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Leaf-level isoprene and monoterpene emissions were collected and analyzed from five of the most abundant oak (Quercus) species in Central Missouri's Ozarks Region in 2012 during PINOT NOIR (Particle Investigations at a Northern Ozarks Tower - NOx, Oxidants, Isoprene Research). June measurements, prior to the onset of severe drought, showed isoprene emission rates and leaf temperature responses similar to those previously reported in the literature and used in Biogenic Volatile Organic Compound (BVOC) emission models. During the peak of the drought in August, isoprene emission rates were substantially reduced, and response to temperature was dramatically altered, especially for the species in the red oak subgenus (Erythrobalanus). Quercus stellata (in the white oak subgenus Leucobalanus), on the other hand, increased its isoprene emission rate during August, and showed no decline at high temperatures during June or August, consistent with its high tolerance to drought and adaptation to xeric sites at the prairie-deciduous forest interface. Mid-late October measurements were conducted after soil moisture recharge, but were affected by senescence and cooler temperatures. Isoprene emission rates were considerably lower from all species compared to June and August data. The large differences between the oaks in response to drought emphasizes the need to consider BVOC emissions at the species level instead of just the whole canopy. Monoterpene emissions from Quercus rubra in limited data were highest among the oaks studied, while monoterpene emissions from the other oak species were 80-95% lower and less than assumed in current BVOC emission models. Major monoterpenes from Q. rubra (and in ambient air) were p-cymene, α-pinene, β-pinene, d-limonene, γ-terpinene, β-ocimene (predominantly1,3,7-trans-β-ocimene, but also 1,3,6-trans-β-ocimene), tricyclene, α-terpinene, sabinene, terpinolene, and myrcene. Results are discussed in the context of canopy flux studies conducted at the site during PINOT NOIR, which are described elsewhere. The leaf isoprene emissions before and during the drought were consistent with above canopy fluxes, while leaf and branch monoterpene emissions were an order of magnitude lower than the observed above canopy fluxes, implying that other sources may be contributing substantially to monoterpene fluxes at this site. This strongly demonstrates the need for further simultaneous canopy and enclosure BVOC emission studies., (Published by Elsevier Ltd.)

Considerable amounts and varieties of biogenic volatile organic compounds (BVOCs) are exchanged between vegetation and the surrounding air. These BVOCs play key ecological and atmospheric roles that must be adequately represented for accurately modeling the coupled biosphere-atmosphere-climate earth system. One key uncertainty in existing models is the response of BVOC fluxes to an important global change process: drought. We describe the diurnal and seasonal variation in isoprene, monoterpene, and methanol fluxes from a temperate forest ecosystem before, during, and after an extreme 2012 drought event in the Ozark region of the central USA. BVOC fluxes were dominated by isoprene, which attained high emission rates of up to 35.4 mg m(-2)  h(-1) at midday. Methanol fluxes were characterized by net deposition in the morning, changing to a net emission flux through the rest of the daylight hours. Net flux of CO2 reached its seasonal maximum approximately a month earlier than isoprenoid fluxes, which highlights the differential response of photosynthesis and isoprenoid emissions to progressing drought conditions. Nevertheless, both processes were strongly suppressed under extreme drought, although isoprene fluxes remained relatively high compared to reported fluxes from other ecosystems. Methanol exchange was less affected by drought throughout the season, confirming the complex processes driving biogenic methanol fluxes. The fraction of daytime (7-17 h) assimilated carbon released back to the atmosphere combining the three BVOCs measured was 2% of gross primary productivity (GPP) and 4.9% of net ecosystem exchange (NEE) on average for our whole measurement campaign, while exceeding 5% of GPP and 10% of NEE just before the strongest drought phase. The meganv2.1 model correctly predicted diurnal variations in fluxes driven mainly by light and temperature, although further research is needed to address model BVOC fluxes during drought events., (© 2015 John Wiley & Sons Ltd.)

The Estate of Castelporziano (Rome, Italy) hosts many ecosystems representative of Mediterranean vegetation, especially holm oak and pine forests and dune vegetation. In this work, basal emission factors (BEFs) of biogenic volatile organic compounds (BVOCs) obtained by Eddy Covariance in a field campaign using a proton transfer reaction-time-of-flight-mass spectrometer (PTR-TOF-MS) were compared to BEFs reported in previous studies that could not measure fluxes in real-time. Globally, broadleaf forests are dominated by isoprene emissions, but these Mediterranean ecosystems are dominated by strong monoterpene emitters, as shown by the new BEFs. The original and new BEFs were used to parametrize the model of emissions of gases and aerosols from nature (MEGAN v2.1), and model outputs were compared with measured fluxes. Results showed good agreement between modeled and measured fluxes when a model was used to predict radiative transfer and energy balance across the canopy. We then evaluated whether changes in BVOC emissions can affect the chemistry of the atmosphere and climate at a regional level. MEGAN was run together with the land surface model (community land model, CLM v4.0) of the community earth system model (CESM v1.0). Results highlighted that tropospheric ozone concentration and air temperature predicted from the model are sensitive to the magnitude of BVOC emissions, thus demonstrating the importance of adopting the proper BEF values for model parametrization.

Emissions of volatile organic compounds (VOC) from vegetation are believed to be a major source of secondary organic aerosol (SOA), which in turn comprises a large fraction of fine particulate matter in many areas. Sesquiterpenes are a class of biogenic VOC with high chemical reactivity and SOA yields. Sesquiterpenes have only recently been quantified in emissions from a wide variety of plants. In this study, a new sesquiterpene emission inventory is used to provide input to the Models-3 Community Multiscale Air Quality (CMAQ) model. CMAQ is used to estimate the contribution of sesquiterpenes and monoterpenes to SOA concentrations over the contiguous United States. The gas-particle partitioning module of CMAQ was modified to include condensable products of sesquiterpene oxidation and to update values of the enthalpy of vaporization. The resulting model predicts July monthly average surface concentrations of total SOA in the eastern U.S. ranging from about 0.2-0.8 microg m(-3). This is roughly double the amount of SOA produced in this region when sesquiterpenes are not included. Even with sesquiterpenes included, however, the model significantly underpredicts surface concentrations of particle-phase organic matter compared to observed values. Treating all SOA as capable of undergoing polymerization increases predicted monthly average surface concentrations in July to 0.4-1.2 microg m(-3), in closer agreement with observations. Using the original enthalpy of vaporization value in CMAQ in place of the values estimated from the recent literature results in predicted SOA concentrations of about 0.3-1.3 microg m(-3).

Biogenic volatile organic compounds (BVOC) contribute significantly to the formation of ozone and secondary organic aerosol (SOA). The Model of Emissions of Gases and Aerosols from Nature (MEGANv2.02) is used to estimate emissions of isoprene, monoterpenes (MT), and sesquiterpenes (SQT) across the United States. Compared to the Biogenic Emission Inventory System (BEIS3.0), MEGANv2.02 estimates higher isoprene but lower MT emissions for July 2001 and January 2002. A sensitivity study of SQT and MT emission factors and algorithm parameters was conducted by assigning values to four plant functional types (PFTs) using both recent measurements and literature values. The standard deviations of the emissions factors within these PFTs were two to four times the averages because of the variation in experimental basal emissions rate data. More recently published SQT and MT basal emission rates are generally lower than those reported in the literature through 2004. With the new emissions factors, monthly average SQT emission rates for the contiguous United States are equal to 16% of the MT emissions during July and 9% of the emissions during January. The SQT emissions distribution is strongly influenced by the grass and crop PFT, for which SQT emissions data are quite limited.

Emissions of Biogenic Volatile Organic Compounds (BVOCs) observed during 2007 from an experimental Pinus taeda plantation in Central North Carolina are compared with model estimates from the Model of Emissions of Gases and Aerosols from Nature (MEGAN) version 2.1. Relaxed eddy accumulation (REA) estimates of 2-methyl-3-buten-2-ol (MBO) fluxes are a factor of 3-4 higher than MEGAN estimates. MEGAN monoterpene emission estimates were a factor of approximately two higher than REA flux measurements. MEGAN β-caryophyllene emission estimates were within 60% of growing season REA flux estimates but were several times higher than REA fluxes during cooler, dormant season periods. The sum of other sesquiterpene emissions estimated by MEGAN was several times higher than REA estimates throughout the year. Model components are examined to understand these discrepancies. Measured summertime leaf area index (LAI) (and therefore foliar biomass) is a factor of two higher than assumed in MEGAN for the P. taeda default. Increasing the canopy mean MBO emission factor from 0.35 to 1.0mgm(-2)h(-1) also reduces MEGAN vs. REA flux differences. This increase is within current MBO emission factor uncertainties. The algorithm within MEGAN which adjusts isoprene emission estimates as a function of the temperature and light of the previous 24h seems also to improve the seasonal MEGAN MBO correlation with REA fluxes. Including the effects of the previous 240h, however, seems to degrade temporal model correlation with fluxes. Monoterpene and sesquiterpene composition data from the REA are compared with MEGAN2.1 estimates and also branch enclosure and needle extract data collected at this site. To our knowledge, the flux data presented here are the first reported for MBO and sesquiterpenes from a P. taeda ecosystem.

Oxidation products of monoterpenes and isoprene have a major influence on the global secondary organic aerosol (SOA) burden and the production of atmospheric nanoparticles and cloud condensation nuclei (CCN). Here, we investigate the formation of extremely low volatility organic compounds (ELVOC) from O3 and OH radical oxidation of several monoterpenes and isoprene in a series of laboratory experiments. We show that ELVOC from all precursors are formed within the first minute after the initial attack of an oxidant. We demonstrate that under atmospherically relevant concentrations, species with an endocyclic double bond efficiently produce ELVOC from ozonolysis, whereas the yields from OH radical-initiated reactions are smaller. If the double bond is exocyclic or the compound itself is acyclic, ozonolysis produces less ELVOC and the role of the OH radical-initiated ELVOC formation is increased. Isoprene oxidation produces marginal quantities of ELVOC regardless of the oxidant. Implementing our laboratory findings into a global modeling framework shows that biogenic SOA formation in general, and ELVOC in particular, play crucial roles in atmospheric CCN production. Monoterpene oxidation products enhance atmospheric new particle formation and growth in most continental regions, thereby increasing CCN concentrations, especially at high values of cloud supersaturation. Isoprene-derived SOA tends to suppress atmospheric new particle formation, yet it assists the growth of sub-CCN-size primary particles to CCN. Taking into account compound specific monoterpene emissions has a moderate effect on the modeled global CCN budget.

Terpenoid emissions from ponderosa pine (Pinus ponderosa subsp. scopulorum) were measured in Colorado, USA over two growing seasons to evaluate the role of incident light, needle temperature, and stomatal conductance in controlling emissions of 2-methyl-3-buten-2-ol (MBO) and several monoterpenes. MBO was the dominant daylight terpenoid emission, comprising on average 87% of the total flux, and diurnal variations were largely determined by light and temperature. During daytime, oxygenated monoterpenes (especially linalool) comprised up to 75% of the total monoterpenoid flux from needles. A significant fraction of monoterpenoid emissions was dependent on light and 13CO2 labeling studies confirmed de novo production. Thus, modeling of monoterpenoid emissions required a hybrid model in which a significant fraction of emissions was dependent on both light and temperature, while the remainder was dependent on temperature alone. Experiments in which stomata were forced to close using abscisic acid demonstrated that MBO and a large fraction of the monoterpene flux, presumably linalool, could be limited at the scale of seconds to minutes by stomatal conductance. Using a previously published model of terpenoid emissions, which explicitly accounts for the physicochemical properties of emitted compounds, we were able to simulate these observed stomatal effects, whether induced experimentally or arising under naturally fluctuation conditions of temperature and light. This study shows unequivocally that, under naturally occurring field conditions, de novo light-dependent monoterpenes comprise a significant fraction of emissions in ponderosa pine. Differences between the monoterpene composition of ambient air and needle emissions imply a significant non-needle emission source enriched in Δ-3-carene.

Monoterpene emissions from the dominant understory species Pteridium aquilinum (Bracken fern) in a mixed temperate forest were measured in the field during the summers of 2006, 2007 and 2008. The results showed that Bracken fern emitted monoterpenes at different rates depending if the plants were located in the understory or in open areas. Understory plants emitted monoterpene levels ranging from 0.002 to 13 μgC g dw-1 h -1. Open area plants emitted monoterpene levels ranging from 0.005 to 2.21 μgC g dw-1 h -1. During the summer of 2008 greenhouse studies were performed to complement the field studies. Only 3% of the greenhouse Bracken fern plants emitted substantial amounts of monoterpenes. The average emission, 0.15 μgC g dw-1 h -1 ± 0.9 μgC g dw-1 h -1, was much lower than that observed in the field. The factors controlling monoterpene emissions are not clear, but this study provides evidence of the potential importance of understory vegetation to ecosystem total hydrocarbon emissions and emphasizes the need for longer-term field studies. © 2012 Elsevier Ltd.

Biogenic volatile organic compounds (BVOCs) in the atmosphere react to form ozone and secondary organic aerosols, which deteriorate air quality, affect human health, and indirectly influence global climate changes. The present study aims to provide a preliminary assessment of BVOC emissions in Hong Kong (HKSAR). Thriteen local tree species were measured for their isoprene emission potential. Tree distribution was estimated for country park areas based on field survey data. Plant emission data obtained from measurements and the literature, tree distribution estimation data, land use information, and meteorological data were combined to estimate annual BVOC emissions of 8.6×109 g C for Hong Kong. Isoprene, monoterpenes, and other VOCs contributed about 30%, 40%, and 30% of the estimated total annual emissions, respectively. Although hundreds of plant species are found in Hong Kong country parks, the model results indicate that only 10 tree species contribute about 76% of total annual VOC emissions. Prominent seasonal and diurnal variations in emissions were also predicted by the model. The present study lays a solid foundation for future local research, and results can be applied for studying BVOC emissions in nearby southern China and Asian regions that share similar climate and plant distributions. © 2008 Elsevier Ltd. All rights reserved.

The focus of the studies presented in the preceding companion paper (Part A: Review) and here (Part B: Applications) is on defining representative emission rates from vegetation for determining the roles of biogenic volatile organic compound (BVOC) emissions in atmospheric chemistry and aerosol processes. The review of previously published procedures for identifying and quantifying BVOC emissions has revealed a wide variety of experimental methods used by various researchers. Experimental details become increasingly critical for quantitative emission measurements of low volatility monoterpenes (MT) and sesquiterpenes (SQT). These compounds are prone to be lost inadvertently by uptake to materials in contact with the sample air or by reactions with atmospheric oxidants. These losses become more prominent with higher molecular weight compounds, potentially leading to an underestimation of their emission rates. We present MT and SQT emission rate data from numerous experiments that include 23 deciduous tree species, 14 coniferous tree species, 8 crops, and 2 shrubs. These data indicate total, normalized (30 degrees C) basal emission rates from

Canopy scale fluxes of isoprene and monoterpenes were investigated in both wet and dry seasons above a rubber tree (Hevea brasiliensis)/secondary tropical forest in the Yunnan province of southwestern China. Drought conditions were unusually high during the dry season experiment. The eddy covariance measurement technique was used to measure isoprene fluxes, while monoterpene fluxes were modeled based on leaf level emission measurements. Maximum observed isoprene fluxes occurred during the wet season and daytime average fluxes were about 1 mg C m -2 h -1. Dry season fluxes were much lower with a daytime average of 0.15 mg C m -2 h -1. Wet season isoprene fluxes compare quite well with isoprene fluxes observed from other tropical forests. Monoterpene fluxes came, almost entirely, from Hevea brasiliensis, which is a light-dependent monoterpene emitter. Modeled wet season total monoterpene fluxes were about 2 mg C m -2 h -1 (average for the daytime), and in the dry season were undetectable. Extreme drought conditions, and the drought deciduous nature of Hevea brasiliensis may be the cause of the low dry season fluxes. © 2004 Elsevier Ltd. All rights reserved.

The magnitudes, distributions, controlling processes and uncertainties associated with North American natural emissions of oxidant precursors are reviewed. Natural emissions are responsible for a major portion of the compounds, including non-methane volatile organic compounds (NMVOC), carbon monoxide (CO) and nitric oxide (NO), that determine tropospheric oxidant concentrations. Natural sources include soil microbes, vegetation, biomass burning, and lightning. These sources are strongly influenced by human activities that have led to significant changes in the magnitude and distribution of natural emissions in the past two centuries. The total NMVOC flux of about 84 × 1012 g of carbon (Tg C) is comprised primarily of isoprene (35%), 19 other terpenoid compounds (25%) and 17 non-terpenoid compounds (40%). Vegetation is predicted to contribute about 98% of the total annual natural NMVOC emission. The estimated annual natural NO emission of 2.1 × 1012 g of nitrogen (Tg N) from North America is primarily due to soils and lightning, while the estimated 10 Tg C of CO arises from biomass burning and vegetation. Field measurements of ambient concentrations and above canopy fluxes have validated emission estimates for a few compounds from some important landscapes. The uncertainty associated with natural emission estimates ranges from less than 50% for midday summer isoprene emission from some locations to about a factor of 10 for some compounds and landscapes.

The alteration of climate is driven not only by anthropogenic activities, but also by biosphere processes that change in conjunction with climate. Emission of volatile organic compounds (VOCs) from vegetation may be particularly sensitive to changes in climate and may play an important role in climate forcing through their influence on the atmospheric oxidative balance, greenhouse gas concentration, and the formation of aerosols. Using the VEMAP vegetation database and associated vegetation responses to climate change, this study examined the independent and combined effects of simulated changes in temperature, CO2 concentration, and vegetation distribution on annual emissions of isoprene, monoterpenes, and other reactive VOCs (ORVOCs) from potential vegetation of the continental United States. Temperature effects were modelled according to the direct influence of temperature on enzymatic isoprene production and the vapour pressure of monoterpenes and ORVOCs. The effect of elevated CO2 concentration was modelled according to increases in foliar biomass per unit of emitting surface area. The effects of vegetation distribution reflects simulated changes in species spatial distribution and areal coverage by 21 different vegetation classes. Simulated climate warming associated with a doubled atmospheric CO2 concentration enhanced total modelled VOC emission by 81.8% (isoprene +82.1%, monoterpenes +81.6%, ORVOC +81.1%), whereas a simulated doubled CO2 alone enhanced total modelled VOC emission by only +11.8% (isoprene +13.7%, monoterpenes +4.1%, ORVOC +11.7%). A simulated redistribution of vegetation in response to altered temperatures and precipitation patterns caused total modelled VOC emission to decline by 10.4% (isoprene -11.7%, monoterpenes -18.6%, ORVOC 0.0%) driven by a decline in area covered by vegetation classes emitting VOCs at high rates. Thus, the positive effect of leaf-level adjustments to elevated CO2 (i.e. increases in foliar biomass) is balanced by the negative effect of ecosystem-level adjustments to climate (i.e. decreases in areal coverage of species emitting VOC at high rates).

We present reactive VOC observations over the North Pacific and the Arctic Ocean from airborne and shipborne measurements, investigating, in particular, distributions of biogenic volatile organic compounds that may be emitted from phytoplankton. In contrast to terrestrial observations, isoprene (C5H8), the most dominant BVOC emission from the terrestrial ecosystem, was mostly present under the lower detection limit along with monoterpenes (C10H16), the second largest emission from the terrestrial ecosystem. However, we consistently detected sesquiterpenes (C15H24) over the Arctic Ocean for the two Arctic cruises. The results of the analysis of sorbent cartridge samplings conducted over the Arctic Ocean on Korean icebreaker R/V Araon in 2016 and 2017 illustrate that few tens ppt levels of sesquiterpenes were present over the Arctic Ocean. Moreover, the concentration variation was positively correlated with the quantitative indicators of ocean biological activities, such as chlorophyll-a, dissolved DMS, and the ratio of dissolved O2 and Ar. This suggests that further investigations on sesquiterpene's emission and atmospheric transformation processes over the marine boundary layer are required. [ABSTRACT FROM AUTHOR]

Isoprene and monoterpene emission rates are essential inputs for atmospheric chemistry models that simulate atmospheric oxidant and particle distributions. Process studies of the biochemical and physiological mechanisms controlling these emissions are advancing our understanding and the accuracy of model predictions but efforts to quantify regional emissions have been limited by a lack of constraints on regional distributions of ecosystem emission capacities. We used an airborne wavelet-based eddy covariance measurement technique to characterize isoprene and monoterpene fluxes with high spatial resolution during the 2013 SAS (Southeast Atmosphere Study) in the southeastern United States. The fluxes measured by direct eddy covariance were comparable to emissions independently estimated using an indirect inverse modeling approach. Isoprene emission factors based on the aircraft wavelet flux estimates for high isoprene chemotypes (e.g., oaks) were similar to the MEGAN2.1 biogenic emission model estimates for landscapes dominated by oaks. Aircraft flux measurement estimates for landscapes with fewer isoprene emitting trees (e.g., pine plantations), were about a factor of two lower than MEGAN2.1 model estimates. The tendency for high isoprene emitters in these landscapes to occur in the shaded understory, where light dependent isoprene emissions are diminished, may explain the lower than expected emissions. This result demonstrates the importance of accurately representing the vertical profile of isoprene emitting biomass in biogenic emission models. Airborne measurement-based emission factors for high monoterpene chemotypes agreed with MEGAN2.1 in landscapes dominated by pine (high monoterpene chemotype) trees but were more than a factor of three higher than model estimates for landscapes dominated by oak (relatively low monoterpene emitting) trees. This results suggests that unaccounted processes, such as floral emissions or light dependent monoterpene emissions, or vegetation other than high monoterpene emitting trees may be an important source of monoterpene emissions in those landscapes and should be identified and included in biogenic emission models. [ABSTRACT FROM AUTHOR]

Measurements of BVOC emissions, ozone concentration and environmental parameters were carried out from May 2013 to January 2016 in a subtropical Pinus plantation in China. Isoprene and monoterpene emissions were measured using a relaxed eddy accumulation (REA) system and a gradient technique on an above-canopy tower. In 2013, isoprene comprised 21.2% of total terpenoid emissions, while α-pinene, camphene, β-pinene and limonene constituting 51.5%, 2.4%, 9.1%, and 13.0% of total emissions, respectively. Monoterpenes together were the dominant VOCs measured contributing 71.6%. α-pinene, camphene, β-pinene and limonene constituted 67.7%, 3.2%, 11.9%, 17.2% of total monoterpene emissions. Isoprene and monoterpene emissions displayed strong diurnal variations, with lower emissions in the morning and late evening, and the highest emissions around noon. BVOC peak emissions typically occurred a few hours after the noon PAR peak. Isoprene and monoterpene emissions varied with season and were the highest in summer, contributing more than half of the total annual emission, and the lowest emissions were in winter. Evident interannual variations of isoprene, monoterpenes and total BVOCs were observed. Compared to 2013, annual BVOC emissions decreased in 2015, associated with decreases of PAR, Temperature, water vapor, and an increase of all substances in gas, liquid and solid phases in the atmospheric column (e.g., S/Q, the ratio of solar scattered radiation to global radiation). Ozone concentration showed clear diurnal variation with PAR, higher around noon and lower in the early morning and late evening. Generally, there were no evident correlations between ozone concentrations and BVOC emissions, or the vertical gradients of ozone concentrations and BVOC concentrations. Under all sky conditions (including cloudy skies), no strong correlations at a high confidence level or very similar variation patterns were observed between any two following parameters, BVOC emissions, PAR, temperature, water vapor, and S/Q. The major factors controlling BVOC emissions were PAR and temperature but biomass burning smoke and phenology (pine florescence) may also play a role. The mean emission factors at standard conditions determined using the MEGAN model emission algorithms and empirical model of BVOC emissions were 0.71 and 1.19 mg m −2 h −1 for isoprene and 1.39 and 1.65 mg m −2 h −1 for total monoterpenes, respectively. [ABSTRACT FROM AUTHOR]

We quantified springtime ecosystem-scale monoterpene fluxes from two similar Aleppo pine ( Pinus halepensis Mill.) forests, located in Israel, that differed in the amount of received precipitation: Yatir in the arid south and Birya in the northern part of Israel (291 and 755 mm annual average rainfall, respectively). In addition to the lower water availability, during our measurement campaign the Yatir site suffered from a heat wave with temperatures up to 35 °C, which made the campaign-average net CO 2 assimilation to occur in the morning (1 μmol m −2 s −1 ), with the rest of the daytime hours mainly dominated by net release of CO 2 . The milder conditions at Birya favored a higher net CO 2 assimilation during all daytime hours (with average peaks higher than 10 μmol m −2 s −1 ). Despite these large differences in ambient conditions and CO 2 net assimilation, daytime monoterpene emission capacities at both sites were comparable. While observed monoterpene fluxes were lower at Yatir than at Birya (hourly averages up to 0.4 and 1 mg m −2 h −1 , respectively), the standardized hourly fluxes, after accounting for the differences in light, temperature and stand density between both sites, were comparable (0–1.3 mg m −2 h −1 ). The approach typically used by biogenic emission models overestimated monoterpene fluxes at Yatir when temperatures rose during the heat wave. This result, together with complementary leaf-level measurements showing that summertime monoterpene fluxes almost completely ceased at Yatir while being enhanced at Birya, highlight the interaction of water scarcity and high temperatures that drive monoterpene emissions from vegetation in such extreme climate zones and the need to further improve model performance. [ABSTRACT FROM AUTHOR]

The biogenic emissions of isoprene and monoterpenes are one of the main drivers of atmospheric photochemistry, including oxidant and secondary organic aerosol production. In this paper, the emission rates of isoprene and monoterpenes from Australian vegetation are investigated for the first time using the Model of Emissions of Gases and Aerosols from Nature version 2.1 (MEGANv2.1); the CSIRO chemical transport model; and atmospheric observations of isoprene, monoterpenes and isoprene oxidation products (methacrolein and methyl vinyl ketone). Observations from four field campaigns during three different seasons are used, covering urban, coastal suburban and inland forest areas. The observed concentrations of isoprene and monoterpenes were of a broadly similar magnitude, which may indicate that southeast Australia holds an unusual position where neither chemical species dominates. The model results overestimate the observed atmospheric concentrations of isoprene (up to a factor of 6) and underestimate the monoterpene concentrations (up to a factor of 4). This may occur because the emission rates currently used in MEGANv2.1 for Australia are drawn mainly from young eucalypt trees (< 7 years), which may emit more isoprene than adult trees. There is no single increase/decrease factor for the emissions which suits all seasons and conditions studied. There is a need for further field measurements of in situ isoprene and monoterpene emission fluxes in Australia. [ABSTRACT FROM AUTHOR]

Isoprene and monoterpene emissions and environmental conditions were measured over a six month period in a Lei bamboo (Phyllostachys violascens) forest in a subtropical region in China. Isoprene and monoterpene emissions were measured using a relaxed eddy accumulation (REA) system on an above-canopy tower. From July to November of 2012, isoprene contributed 99.1% of terpenoid emissions. α-pinene, constituting 0.8% of total observed terpenoid emissions, was the only monoterpene for which a significant flux was detected. Emissions of the sesquiterpenes longifolene and α-cedrene were observed at very low rates. Isoprene and α-pinene emissions exhibited strong diurnal variations, with lower emissions in the morning and late evening, and the highest emissions around noon. BVOC peak emissions typically occurred a few hours after the noon PAR peak and coincided with the daily temperature peak. This behavior can be described reasonably well by the MEGANv2.1 biogenic emission model. During the campaign (i.e., from 7 July, 2012 to 19 Jan., 2013), the mean (and maximum) emission fluxes (mg m −2 h −1 ) were 0.95 (10.32) for isoprene, 0.010 (0.176) for α pinene, 0.001 (0.063) for longifolene, and 2.6 × 10 −4 (0.009) for α-cedrene, respectively. During the winter season, when the ground was covered by organic mulch to increase soil temperature and to increase the yield of bamboo shoot, there was no evident impact on BVOC emissions. The observed seasonal variation followed the general behavior predicted by the MEGANv2.1 model, with lower emissions associated with cooler conditions, but the magnitude of the emission decrease was greater than expected indicating driving variables are missing from the model. Emission factors, representing the emission expected for a Leaf Area Index of 5 at a temperature of 30 °C and PAR of 1500 μmol m −2 s −1 , during the peak growing season for this site were 0.008 mg m −2 h −1 for α-pinene and 3.3 mg m −2 h −1 for isoprene. The isoprene emission factor is similar to the value (3.6 mg m −2 h −1 ) for this location in the MEGANv2.1 global biogenic emission model. A second bamboo plantation, containing Moso bamboo ( Phyllostachys heterocycla ), was investigated and found to have similar isoprene and monoterpene emission rates as Lei bamboo forest. The emission data obtained in this study are the first canopy-scale flux measurements reported for bamboo plantations and demonstrate the potential importance of bamboo isoprene emissions for regional ozone and organic aerosol production. [ABSTRACT FROM AUTHOR]

Diurnal branch-level emission rates of biogenic volatile organic compounds (BVOC) including isoprene, monoterpenes (MT), and sesquiterpenes (SQT) were determined at the University of Michigan Biological Station for the tree species red maple (Acer rubrum), red oak (Quercus rubra), paper birch (Betula papyrifera), white pine (Pinus strobus), and big tooth aspen (Populus grandidentata). These emission rates were combined with detailed biomass distribution and meteorological data and incorporated into the canopy model, model of emissions of gasses and aerosols from nature (MEGAN), for estimating whole-canopy fluxes of isoprene. The modeled half-hour fluxes and cumulative seasonal fluxes compared favorably with results from direct, canopy-level eddy covariance (EC) isoprene measurements; modeled cumulative seasonal flux deviated less than 30% from the EC results. Significant MT emissions were found from four of the five tree species. MT emissions from three of these were both light-and temperature-dependent and were approximately one order of magnitude greater than light-independent MT emissions. SQT emissions were identified from three of the five tree species. The model was modified to incorporate SQT and both light-dependent and light-independent MT emissions for determining fluxes. Isoprene comprised of the total terpenoid flux with MT and SQT comprising 4% and 0.3%, respectively. The average cumulative fluxes (in ) from June through September were 2490 for isoprene, 105 for MT, and 7 for SQT. A simple box model analysis was used to estimate the contribution of the isoprene, MT, and SQT emissions to the total OH reactivity. These results confirm that isoprene dominates OH reactions especially during daytime hours. Emissions of reactive MT and SQT increase the BVOC+OH reactivity, but are still lower than estimates of BVOC fluxes at this site necessary for affecting OH reactivity to the significant degree suggested by recent reports. [Copyright &y& Elsevier]

• Sunflower, western redcedar, and American sweetgum were exposed to heat stress. • Heat stress transformed the plants from negligible into high monoterpene emitters. • In some plants, monoterpene emissions were elevated for 1–7 days after heat stress. • Redcedar and sweetgum are not ideal for urban greening in heatwave-prone regions. Four different plant species, sunflower (Helianthus annuus), western redcedar (Thuja plicata), American sweetgum (Liquidambar styraciflua), and red ironbark (Eucalyptus sideroxylon), were enclosed in glass or Teflon chambers, exposed to different temperatures typically ranging from 30 °C to 43 °C, and their terpenoid emission rates were quantified using gas chromatography techniques. The results show that sunflower, western redcedar, and American sweetgum were not significant constitutive monoterpene emitters at 30 °C. Monoterpene emissions from western redcedar and American sweetgum remained negligibly low up to 35 °C–37 °C, however, at higher temperatures, the emissions began to increase at a much more rapid rate than predicted by current emission models. The monoterpene emissions from sunflower, western redcedar, and American sweetgum increased by an average factor of 22, 98, and 5900 for a temperature increase (ΔT) of 13 °C, 13 °C, and 15 °C, respectively, thereby transforming them from negligible into high monoterpene emitters. The observed emission increase was substantially higher than the model-predicted increase of 3.7-fold (ΔT = 13 °C) and 4.5-fold (ΔT = 15 °C). We demonstrate that current emission algorithms cannot accurately model the emission behavior of these plant species at the high temperatures that are characteristic of severe heatwaves. Furthermore, monoterpene emissions from western redcedar and American sweetgum remained elevated for 1–7 days after the heat stress. In contrast to the other three species, red ironbark was a significant constitutive monoterpene emitter (at 30 °C) and its emission response to temperature was close to model predictions. Each of the four species investigated in this study possesses specialized foliar terpene-storing structures, for example, glandular trichomes and oil glands. The strong monoterpene emission induction observed in some of these species may be attributed to heat-induced damage to these storage structures. However, the wall permeability of these structures may vary by species and could limit the magnitude of the heat-induced emission response. [ABSTRACT FROM AUTHOR]

Emissions of Biogenic Volatile Organic Compounds (BVOCs) observed during 2007 from an experimental Pinus taeda plantation in Central North Carolina are compared with model estimates from the Model of Emissions of Gases and Aerosols from Nature (MEGAN) version 2.1. Relaxed eddy accumulation (REA) estimates of 2-methyl-3-buten-2-ol (MBO) fluxes are a factor of 3–4 higher than MEGAN estimates. MEGAN monoterpene emission estimates were a factor of approximately two higher than REA flux measurements. MEGAN β-caryophyllene emission estimates were within 60% of growing season REA flux estimates but were several times higher than REA fluxes during cooler, dormant season periods. The sum of other sesquiterpene emissions estimated by MEGAN was several times higher than REA estimates throughout the year. Model components are examined to understand these discrepancies. Measured summertime leaf area index (LAI) (and therefore foliar biomass) is a factor of two higher than assumed in MEGAN for the P. taeda default. Increasing the canopy mean MBO emission factor from 0.35 to 1.0 mg m − 2 h − 1 also reduces MEGAN vs. REA flux differences. This increase is within current MBO emission factor uncertainties. The algorithm within MEGAN which adjusts isoprene emission estimates as a function of the temperature and light of the previous 24 h seems also to improve the seasonal MEGAN MBO correlation with REA fluxes. Including the effects of the previous 240 h, however, seems to degrade temporal model correlation with fluxes. Monoterpene and sesquiterpene composition data from the REA are compared with MEGAN2.1 estimates and also branch enclosure and needle extract data collected at this site. To our knowledge, the flux data presented here are the first reported for MBO and sesquiterpenes from a P. taeda ecosystem. [ABSTRACT FROM AUTHOR]

Abstract: Monoterpene emissions from the dominant understory species Pteridium aquilinum (Bracken fern) in a mixed temperate forest were measured in the field during the summers of 2006, 2007 and 2008. The results showed that Bracken fern emitted monoterpenes at different rates depending if the plants were located in the understory or in open areas. Understory plants emitted monoterpene levels ranging from 0.002 to 13 μgC gdw −1 h−1. Open area plants emitted monoterpene levels ranging from 0.005 to 2.21 μgC gdw −1 h−1. During the summer of 2008 greenhouse studies were performed to complement the field studies. Only 3% of the greenhouse Bracken fern plants emitted substantial amounts of monoterpenes. The average emission, 0.15 μgC gdw −1 h−1 ± 0.9 μgC gdw −1 h−1, was much lower than that observed in the field. The factors controlling monoterpene emissions are not clear, but this study provides evidence of the potential importance of understory vegetation to ecosystem total hydrocarbon emissions and emphasizes the need for longer-term field studies. [Copyright &y& Elsevier]

Abstract: A three-part study was conducted to quantify the impact of landscaped vegetation on air quality in a rapidly expanding urban area in the arid southeastern United States. The study combines in situ, plant-level measurements, a spatial emissions inventory, and a photochemical box model. Maximum plant-level basal emission rates were moderate: 18.1 μgC gdw−1 h−1 (Washingtonia spp., palms) for isoprene and 9.56 μgC gdw−1 h−1 (Fraxinus velutina, Arizona ash) for monoterpenes. Sesquiterpene emission rates were low for plant species selected in this study, with no measurement exceeding 0.1 μgC gdw−1 h−1. The high ambient temperatures combined with moderate plant-level emission factors resulted in landscape emission factors that were low (250–640 μgC m−2 h−1) compared to more mesic environments (e.g., the southeastern United States). The Regional Atmospheric Chemistry Mechanism (RACM) was modified to include a new reaction pathway for ocimene. Using measured concentrations of anthropogenic hydrocarbons and other reactive air pollutants (NO x , ozone), the box model employing the RACM mechanism revealed that these modest emissions could have a significant impact on air quality. For a suburban location that was downwind of the urban core (high NO x ; low anthropogenic hydrocarbons), biogenic terpenes increased time-dependent ozone production rates by a factor of 50. Our study demonstrates that low-biomass density landscapes emit sufficient biogenic terpenes to have a significant impact on regional air quality. [Copyright &y& Elsevier]

Biogenic volatile organic compound (BVOC) emissions were studied using vegetation enclosure experiments. Particular emphasis was given to sesquiterpene compounds (SQT), although monoterpenes(MT)were also characterized. SQT were detected in emissions from seven (out of eight) pine species that were examined. Thirteen SQT compounds were identified; the most abundant ones were β-caryophyllene, α-bergamotene, β-farnesene, and α-farnesene, with emission rates increasing exponentially with temperature. Regression analysis yielded exponential dependencies of both MT and SQT emissions on temperature of the form E = E0 × exp(×(T - T0)). This resulted in SQT basal emission rates (E0 defined at T0 = 30 °C) ranging between <4 and 620 ng (carbon) gdw-1 h-1 (gdw = gram dry weight). The average value of the exponential temperature response factor × for SQT emissions,taken from all experiments, was 0.17 °C-1, whereas the value for monoterpenes was 0.11 °C-1. The average, total SQT emissions from pines were estimated to be 9, 16, and 29% of the MT emissions at 20, 30, and 40°C respectively. The emission factors and β-factors determined from these measurements were used to estimate pine tree MT and SQT emission distributions for the contiguous United States using MEGAN (model of emissions of gases and aerosols from nature, Guenther et al., 2006). SQT fluxes reaching 10-40 mg m-2 for the month of July were estimated for extensive areas of most western and southern U.S. states. [ABSTRACT FROM AUTHOR]

Measurements of natural hydrocarbon emission rates are reported for an old-growth Pacific Northwest coniferous forest. The emission data were collected for the two dominant species Douglas-fir (Pseudotsuga menziesii) and western hemlock (Tsuga heterophylla) during the growing season in 1997 and 1998 using branch enclosure techniques. Samples were collected at different heights from 13 to 51 m within the canopy using the Wind River Canopy Crane facility. The standard emission factor at a temperature of 30°C and the temperature coefficient for Douglas-fir is Es=0.39±0.14 μg C g-1 h-1 and β=0.14±0.05°C-1 and for western hemlock Es=0.95±0.17 μg C g-1 h-1 and β=0.06±0.02°C-1. There was considerable variability among all the emission factors due to seasonal and branch-to-branch variations. Within season emission factors appear to decline from May to September for the Douglas-fir, although there was no corresponding decrease for the western hemlock. There was no significant difference in standard emission factors (Es) or temperature coefficients as a function of sunlit versus shady growth environment (different heights) for Douglas-fir, but western hemlock emission samples collected low in the canopy showed no exponential correlation with temperature. Applying the standard emission factors from this study to a Pacific Northwest domain and comparing the modified emission inventory to the current regulatory-based emission inventory yielded a net decrease of 19% in the domain wide monoterpene emissions. The relatively small difference in biogenic emissions is slightly misleading, as the difference in standard emission rates between this study and current regulatory rates is quite significant, and they offset each other when combined in this domain. When this inventory was input into a regional photochemical air quality simulation using the MM5/CMAQ system, the reduction in biogenic emissions resulted in an insignificant decrease of O3 and a significant decrease in the secondary organic aerosol (domain wide -20%). The emission measurements reported here represent one of the first extensive data sets for an old-growth forest, where sampling conditions are limited to in situ enclosure techniques within the tall, elevated canopy. [Copyright &y& Elsevier]

Quantification of biogenic volatile organic compound (BVOC) fluxes into the atmosphere is crucial to understand their role in atmospheric oxidation and biogeochemical cycles. BVOC flux measurements were carried out in nine forest ecosystems using a relaxed eddy accumulation (REA) based sampling system, which is easily transportable, simple to operate and designed to be low-cost and therefore can easily be deployed at multiple remote locations. The REA measurements were carried out during daytime between 06:00 and 18:30 (Local Time) with a flux averaging period of 30 min. A detailed description of the REA sampling setup, operational procedure and validation by comparison with full eddy covariance (EC) BVOC flux measurements is provided. BVOC flux measurements from established long-term carbon and water flux tower sites in nine forest ecosystems are compared including Manitou Forest Observatory in Colorado, USA (pine woodland forest), Niwot Ridge AmeriFlux site in Colorado, USA (subalpine forest), Deer Canyon Preserve in New Mexico, USA (pinyon-juniper forest), Lei bamboo forest site near Taihuyuan, China, Qianyanzhou ChinaFLUX site in China (pine forest), Baskett Wildfire Refuge MOFlux site in Missouri, USA (deciduous oak forest), University of Michigan Biological Station PROPHET site in Michigan, USA (mixed deciduous forest), Changbai Mountain Forest Research Station in China (mixed deciduous forest) and the Guyaflux site (GF-Guy) in French Guiana (tropical rainforest). BVOC flux measurements using our REA setup confirm dominance of 2,3,2- methylbutenol (2,3,2-MBO) at the Manitou Forest Observatory and Niwot Ridge sites in Colorado. Monoterpene fluxes measured by REA showed good agreement (within ±10%) with monoterpene fluxes measured by PTR-MS at the Manitou Forest Observatory. The MOFlux site in Missouri was dominated by isoprene emissions (average flux of ~ 9.5 mg m−2 h−1) whereas the Deer Canyon site was dominated by α-pinene emissions (average flux ~ 0.73 mg m−2 h−1). Mixed deciduous forest sites at the PROPHET Station in Michigan and Changbai Mountain Forest Research Station in China primarily emitted isoprene along with some α-pinene, β-pinene and d-Limonene. Isoprene and α-pinene were the dominant BVOCs emitted from the subtropical Lei bamboo plantation at the Taihuyuan site in China while the pine forest site at Qianyanzhou in China were dominated by α-pinene emissions along with significant isoprene. BVOC measurements across different seasons (during 2009–2011) at a tropical forest site in French Guiana (Guyaflux site) revealed the dominance of isoprene emissions during all seasons. Irrespective of the type of the forest ecosystem, α-pinene was among the dominant monoterpenes emitted from all nine forests. [ABSTRACT FROM AUTHOR]