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

Sesquiterpenes (C15H24) 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 parameterisation 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 parameterisation that incorporates heat and oxidative stress as the main driving forces of the induced emissions. Therefore, a healthy ca. 80 yr old Norway spruce (Picea abies) 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 (βspring = 0.09 ± 0.01, βsummer = 0.12 ± 0.02, βautumn = 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. 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) ppbv, the emissions become primarily correlated with ozone. Considering the complete dataset, cross correlation analysis resulted in highest correlation with ambient ozone mixing ratios (CCO3 = 0.63 ± 0.01; CCT = 0.47 ± 0.02 at t = 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; R2 = 0.31). However, the addition of an ozone dependent term improved substantially the fitting between measured and modelled emissions (81% of the modelled emissions could be explained by the measurements; R2 = 0.63), providing confidence about the reliability of the suggested parameterisation for the spruce forest site investigated

Ozone stress as a driving force of sesquiterpene emissions: a suggested parameterization [Discussion paper]

Sesquiterpenes (C15H24) 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 (Picea abies) 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 (βspring=0.09 ± 0.01, βsummer=0.12 ± 0.02, βautumn=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. 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) ppbv, the emissions become primarily correlated with ozone. Considering the complete dataset, cross correlation analysis resulted in highest correlation with ambient ozone mixing ratios (CCO3=0.63 ± 0.01; CCT=0.47 ± 0.02 at t=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; R2=0.31). However, the addition of an ozone dependent term improved substantially the fitting between measured and modeled emissions (81 % of the measured; R2=0.63), providing confidence about the reliability of the suggested parameterization for the spruce forest site investigated

Seasonal measurements of total OH reactivity emission rates from Norway spruce in 2011

Numerous reactive volatile organic compounds (VOCs) are emitted into the atmosphere by vegetation. Most biogenic VOCs are highly reactive towards the atmosphere's most important oxidant, the hydroxyl (OH) radical. One way to investigate the chemical interplay between biosphere and atmosphere is through the measurement of total OH reactivity, the total loss rate of OH radicals. This study presents the first determination of total OH reactivity emission rates (measurements via the comparative reactivity method) based on a branch cuvette enclosure system mounted on a Norway spruce (Picea abies) throughout spring, summer and autumn 2011. In parallel VOC emission rates were monitored by a second proton-transfer-reaction mass spectrometer (PTR-MS), and total ozone (O3) loss rates were obtained inside the cuvette. Total OH reactivity emission rates were in general temperature and light dependent, showing strong diel cycles with highest values during daytime. Monoterpene emissions contributed most, accounting for 56–69% of the measured total OH reactivity flux in spring and early summer. However, during late summer and autumn the monoterpene contribution decreased to 11–16%. At this time, a large missing fraction of the total OH reactivity emission rate (70–84%) was found when compared to the VOC budget measured by PTR-MS. Total OH reactivity and missing total OH reactivity emission rates reached maximum values in late summer corresponding to the period of highest temperature. Total O3 loss rates within the closed cuvette showed similar diel profiles and comparable seasonality to the total OH reactivity fluxes. Total OH reactivity fluxes were also compared to emissions from needle storage pools predicted by a temperature-only-dependent algorithm. Deviations of total OH reactivity fluxes from the temperature-only-dependent emission algorithm were observed for occasions of mechanical and heat stress. While for mechanical stress, induced by strong wind, measured VOCs could explain total OH reactivity emissions, during heat stress they could not. The temperature-driven algorithm matched the diel variation of total OH reactivity emission rates much better in spring than in summer, indicating a different production and emission scheme for summer and early autumn. During these times, unmeasured and possibly unknown primary biogenic emissions contributed significantly to the observed total OH reactivity flux

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

Numerous reactive volatile organic compounds (VOCs) are emitted into the atmosphere by vegetation. Most biogenic VOCs are highly reactive towards the atmosphere's most important oxidant, the hydroxyl (OH) radical. One way to investigate the chemical interplay between biosphere and atmosphere is through the measurement of total OH reactivity, the total loss rate of OH radicals. This study presents the first determination of total OH reactivity emission rates (measurements via the Comparative Reactivity Method) based on a branch cuvette enclosure system mounted on a Norway spruce (Picea abies) throughout spring, summer and autumn 2011. In parallel separate VOC emission rates were monitored by a Proton Transfer Reaction-Mass Spectrometer (PTR-MS), and total ozone (O3) loss rates were obtained inside the cuvette. Total OH reactivity emission rates were in general temperature and light dependent, showing strong diel cycles with highest values during daytime. Monoterpene emissions contributed most, accounting for 56–69% of the measured total OH reactivity flux in spring and early summer. However, during late summer and autumn the monoterpene contribution decreased to 11–16%. At this time, a large missing fraction of the total OH reactivity emission rate (70–84%) was found when compared to the VOC budget measured by PTR-MS. Total OH reactivity and missing total OH reactivity emission rates reached maximum values in late summer corresponding to the period of highest temperature. Total O3 loss rates within the closed cuvette showed similar diel profiles and comparable seasonality to the total OH reactivity fluxes. Total OH reactivity fluxes were also compared to emissions from needle storage pools predicted by a temperature-only dependent algorithm. Deviations of total OH reactivity fluxes from the temperature-only dependent emission algorithm were observed for occasions of mechanical and heat stress. While for mechanical stress, induced by strong wind, measured VOCs could explain total OH reactivity emissions, during heat stress they could not. The temperature driven algorithm matched the diel course much better in spring than in summer, indicating a different production and emission scheme for summer and early autumn. During these times, unmeasured and possibly unknown primary biogenic emissions contributed significantly to the observed total OH reactivity flux

Ambient new particle formation parameter indicates potential rise in future events

Atmospheric new particle formation is a general phenomenon observed over coniferous forests. So far nucleation is described as a function of gaseous sulfuric acid concentration only, which is unable to explain the observed seasonality of nucleation events at different measurement sites. Here we introduce a new nucleation parameter including ozone and water vapor concentrations as well as UV-B radiation as a proxy for OH radical formation. Applying this new parameter to field studies conducted at Finnish and German measurement sites it is found capable to predict the occurrence of nucleation events and their seasonal and annual variation indicating a significant role of organics. Extrapolation to possible future conditions of ozone, water vapor and organic concentrations leads to a significant potential increase in nucleation event number

A new parametrization for ambient particle formation over coniferous forests and its potential implications for the future

Atmospheric new particle formation is a general phenomenon observed over coniferous forests. So far nucleation is either parameterised as a function of gaseous sulphuric acid concentration only, which is unable to explain the observed seasonality of nucleation events at different measurement sites, or as a function of sulphuric acid and organic molecules. Here we introduce different nucleation parameters based on the interaction of sulphuric acid and terpene oxidation products and elucidate the individual importance. They include basic trace gas and meteorological measurements such as ozone and water vapour concentrations, temperature (for terpene emission) and UV B radiation as a proxy for OH radical formation. We apply these new parameters to field studies conducted at conducted at Finnish and German measurement sites and compare these to nucleation observations on a daily and annual scale. General agreement was found, although the specific compounds responsible for the nucleation process remain speculative. This can be interpreted as follows: During cooler seasons the emission of biogenic terpenes and the OH availability limits the new particle formation while towards warmer seasons the ratio of ozone and water vapour concentration seems to dominate the general behaviour. Therefore, organics seem to support ambient nucleation besides sulphuric acid or an OH-related compound. Using these nucleation parameters to extrapolate the current conditions to prognosed future concentrations of ozone, water vapour and organic concentrations leads to a significant potential increase in the nucleation event number

On-line field measurements of BVOC emissions from Norway spruce (Picea abies) at the hemiboreal SMEAR-Estonia site under autumn conditions

We investigated biogenic volatile organic compound (BVOC) emissions from a Norway spruce (Picea abies) in a hemiboreal mixed forest in autumn. Measurements were performed at the SMEAR-Estonia forest station, using PTR-MS techniques and a dynamic branch enclosure system. Parallel to BVOC measurements, atmospheric (CO2, CH4, H2O, CO) and meteorological (temperature, relative humidity, global radiation, wind speed, precipitation) parameters were monitored in the ambient atmosphere and inside the enclosure (temperature, relative humidity, ozone). Prior to the measuring period, a new inlet line consisting of 19.4 m of heated and isolated glass tube was constructed. The new inlet system allowed the on-line detection and calculation of sesquiterpene (SQT) emission rates for the first time for a hemiboreal forest site. In total, 12 atmospherically relevant BVOCs were continuously monitored during the measurement campaign and the emission rates of terpenoid species and predominant oxygenated VOCs were estimated, with monoterpenes to be emitted the most, followed by acetone, acetaldehyde and sesquiterpenes

Nocturnal nitrogen oxides at a rural mountain-site in south-western Germany

A new, two-channel instrument for simultaneous NO3 and N2O5 monitoring was used to make the first comprehensive set of nocturnal NOx measurements (NO, NO2, NO3 and N2O5) at the Taunus Observatory, a rural mountain site (Kleiner Feldberg) in South-western Germany. In May 2008, NO3 and N2O5 mixing ratios were well above the instrumental detection limit (a few ppt) on all nights of the campaign and were characterised by large variability. The concentrations of NO3, N2O5 and NO2 were consistent with the equilibrium constant, K2, defining the rates of formation and thermal dissociation of N2O5. A steady-state lifetime analysis is consistent with the loss of nocturnal NOx being dominated by the reaction of NO3 with volatile organic compounds in this forested region, with N2O5 uptake to aerosols of secondary importance. Analysis of a limited dataset obtained at high relative humidity indicated that the loss of N2O5 by reaction with water vapour is less efficient (>factor 3) than derived using laboratory kinetic data. The fraction of NOx present as NO3 and N2O5 reached ~20% on some nights, with night-time losses of NOx competing with daytime losses

Understanding the formation of biogenic secondary organic aerosol formation from alpha-pinene in smog chamber studies: role of organic peroxy radicals

Non peer reviewe

Explaining global surface aerosol number concentrations in terms of primary emissions and particle formation

We use observations of total particle number concentration at 36 worldwide sites and a global aerosol model to quantify the primary and secondary sources of particle number. We show that emissions of primary particles can reasonably reproduce the spatial pattern of observed condensation nuclei (CN) (R2=0.51) but fail to explain the observed seasonal cycle at many sites (R2=0.1). The modeled CN concentration in the free troposphere is biased low (normalised mean bias, NMB=&#8722;88%) unless a secondary source of particles is included, for example from binary homogeneous nucleation of sulfuric acid and water (NMB=&#8722;25%). Simulated CN concentrations in the continental boundary layer (BL) are also biased low (NMB=&#8722;74%) unless the number emission of anthropogenic primary particles is increased or an empirical BL particle formation mechanism based on sulfuric acid is used. We find that the seasonal CN cycle observed at continental BL sites is better simulated by including a BL particle formation mechanism (R2=0.3) than by increasing the number emission from primary anthropogenic sources (R2=0.18). Using sensitivity tests we derive optimum rate coefficients for this nucleation mechanism, which agree with values derived from detailed case studies at individual sites