvette and diluted the mixture (Ruuskanen et al, 2005). After correcting for the modest dilution effect of ambient air (k dil = (2.9±0.2)×10 −3 s −1 , see Bourtsoukidis et al, 2012), the data were fitted by a linear regression. Mono-and sesquiterpene emission rates from the branch enclosure have been reported in Bourtsoukidis et al (2012).…”

“…After correcting for the modest dilution effect of ambient air (k dil = (2.9±0.2)×10 −3 s −1 , see Bourtsoukidis et al, 2012), the data were fitted by a linear regression. Mono-and sesquiterpene emission rates from the branch enclosure have been reported in Bourtsoukidis et al (2012). Total OH reactivity emission rates (TOHRE) were obtained in similar fashion to those of the single compounds.…”