Isoprene Emission Response to Drought and the Impact on Global Atmospheric Chemistry.

Biogenic isoprene emissions play a very important role in atmospheric chemistry. These emissions are strongly dependent on various environmental conditions, such as temperature, solar radiation, plant water stress, ambient ozone and CO2 concentrations, and soil moisture. Current biogenic emission models (i.e., Model of Emissions of Gases and Aerosols from Nature, MEGAN) can simulate emission responses to some of the major driving variables, such as short-term variations in temperature and solar radiation, but the other factors are either missing or poorly represented. In this paper, we propose a new modeling approach that considers the physiological effects of drought stress on plant photosynthesis and isoprene emissions for use in the MEGAN3 biogenic emission model. We test the MEGAN3 approach by integrating the algorithm into the existing MEGAN2.1 biogenic emission model framework embedded into the global Community Land Model of the Community Earth System Model (CLM4.5/CESM1.2). Single-point simulations are compared against available field measurements at the Missouri Ozarks AmeriFlux (MOFLUX) field site. The modeling results show that the MEGAN3 approach of using of a photosynthesis parameter (Vcmax) and soil wetness factor (ßt) to determine the drought activity factor leads to better simulated isoprene emissions in non-drought and drought periods. The global simulation with the MEGAN3 approach predicts a 17% reduction in global annual isoprene emissions, in comparison to the value predicted using the default CLM4.5/MEGAN2.1 without any drought effect. This reduction leads to changes in surface ozone and oxidants in the areas where the reduction of isoprene emissions is observed. Based on the results presented in this study, we conclude that it is important to simulate the drought-induced response of biogenic isoprene emission accurately in the coupled Earth System model.

Ecosystem-scale volatile organic compound fluxes during an extreme drought in a broadleaf temperate forest of the Missouri Ozarks (central USA).

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.

Chromatography related performance of the Monitor for AeRosols and GAses in ambient air (MARGA): laboratory and field-based evaluation.

Evaluation of the semi-continuous Monitor for AeRosols and GAses in ambient air (MARGA, Metrohm Ap-plikon B.V.) was conducted with an emphasis on examination of accuracy and precision associated with processing of chromatograms. Using laboratory standards and atmospheric measurements, analytical accuracy, precision and method detection limits derived using the commercial MARGA software were compared to an alternative chromatography procedure consisting of a custom Java script to reformat raw MARGA conductivity data and Chromeleon (Thermo Scientific Dionex) software for peak integration. Our analysis revealed issues with accuracy and precision resulting from misidentification and misintegration of chromatograph peaks by the MARGA automated software as well as a systematic bias at low concentrations for anions. Reprocessing and calibration of raw MARGA data using the alternative chromatography method lowered method detection limits and re-duced variability (precision) between parallel sampler boxes. Instrument performance was further evaluated during a 1-month intensive field campaign in the fall of 2014, including analysis of diurnal patterns of gaseous and particulate water-soluble species (NH3, SO2, HNO3, N H 4 + , S O 4 2 - and N O 3 - , gas-to-particle partitioning and particle neutralization state. At ambient concentrations below ~ 1 µg m-3, concentrations determined using the MARGA software are biased +30 and +10 % for N O 3 - and S O 4 2 - , respectively, compared to concentrations determined using the alternative chromatography procedure. Differences between the two methods increase at lower concentrations. We demonstrate that positively biased N O 3 - and S O 4 2 - measurements result in overestimation of aerosol acidity and introduce nontrivial errors to ion balances of inorganic aerosol. Though the source of the bias is uncertain, it is not corrected by the MARGA online single-point internal LiBr standard. Our results show that calibra-tion and verification of instrument accuracy by multilevel external standards is required to adequately control analytical accuracy. During the field intensive, the MARGA was able to capture rapid compositional changes in PM2.5 due to changes in meteorology and air mass history relative to known source regions of PM precursors, including a fine N O 3 - aerosol event associated with intrusion of Arctic air into the southeastern US.

Airborne measurements of isoprene and monoterpene emissions from southeastern U.S. forests.

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.

Large drought-induced variations in oak leaf volatile organic compound emissions during PINOT NOIR 2012.

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, ß-pinene, d-limonene, γ-terpinene, ß-ocimene (predominantly1,3,7-trans-ß-ocimene, but also 1,3,6-trans-ß-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.

Nitrous oxide emissions from the gulf of Mexico hypoxic zone.

The production of nitrous oxide (N(2)O), a potent greenhouse gas, in hypoxic coastal zones remains poorly characterized due to a lack of data, though large nitrogen inputs and deoxygenation typical of these systems create the potential for large N(2)O emissions. We report the first N(2)O emission measurements from the Gulf of Mexico Hypoxic Zone (GOMHZ), including an estimate of the emission "pulse" associated with the passage of Tropical Storm Edouard in August, 2008. Prestorm emission rates (25-287 nmol m(-2) hr(-1)) and dissolved N(2)O concentrations (5 - 30 nmol L(-1)) were higher than values reported for the Caribbean and western Tropical Atlantic, and on the lower end of the range of observations from deeper coastal hypoxic zones. During the storm, N(2)O rich subsurface water was mixed upward, increasing average surface concentrations and emission rates by 23% and 61%, respectively. Approximately 20% of the N(2)O within the water column vented to the atmosphere during the storm, equivalent to 13% of the total "hypoxia season" emission. Relationships between N(2)O, NO(3)(-), and apparent oxygen utilization (AOU) suggest enhanced post storm N(2)O production, most likely in response to reoxygenation of the water column and redistribution of organic nitrogen. Our results indicate that mixing related emissions contribute significantly to total seasonal emissions and must therefore be included in emission models and inventories for the GOMHZ and other shallow coastal hypoxic zones.

N2O emissions from streams in the Neuse river watershed, North Carolina.

We present N2O emission data from 11 sites in the Neuse River watershed. Emissions were measured using a static surface enclosure technique deployed on eight sites on the main river channel and three tributary sites. Ancillary data collected included dissolved oxygen, nitrate, total nitrogen, ammonium, dissolved organic carbon, total phosphorus, and temperature. Analysis using standard linear models, and classification and regression trees (CART), indicated nitrate to be the primary driving variable associated with N2O emission, although dissolved organic carbon concentration and water temperature were positively related with N2O emission as well. Relationships between nitrate concentration and N2O emission were consistent with those found in previous studies, although the data presented here represent the lower end of the range for both variables among published studies. Using our measured N2O emission rates along with literature values for the ratio of nitrogen gas to N2O produced during denitrification, we estimate N loss via denitrification in the Neuse River is approximately 17% of the annual N load delivered to the estuary.

Temporal variability in basal isoprene emission factor.

Seasonal variability in basal isoprene emission factor (&mgr;g C g(-1) h(-1) or nmol m(-2) s(-1), leaf temperature at 30 degrees C and photosynthetically active radiation (PAR) at 1000 &mgr;mol m(-2) s(-1)) was studied during the 1998 growing season at Duke Forest in the North Carolina Piedmont. Emissions from eight upper-canopy white oak (Quercus alba L.) leaves were measured periodically from the onset of isoprene emission on Day of Year (DOY) 119 (April 29) to leaf senescence in late October (DOY 299). Emissions from four leaves were measured under basal conditions with a controlled-environment cuvette system equipped with 10-ml gas-tight syringes and a reduction gas detector. Emissions from the other four leaves were measured under ambient conditions with the same system. Emission rates from the four leaves measured under ambient conditions were adjusted to basal conditions based on the PAR and leaf temperature algorithms of Guenther et al. (1993). The seasonal onset of isoprene emission was in agreement with previous studies where cumulative degree days from the date of the last spring frost were used to estimate bud break, leaf expansion, and increase in basal emission factor (EF). Between DOY 141 (May 21) and 240 (August 28), mean meteorological conditions 6 to 18 h prior to the EF measurements (ambient PAR and temperature) explained up to 78% of the variability in mean basal EF between measurement periods. Summertime mean isoprene emission potential was reached on DOY 141 (May 21) and was maintained until DOY 240 (August 28), when isoprene emission began to decline monotonically as leaf senescence approached. The mean value for leaves measured under ambient conditions and adjusted to basal conditions for DOY 141-240 was 75.6 &mgr;g C g(-1) h(-1) (74.2-79.1), whereas the mean value for leaves measured under basal conditions was 72.9 &mgr;g C g(-1) h(-1) (64.7-88.9). Between DOY 141 and 240, daily mean isoprene EFs varied from 54 to 96 &mgr;g C g(-1) h(-1) (27 to 49 nmol m(-2) s(-1)). In agreement with previous work at this and other sites, basal isoprene emission rates of fully exposed leaves at the crown apex of this tree were about 20% higher than those of the selected leaves. The length of the period prior to measurement of isoprene emission, during which meteorology was correlated with basal EF, appeared to be related to the timing and periodicity of meteorological change, and probably explains quantitative differences in the length of this period among studies. The empirical equation that we derived for this effect explained variability in midday EFs at the study site, but its general applicability remains to be tested. Strong diurnal changes in EF (as high as a factor of 2) are implied in this study, and should be examined further.