A disjunct eddy accumulation system for the measurement of BVOC fluxes: instrument characterizations and field deployment

Abstract

Abstract. Biological volatile organic compounds (BVOCs), such as isoprene and monoterpenes, are emitted in large amounts from forests. Quantification of the flux of BVOCs is critical in the evaluation of the impact of these compounds on the concentrations of atmospheric oxidants and on the production of secondary organic aerosol. A disjunct eddy accumulation (DEA) sampler system was constructed for the measurement of speciated BVOC fluxes. Unlike traditional eddy covariance (EC), the relatively new technique of disjunct sampling differs by taking short, discrete samples that allow for slower sampling frequencies. Disjunct sample airflow is directed into cartridges containing sorbent materials at sampling rates proportional to the magnitude of the vertical wind. Compounds accumulated on the cartridges are then quantified by thermal desorption and gas chromatography. Herein, we describe our initial tests to evaluate the disjunct sampler including the application of vertical wind measurements to create optimized sampling thresholds. Measurements of BVOC fluxes obtained from DEA during its deployment above a mixed hardwood forest at the University of Michigan Biological Station (Pellston, MI) during the 2009 CABINEX field campaign are reported. Daytime (09:00 a.m. to 05:00 p.m. LT) isoprene fluxes, when averaged over the footprint of the tower, were 1.31 mg m −2 h −1 which are comparable to previous flux measurements at this location. Speciated monoterpene fluxes are some of the first to be reported from this site. Daytime averages were 26.7 µg m −2 h −1 for α-pinene and 10.6 µg m −2 h −1 for β-pinene. These measured concentrations and fluxes were compared to the output of an atmospheric chemistry model, and were found to be consistent with our knowledge of the variables that control BVOCs fluxes at this site.

Contributions of individual reactive biogenic volatile organic compounds to organic nitrates above a mixed forest

“…Modeled mid-day canopy-scale emission rates of total MTs and SQTs (0.23 [0.04-0.61] mg m −2 h −1 and 0.03 [0.001-0.09] mg m −2 h −1 , respectively) were close to previous estimates (0.21 ± 0.06 mg m −2 h −1 and 0.10 ± 0.05 mg m −2 h −1 , respectively) based on PTR-MS measurements at UMBS (Kim et al, 2009). Considering uncertainties and variability, modeled α-and β-pinene emission rates were also in agreement with measured fluxes at UMBS using a disjunct eddy accumulation system (Edwards et al, 2012); these new speciated MT flux measurements by Edwards et al (2012) will help constrain emissions in future modeling efforts. Comparison between measured (PTR-LIT) and base modeled total MT concentrations ranged from agreement to an under-prediction of up to a factor of ∼ 3; however, these values were still within the range of measured variability (Fig.…”

“…BVOC emission rates are difficult to determine due to tree-to-tree variability and sampling challenges during branch enclosure BVOC emission measurements (e.g., Ortega et al, 2008), leading to large ranges in estimated production rates, shown here to result in a range of oxidation product concentrations. In the future, improved canopy-level flux measurements of speciated monoterpenes, such as those recently completed by Edwards et al (2012) at UMBS, will improve modeling of individual monoterpenes at the canopy level.…”

Bidirectional Ecosystem–Atmosphere Fluxes of Volatile Organic Compounds Across the Mass Spectrum: How Many Matter?

“…The ∼23 m forest includes bigtooth aspen, quaking aspen, and paper birch (upper canopy); white pine, red maple, and American beech (lower canopy); and red oak (both upper and lower canopy). , There are minimal sources of pollution in the surrounding area (median daytime [10:00–20:00 EDT] NO during PROPHET-AMOS was 23 ppt). There has been a long history of atmospheric measurements at the tower since its construction in 1997, including studies focusing on individual VOCs or classes of VOCs such as isoprene and monoterpenes, − sesquiterpenes, oxygenated VOCs, ,− and organic nitrates. , …”

“…Observed fluxes for these species are also within the range of what has been observed previously at PROPHET. 14,21 We see in Figure S5 that the GEOS-Chem simulation underestimates isoprene emissions by 30−40% around mid-day, even after correcting the model temperature fields as described in Section 2.5. The decline from the midday peak also occurs too soon in the model, so that a much larger underestimate is seen in the afternoon.…”