Abstract
Abstract. We present OH reactivity measurements using the comparative reactivity method with a branch enclosure technique for four different tree species (red oak, white pine, beech and red maple) in the UMBS PROPHET tower footprint during the Community Atmosphere Biosphere INteraction EXperiment (CABINEX) field campaign in July of 2009. Proton Transfer Reaction-Mass Spectrometry (PTR-MS) was sequentially used as a detector for OH reactivity and BVOC concentrations including isoprene and monoterpenes (MT) for enclosure air. Therefore, the measurement dataset contains both measured and calculated OH reactivity from well-known BVOC. The results indicate that isoprene and MT, and in one case a sesquiterpene, can account for the measured OH reactivity. Significant discrepancy between measured OH reactivity and calculated OH reactivity from isoprene and MT is found for the red maple enclosure dataset but it can be reconciled by adding reactivity from emission of a sesquiterpene, α-farnesene, detected by GC-MS. This leads us to conclude that no significant unknown BVOC emission contributed to ambient OH reactivity from these trees at least during the study period. However, this conclusion should be followed up by more comprehensive side-by-side intercomparison between measured and calculated OH reactivity and laboratory experiments with controlled temperature and light environments to verify effects of those essential parameters towards unknown/unmeasured reactive BVOC emissions. This conclusion leads us to explore the contribution towards ambient OH reactivity (the dominant OH sink in this ecosystem) oxidation products such as hydroxyacetone, glyoxal, methylglyoxal and C4 and C5-hydroxycarbonyl using recently published isoprene oxidation mechanisms (Mainz Isoprene Mechanism II and Leuven Isoprene Mechanism). Evaluation of conventionally unmeasured first generation oxidation products of isoprene and their possible contribution to ambient missing OH reactivity indicates that the ratio of OH reactivity from unmeasured products over OH reactivity from MVK + MACR is strongly dependent on NO concentrations. The unmeasured oxidation products can contribute ~7.2 % (8.8 % from LIM and 5.6 % by MIM 2 when NO = 100 pptv) of the isoprene contribution towards total ambient OH reactivity. This amount can explain ~8.0 % (9.7 % from LIM and 6.2 % from MIM 2) of missing OH reactivity, reported by Di Carlo et al. (2004) at the same site. Further study on the contribution from further generation of unmeasured oxidation products is needed to constrain tropospheric photochemical reactivity of BVOC that have important implications for both photochemical ozone and secondary organic aerosol formation.
Highlights
Total OH reactivity is defined as the quantity of total atmospheric constituents that can react with OH
Existence of unmeasured BVOC emission is supported by evidence such as: (1) temperature dependence of missing OH reactivity closely following the temperature dependence of terpenoid emission (Di Carlo et al, 2004) (2) high sesquiterpene emissions from some ecosystems that are very reactive and have not been constrained by measurements (BouvierBrown et al, 2009) and (3) total MT concentrations, measured by the gas-chromatography technique (GC-FID) being consistently lower than those measured by proton transfer reaction mass spectrometry (PTR-MS; Lee et al, 2005)
A 5 ◦C running averaged measured and calculated OH reactivity with colorcoding of photosynthetically active radiation (PAR) on the plots clearly indicate that the difference of the measured and calculated OH reactivity in each 5 ◦C temperature bin can be explained by the differences in PAR. This comparison demonstrated the good agreement between measured and calculated OH reactivity. These results suggest that there is no significant missing OH reactivity associated with primary BVOC emission for red oak and white pine, which are the two main tree species dominating isoprene and MT emissions, respectively, in this ecosystem
Summary
Considering the dominance of BVOC emission relative to anthropogenic VOC emission (ten times higher; Goldstein and Galbally, 2007), this uncertainty can be a potential problem in understanding photochemical ozone and secondary organic aerosol (SOA) formation in the atmosphere, two important radiative forcing agents controlling global climate (IPCC, 2007) For this reason, a number of studies have tried to identify the sources of unexplored reactive compounds in BVOC dominated regions. There is a better agreement between sensitivity-corrected PTR-MS mass spectra and speciated BVOC observations by GC for branch enclosure air than the same comparisons with ambient air inside of a ponderosa pine forest canopy (Kim et al, 2010) These studies results strongly suggest that the source of unknown reactive species in the atmosphere, in some of these locations, is poorly characterized oxidation products rather than unconstrained emitted species. We present calculations using two up-to-date isoprene oxidation mechanisms (Mainz Isoprene Mechanism 2 (MIM2) Taraborrelli et al, 2009 and Leuven Isoprene Mechanism (LIM) Stavrakou et al, 2010) to assess possible contributions of unmeasured isoprene oxidation products towards total OH reactivity
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