Volatile Organic Compounds Flux Measurements Research Articles

Gap-filling strategies for annual VOC flux data sets

Abstract. Up to now the limited knowledge about the exchange of volatile organic compounds (VOCs) between the biosphere and the atmosphere is one of the factors which hinders more accurate climate predictions. Complete long-term flux data sets of several VOCs to quantify the annual exchange and validate recent VOC models are basically not available. In combination with long-term VOC flux measurements the application of gap-filling routines is inevitable in order to replace missing data and make an important step towards a better understanding of the VOC ecosystem–atmosphere exchange on longer timescales. We performed VOC flux measurements above a mountain meadow in Austria during two complete growing seasons (from snowmelt in spring to snow reestablishment in late autumn) and used this data set to test the performance of four different gap-filling routines, mean diurnal variation (MDV), mean gliding window (MGW), look-up tables (LUT) and linear interpolation (LIP), in terms of their ability to replace missing flux data in order to obtain reliable VOC sums. According to our findings the MDV routine was outstanding with regard to the minimization of the gap-filling error for both years and all quantified VOCs. The other gap-filling routines, which performed gap-filling on 24 h average values, introduced considerably larger uncertainties. The error which was introduced by the application of the different filling routines increased linearly with the number of data gaps. Although average VOC fluxes measured during the winter period (complete snow coverage) were close to zero, these were highly variable and the filling of the winter period resulted in considerably higher uncertainties compared to the application of gap-filling during the measurement period. The annual patterns of the overall cumulative fluxes for the quantified VOCs showed a completely different behaviour in 2009, which was an exceptional year due to the occurrence of a severe hailstorm, compared to 2011. Methanol was the compound which, at 381.5 mg C m−2 and 449.9 mg C m−2, contributed most to the cumulative VOC carbon emissions in 2009 and 2011, respectively. In contrast to methanol emissions, however, considerable amounts of monoterpenes (−327.3 mg C m−2) were deposited onto the mountain meadow during 2009 caused by a hailstorm. Other quantified VOCs had considerably lower influences on the annual patterns.

Standardization of flux chamber and wind tunnel flux measurements for quantifying volatile organic compound and ammonia emissions from area sources at animal feeding operations

A variety of portable wind tunnels and flux chambers have been used to measure fluxes of volatile organic compounds (VOC) and ammonia (NH3) at animal feeding operations (AFO). However, there has been little regard to the extreme variation and potential inaccuracies caused by air velocity or sweep air flow rates that are either too low or too high to simulate field conditions. There is a need for correction factors to standardize flux chamber and wind tunnel measurements. In this manuscript, we present results of water evaporative flux and VOC flux measurements with the EPA flux chamber and a small wind tunnel. In the EPA flux chamber, water evaporative flux was positively correlated with sweep air flow rate (SAFR) between 1 and 20 L min−1 (r2 = 0.981–0.999) and negatively correlated with sweep air relative humidity between 0 and 80% (r2 = 0.982–0.992). Emissions of gas-film controlled compounds like NH3 and VOC at AFOs were positively correlated with evaporation rates between 0.6 and 2.8 mm d−1. We demonstrate a simple methodology for standardizing and comparing different chamber types by measuring water evaporation within the chamber using a gravimetric mass balance approach under controlled laboratory conditions. A water evaporative flux ratio correction factor (EFRCF) was used to improve the accuracy of field-measured VOC and NH3 chamber flux measurements. In a field study, both the EPA flux chamber (SAFR = 5 L min−1) and small wind tunnel (SAFR = 1 L min−1) underestimated the true field emissions of VOC, with EFRCFs of 2.42 and 3.84, respectively. EFRCFs are recommended for all but the driest of soil and manure conditions.

Cut‐induced VOC emissions from agricultural grasslands

The introduction of proton transfer reaction mass spectrometry (PTR-MS) for fast response measurements of volatile organic compounds (VOC) has enabled the use of eddy covariance methods to investigate VOC fluxes on the ecosystem scale. In this study PTR-MS flux measurements of VOC were performed over agricultural grassland during and after a cut event. Selected masses detected by the PTR-MS showed fluxes of methanol, acetaldehyde, and acetone. They were highest directly after cutting and during the hay drying phase. Simultaneously, significant fluxes of protonated ion masses 73, 81, and 83 were observed. Due to the limited identification of compounds with the PTR-MS technique, GC-MS and GC-FID-PTR-MS techniques were additionally applied. In this way, ion mass 73 could be identified as 2-butanone, mass 81 mainly as (Z)-3-hexenal, and mass 83 mainly as the sum of (Z)-3-hexenol and hexenyl acetates. Hexenal, hexenols, and the hexenyl acetates are mostly related to plant wounding during cutting. It was found that legume plants and forbs emit a higher number of different VOC species than graminoids.

Design and field application of an automated cartridge sampler for VOC concentration and flux measurements

One of the major limitations in advancing the understanding of tropospheric ozone and aerosol generation and developing strategies for their control is the technical ability to accurately measure volatile organic compounds (VOCs). This paper describes the design of a constant flow VOC sampler. The versatile sampler can be used for fully automated concentration and flux measurements of VOCs. The sampler incorporates a microprocessor control unit and provides highly accurate mass flow control and significant ease of operation. Sampling sequences can be programmed directly or by remote control through a PC. All important operational parameters necessary for a complete sampling audit trail are logged. Compact weatherproof housings and low power consumption allow operation at remote sites and locations which are sensitive to disturbances or have restricted access. Inner wetted surfaces of the sampler are constructed from non-contaminating materials that do not sorb or emit VOC, and thus permit the collection of representative samples even in environments with very low VOC concentrations. The cartridge magazine provides a maximum of 20 sequential cartridge samples, which allows for long-term air quality assessments. In the dual channel mode, two samples can be collected simultaneously through two independent sample loops, providing ten sequential sample pairs. This design allows the parallel collection of (a) quality assurance backup samples, (b) samples on two different types of cartridges/sorbents to allow a variety of analyses, or (c) differential samples for flux measurements using enclosure, aerodynamic profile, or relaxed eddy accumulation (REA) methods. Field applications including airplane profile measurements above a tropical rainforest area, as well as gradient and REA measurements over a mid-latitude mixed forest stand are described, and demonstrate the validity and flexibility of the system. In particular, the application of the VOC sampler as an integrated part of a REA system is emphasized.

Aircraft-based volatile organic compounds flux measurements with relaxed eddy accumulation

Regional estimates of fluxes of volatile organic compounds (VOCs) are required to improve our understanding of their role in the chemistry of the atmosphere. Flux measurements on such a scale can best be obtained using aircraft-based systems. These systems usually rely on the eddy covariance technique, which requires fast response gas sensors for flux measurement, but such sensors are not available for most organic compounds, therefore, the relaxed eddy-accumulation (REA) technique was selected. An aircraft-based REA sampling system was developed and used to measure isoprene emission over the boreal forest during the 1996 summer. Over a short period in July at the Boreal Ecosystem/Atmosphere Study (BOREAS) southern study area (SSA), the isoprene fluxes ranged from −0.06 to 1.79 μg m -2 s -1, with a mean of 0.59±0.34 μg m -2 s -1, while in August at the BOREAS northern study area (NSA) the isoprene fluxes ranged from 0.00 to 0.26 μg m -2 s -1, with a mean of 0.14±0.09 μg m -2 s -1. In the SSA, the isoprene fluxes over aspen ranged from 0.44 to 1.79 μg m -2 s -1, with a mean of 0.92±0.33 μg m -2 s -1, whereas over black spruce, isoprene fluxes ranged from −0.06 to 0.54 μg m -2 s -1, with a mean of 0.36±0.21 μg m -2 s -1. The isoprene fluxes were exponentially correlated with solar radiation and radiative surface temperature. High correlations between isoprene fluxes and the fluxes of CO 2 and latent heat were also observed. Carbon lost through isoprene emissions was about 0.7 and 0.8% of the CO 2 assimilation rate for aspen and black spruce, respectively. The results demonstrate that the aircraft-based relaxed eddy-accumulation technique is a promising approach for quantifying the atmosphere–surface exchange of VOCs on a regional scale.