Ambient biogenic hydrocarbons and isoprene emissions from a mixed deciduous forest

Abstract

Experiments were conducted during the growing season of 1993 at a mixed deciduous forest in southern Ontario, Canada to investigate the atmospheric abundance of hydrocarbons from phytogenic origins, and to measure emission rates from foliage of deciduous trees. The most abundant phytogenic chemical species found in the ambient air were isoprene and the monoterpenes a-pinene and/3-pinene. Prior to leaf-bud break during spring, ambient hydrocarbon mixing ratios above the forest remained barely above instrument detection limit (~20 parts per trillion), but they became abundant during the latter part of the growing season. Peak isoprene mixing ratios reached nearly 10 parts per billion (ppbv) during mid-growing season while maximum monoterpene mixing ratios were close to 2 ppbv. Both isoprene and monoterpene mixing ratios exhibited marked diurnal variations. Typical isoprene mixing ratios were highest during mid-afternoon and were lowest during nighttime. Peak isoprene mixing ratios coincided with maximum canopy temperature. The diurnal pattern of ambient isoprene mixing ratio was closely linked to the local emissions from foliage. Isoprene emission rates from foliage were measured by enclosing branches of trees inside environmentcontrolled cuvette systems and measuring the gas mixing ratio difference between cuvette inlet and outlet airstream. Isoprene emissions depended on tree species, foliage ontogeny, and environmental factors such as foliage temperature and intercepted photosynthetically active radiation (PAR). For instance, young (< 1 month old) aspen leaves released approximately 80 times less isoprene than mature (> 3 months old) leaves. During the latter part of the growing season the amount of carbon released back to the atmosphere as isoprene by big-tooth and trembling aspen leaves accounted for approximately 2% of the photosynthetically fixed carbon. Significant isoprene mixing ratio gradients existed between the forest crown and at twice canopy height above the ground. The gradient diffusion approach coupled with similarity theory was used to estimate canopy isoprene flux densities. These canopy fluxes compared favorably with values obtained from a multilayered canopy model that utilized locally measured plant microclimate, biomass distribution and leaf isoprene emission rate data. Modeled isoprene fluxes were approximately 30% higher compared to measured fluxes. Further comparisons between measured and modeled canopy biogenic hydrocarbon flux densities are required to assess uncertainties in modeling systems that provide inventories of biogenic hydrocarbons.

Observations of HNO&lt;sub&gt;3&lt;/sub&gt;, ΣAN, ΣPN and NO&lt;sub&gt;2&lt;/sub&gt; fluxes: evidence for rapid HO&lt;sub&gt;x&lt;/sub&gt; chemistry within a pine forest canopy

“…We estimate the gradients in NO x , PNs, ANs and HNO 3 from the chemical fluxes using three methods: 1) assuming similarity to CO 2 fluxes and gradients, 2) using Monin-Obukhov similarity theory, as described in detail by Fuentes et al (1996) and 3.) using the surface renewal model described by Holzinger et al (2005b).…”

“…These systems can exhibit chemical flux divergence depending on the timescale of turbulence relative to the timescale of chemistry (Fitzjarrald and Lenschow, 1983;Brost et al, 1988;Kramm et al, 1995;Rummel et al, 2002;Nemitz et al, 2004). Prior models of reactive within-canopy chemistry and the associated trace gas fluxes typically calculate or assume OH in the range 5×10 4 to 1×10 6 molec cm −3 within the forest canopy, which is indeed low enough that there is little effect of OH within the canopy on fluxes (Kramm et al, 1991;Gao et al, 1993;Fuentes et al, 1996;Stroud et al, 2005;Forkel et al, 2006;Rinne et al, 2007). These concentrations are much lower than those in the boundary layer above the canopy because the shade of the canopy reduces sunlight that initiates OH production through photochemistry.…”

Disjunct eddy covariance technique for trace gas flux measurements

“…Its use has been restricted to relatively few atmospheric compounds due to the requirement of fast response sensors. Other micrometeorological methods for obtaining data on the trace gas exchange between atmosphere and surfaces include gradient and eddy accumulation techniques [Fuentes et al, 1996;Desjardins, 1977;Businger and Oncley, 1990]. These are however more indirect, i.e., they are based on empirical parameterizations, except the true eddy accumulation method.…”

A Biomass-Based Model to Estimate the Plausibility of Exoplanet Biosignature Gases

“…The biomass model estimates are intended to be a step towards a more general framework for biosignature gases, enabling the move beyond the dominant terracentric gases. We hope this new approach will help ensure that out of the handful of anticpated potentially habitable worlds suitable for followup spectral observations, we can broaden our chances to identify an inhab- Moore et al (1996); (2) Morrison & Hines (1990); (7) Fuentes et al (1996); (8) Nykanen et al (1995 …”