[1] Isoprene, monoterpene, and other volatile organic compound (VOC) emissions from grasslands, shrublands, forests, and peatlands in China were characterized to estimate their regional magnitudes and to compare these emissions with those from landscapes of North America, Europe, and Africa. Ecological and VOC emission sampling was conducted at 52 sites centered in and around major research stations located in seven different regions of China: Inner Mongolia (temperate), Changbai Mountain (boreal-temperate), Beijing Mountain (temperate), Dinghu Mountain (subtropical), Ailao Mountain (subtropical), Kunming (subtropical), and Xishuangbanna (tropical). Transects were used to sample plant species and growth form composition, leafy (green) biomass, and leaf area in forests representing nearly all the major forest types of China. Leafy biomass was determined using generic algorithms based on tree diameter, canopy structure, and absolute cover. Measurements of VOC emissions were made on 386 of the 541 recorded species using a portable photo-ionization detector method. For 105 species, VOC emissions were also measured using a flow-through leaf cuvette sampling/gas chromatography analysis method. Results indicate that isoprene and monoterpene emissions, as well as leafy biomass, vary systematically along gradients of ecological succession in the same manner found in previous studies in the United States, Canada, and Africa. Applying these results to a regional VOC emissions model, we arrive at a value of 21 Tg C for total annual biogenic VOC emissions from China, compared to 5 Tg C of VOCs released annually from anthropogenic sources there. The isoprene and monoterpene emissions are nearly the same as those reported for Europe, which is comparable in size to China.
[1] A one-dimensional canopy model was used to quantify the impact of photochemistry in modifying biosphere-atmosphere exchange of trace gases. Canopy escape efficiencies, defined as the fraction of emission that escapes into the well-mixed boundary layer, were calculated for reactive terpene species. The modeled processes of emission, photochemistry, diffusive transport, and deposition were highly constrained based on intensive observations collected in a Loblolly Pine plantation at Duke Forest, North Carolina, during the CELTIC field study. Canopy top fluxes for isoprene and a,b-pinene were not significantly altered by photochemistry as calculated escape efficiencies were greater than 0.90 for both species. b-caryophyllene emission and photochemistry were added to the canopy model as a surrogate for the reactive sesquiterpene class of species. b-caryopyllene escape efficiencies of 0.30 were calculated for midday summertime conditions. Urbanization scenarios were also performed to assess the impact of pollution on modifying biosphere-atmosphere exchange. Modest changes in escape efficiencies were calculated for a wide range of anthropogenic hydrocarbon and NO x mixing ratios suggesting a simple parameterization of escape efficiency in terms of grid cell NO x may be possible for incorporating the impact of canopy scale photochemistry within biogenic emission processing systems for regional air quality and climate models. The inferred magnitude of sesquiterpene ozonolysis reactions has important implications on both daytime and nighttime radical formation in the canopy.
Abstract. Using new in-situ field observations of the most abundant oxygenated VOCs (methanol, acetaldehyde, acetone, C 3 /C 4 carbonyls, MVK+MAC and acetic acid) we were able to constrain emission and deposition patterns above and within a loblolly pine (Pinus taeda) plantation with a sweetgum (Liquidambar styraciflua) understory. During the day canopy scale measurements showed significant emission of methanol and acetone, while methyl vinyl ketone and methacrolein, acetaldehyde and acetic acid were mainly deposited during the day. All oxygenated compounds exhibited strong losses during the night that could not be explained by conventional dry deposition parameterizations. Accompanying leaf level measurements indicated substantial methanol and acetone emissions from loblolly pine. The exchange of acetaldehyde was more complex. Laboratory measurements made on loblolly pine needles indicated that acetaldehyde may be either emitted or taken up depending on ambient concentrations, with the compensation point increasing exponentially with temperature, and that mature needles tended to emit more acetaldehyde than younger needles. Canopy scale measurements suggested mostly deposition. Short-term (approx. 2 h) ozone fumigation in the laboratory had no detectable impact on post-exposure emissions of methanol and acetone, but decreased the exchange rates of acetaldehyde. The emission of a variety of oxygenated compounds (e.g. carbonyls and alcohols) was triggered or significantly enhanced during laboratory ozone fumigation experiments. These results suggest that higher ambient ozone levels in the future might enhance the biogenic contribution of some oxygenated compounds. Those with sufficiently low vapor pressures may potentially influence secondary organic aerosol growth. Compounds recently hypothesized to be primarily produced in the canopy atmosphere via ozone plus terpenoid-type reactions can also originate from the oxidaCorrespondence to: T. Karl (tomkarl@ucar.edu) tion reaction of ozone with leaf surfaces and inside the leaf. This needs to be taken into account when scaling up very reactive biogenic compounds.
Twenty common plant species were screened for emissions of biogenic volatile organic compounds (BVOCs) at a lowland tropical wet forest site in Costa Rica. Ten of the species examined emitted substantial quantities of isoprene. These species accounted for 35-50% of the total basal area of old-growth forest on the major edaphic site types, indicating that a high proportion of the canopy leaf area is a source of isoprene. A limited number of canopy-level BVOC flux measurements were also collected by relaxed eddy accumulation (REA). These measurements verify that the forest canopy in this region is indeed a significant source of isoprene. In addition, REA fluxes of methanol and especially acetone were also significant, exceeding model estimates and warranting future investigation at this site. Leaf monoterpene emissions were non-detectable or very low from the species surveyed, and ambient concentrations and REA fluxes likewise were very low. Although the isoprene emission rates reported here are largely consistent with phylogenetic relations found in other studies (at the family, genus, and species levels), two species in the family Mimosaceae, a group previously found to consist largely of non-isoprene emitters, emitted significant quantities of isoprene. One of these, Pentaclethra macroloba (Willd.) Kuntze, is by far the most abundant canopy tree species in the forests of this area, composing 30-40% of the total basal area. The other, Zygia longifolia (Humb. & Bonpl.) Britton & Rose is a common riparian species. Our results suggest that the source strength of BVOCs is important not only to tropical atmospheric chemistry, but also may be important in determining net ecosystem carbon exchange. Published by
The wounding and drying of plant material during crop harvest could be a significant source of volatile organic compounds (VOCs) that enter the atmosphere. Here, we show that these primarily oxygenated VOCs can be measured using proton-transfer chemical-ionization mass spectrometry (PT-CIMS), a method that allows online and simultaneous monitoring of oxygenated VOC levels. For clover, alfalfa, and corn, leaf wounding and in particular drying were shown to lead to strongly enhanced emissions of a series of C6 aldehydes, alcohols, and esters derived from (Z)-3-hexenal. Additionally, for the forage crops clover and alfalfa, enhanced emissions of methanol, acetaldehyde, acetone, and butanone were observed. The identities of the measured carbonyl compounds were confirmed using high-pressure liquid chromatography. For clover, initial cutting led to a VOC release of about 175 μg of C (g dry wt)-1, while during drying the cut clover released >1000 μg of C (g dry wt)-1; qualitatively, similar amounts of VOCs were released from alfalfa, the major hay crop in the United States. The atmospheric implications of these findings may include effects on the local air quality in agricultural areas, contributions to long-range transport of pollutants, and effects on the formation of HO x (OH + HO2) radicals in the upper troposphere.
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