PNNL

Abstract

Most vascular plants species, especially trees, emit biogenic volatile organic compounds (BVOC). Global estimates of BVOC emissions from plants range from 1 to 1.5 Pg C yr-1.1 Mediterranean forest trees have been described as high BVOC emitters, with emission depending primarily on light and temperature, and therefore being promoted by the warm Mediterranean climate. In the presence of sufficient sunlight and nitrogen oxides (NOx), the oxidation of BVOCs can lead to the formation of tropospheric ozone, a greenhouse gas with detrimental effects on plant health, crop yields, and human health. BVOCs are also precursors for aerosol formation, accounting for a significant fraction of secondary organic aerosol (SOA) produced in the atmosphere. The presidential Estate of Castelporziano covers an area of about 6000 ha located 25 km SW from the center of Rome, Italy (Figure 1) and hosts representative forest ecosystems typical of Mediterranean areas: holm oak forests, pine forests, dune vegetation, mixed oak and pine forests. Between 1995 and 2011, three intensive field campaigns were carried out on Mediterranean-type ecosystems inside the Estate. These campaigns were aimed at measuring BVOC emissions and environmental parameters, to improve formulation of basal emission factors (BEFs), that is, standardized emissions at 30 °C and 1000 µmol m-2s-1 of photosynthetic active radiation (PAR). BEFs are key input parameters of emission models. The first campaign in Castelporziano was a pioneering integrated study on biogenic emissions (1993- 19964). BVOC fluxes from different forest ecosystems were mainly investigated using plant- and leaf enclosures connected to adsorption tubes followed by GC-MS analysis in the laboratory. This allowed a first screening of Mediterranean species with respect to their BVOC emission potential, environmental control, and emission algorithms. In particular, deciduous oak species revealed high isoprene emissions (Quercus f rainetto, Quercus petrea, Quercus pubescens), while evergreen oaks emitted monoterpenes only, for example, Quercus ilex = holm oak. Differences in constitutive emission patterns discovered in Castelporziano supplied basic information to discriminate oak biodiversity in following studies.Ten years later, a second experimental campaign took place in spring and summer 2007 on a dune-shrubland experimental site. In this campaign, the use of a proton transfer reaction mass spectrometer (PTR-MS14) provided the fast BVOC observations necessary for quasi-real-time flux measurements using Disjunct Eddy Covariance. This allowed for the first time continuous measurements and BEFs calculation at canopy level. Finally, in September 2011 a third campaign was performed with the aim of further characterizing and improving estimates of BVOC fluxes from mixed Mediterranean forests dominated by a mixed holm oak and stone pine forest, using for the first time a proton transfer reaction-time-of-flight-mass spectrometer (PTR-TOF-MS). In contrast to the standard quadrupole PTR-MS, which can only measure one m/z ratio at a discrete time, thus being inadequate to quantify fluxes of more than a handful of compounds simultaneously, PTR-TOF-MS allowed simultaneous measurements (10 Hz) of fluxes of all BVOCs at the canopy level by Eddy Covariance.17-20, 50 In this work, we reviewed BEFs from previous campaigns in Castelporziano and calculated new BEFs from the campaign based on PTR-TOF-MS analysis. The new BEFs were used to parametrize the model of emissions of gases and aerosols from nature (MEGAN v2.11).