Mark J. Potosnak

The tropical forest and fire emissions experiment: Emission, chemistry, and transport of biogenic volatile organic compounds in the lower atmosphere over Amazonia

(7.8 ± 2.3 mg/m 2 /h) and monoterpene fluxes (1.2 ± 0.5 mg/m 2 /h) compared well between ground and airborne measurements and are higher than fluxes estimated in this region during other seasons. The biogenic emission model, Model of Emissions of Gases and Aerosols from Nature (MEGAN), estimates fluxes that are within the model and measurement uncertainty and can describe the large observed variations associated with land-use change in the region north-west of Manaus. Isoprene and monoterpenes accounted for � 75% of the total OH reactivity in this region and are important volatile organic compounds (VOCs) for modeling atmospheric chemistry in Amazonia. The presence of fair weather clouds (cumulus humilis) had an important impact on the vertical distribution and chemistry of VOCs through the planetary boundary layer (PBL), the cloud layer, and the free troposphere (FT). Entrainment velocities between 10:00 and 11:30 local time (LT) are calculated to be on the order of 8–10 cm/s. The ratio of methyl-vinyl-ketone (MVK) and methacrolein (MAC) (unique oxidation products of isoprene chemistry) with respect to isoprene showed a pronounced increase in the cloud layer due to entrainment and an increased oxidative capacity in broken cloud decks. A decrease of the ratio in the lower free troposphere suggests cloud venting through activated clouds. OH modeled in the planetary boundary layer using a photochemical box model is much lower than OH calculated from a mixed layer budget approach. An ambient reactive sesquiterpene mixing ratio of 1% of isoprene would be sufficient to explain most of this discrepancy. Increased OH production due to increased photolysis in the cloud layer balances the low OH values modeled for the planetary boundary layer. The intensity of segregation (Is) of isoprene and OH, defined as a relative reduction of the reaction rate constant due to incomplete mixing, is found to be significant: up to 39 ± 7% in the � 800-m-deep cloud layer. The effective reaction rate between isoprene and OH can therefore vary significantly in certain parts of the lower atmosphere.

Influence of biotic exchange and combustion sources on atmospheric CO2 concentrations in New England from observations at a forest flux tower

Hourly data for concentrations and fluxes of CO2 at 30 m in Harvard Forest (Petersham, Massachusetts) are analyzed using linear modeling to obtain regionally representative CO2 concentrations at a continental site. The time series is decomposed into contributions due to regional combustion, local canopy exchange, monthly average regional biotic exchange (as modulated by the daily cycle of growth and decay of the planetary boundary layer (PBL)), and the regional monthly background concentration. Attributions are derived using time series analysis, data for a tracer for combustion (CO or acetylene (C2H2)), and measurements of indicators of proximate canopy exchange (CO2 flux and momentum flux). Results are compared to observations at Cold Bay, Alaska. Combustion contributes on average 4–5 ppm to ambient CO2 at Harvard Forest in winter and 2–3 ppm in summer. Regional biotic emissions elevate daily mean CO2 by 4–6 ppm in winter, and the covariance of the biotic cycle of uptake and emission with PBL height enhances daily mean CO2 by 1–2 ppm in summer; minimum values in late afternoon average 10 ppm lower than at Cold Bay in summer. The study shows that regionally representative concentrations of CO2 can be determined at continental sites if suitable correlates (tracers, fluxes of CO2, and momentum) are measured simultaneously with CO2 itself.

Coupling between Land Ecosystems and the Atmospheric Hydrologic Cycle through Biogenic Aerosol Pathways

AUTHOR AFFILIATIONS: BARTH, SUN, WIEDINMYER, KARL, KIM, LEVIS, MAHOWALD, MOORE, NANDI, NEMITZ, POTOSNAK, SMITH, AND STROUD—National Center for Atmospheric Research, Boulder, Colorado; MCFADDEN—University of Minnesota, Saint Paul, Minnesota; CHUANG—University of California, Santa Cruz, Santa Cruz, California; COLLINS—Texas A&M University, College Station, Texas; GRIFFIN—University of New Hampshire, Durham, New Hampshire; HANNIGAN—University of Colorado, Boulder, Colorado; LASHER-TRAPP—Purdue University, West Lafayette, Indiana; LITVAK—University of Texas, Austin, Texas; NENES—Georgia Institute of Technology, Atlanta, Georgia; RAYMOND—Bucknell University, Lewisburg, Pennsylvania; STILL—University of California, Santa Barbara, Santa Barbara, California CORRESPONDING AUTHOR: Dr. Mary Barth, NCAR/MMM, P.O. Box 3000, Boulder, CO 80307 E-mail: barthm@ucar.edu

Chicago’s Heat Island and Climate Change: Bridging the Scales via Dynamical Downscaling

AbstractThe interaction of global climate change and urban heat islands (UHI) is expected to have far-reaching impacts on the sustainability of the world’s rapidly growing urban population centers. Given that a wide range of spatiotemporal scales contributed by meteorological forcing and complex surface heterogeneity complicates UHI, a multimodel nested approach is used in this paper to study climate-change impacts on the Chicago, Illinois, UHI, covering a range of relevant scales. One-way dynamical downscaling is used with a model chain consisting of global climate (Community Atmosphere Model), regional climate (Weather Research and Forecasting Model), and microscale (“ENVI-met”) models. Nested mesoscale and microscale models are evaluated against the present-day observations (including a dedicated urban miniature field study), and the results favorably demonstrate the fidelity of the downscaling techniques that were used. A simple building-energy model is developed and used in conjunction with microscal...

Processing arctic eddy-flux data using a simple carbon-exchange model embedded in the ensemble Kalman filter

Continuous time-series estimates of net ecosystem carbon exchange (NEE) are routinely made using eddy covariance techniques. Identifying and compensating for errors in the NEE time series can be automated using a signal processing filter like the ensemble Kalman filter (EnKF). The EnKF compares each measurement in the time series to a model prediction and updates the NEE estimate by weighting the measurement and model prediction relative to a specified measurement error estimate and an estimate of the model-prediction error that is continuously updated based on model predictions of earlier measurements in the time series. Because of the covariance among model variables, the EnKF can also update estimates of variables for which there is no direct measurement. The resulting estimates evolve through time, enabling the EnKF to be used to estimate dynamic variables like changes in leaf phenology. The evolving estimates can also serve as a means to test the embedded model and reconcile persistent deviations between observations and model predictions. We embedded a simple arctic NEE model into the EnKF and filtered data from an eddy covariance tower located in tussock tundra on the northern foothills of the Brooks Range in northern Alaska, USA. The model predicts NEE based only on leaf area, irradiance, and temperature and has been well corroborated for all the major vegetation types in the Low Arctic using chamber-based data. This is the first application of the model to eddy covariance data. We modified the EnKF by adding an adaptive noise estimator that provides a feedback between persistent model data deviations and the noise added to the ensemble of Monte Carlo simulations in the EnKF. We also ran the EnKF with both a specified leaf-area trajectory and with the EnKF sequentially recalibrating leaf-area estimates to compensate for persistent model-data deviations. When used together, adaptive noise estimation and sequential recalibration substantially improved filter performance, but it did not improve performance when used individually. The EnKF estimates of leaf area followed the expected springtime canopy phenology. However, there were also diel fluctuations in the leaf-area estimates; these are a clear indication of a model deficiency possibly related to vapor pressure effects on canopy conductance.

The effect of elevated CO2, soil and atmospheric water deficit and seasonal phenology on leaf and ecosystem isoprene emission

Two cottonwood plantations were grown at different CO2 concentrations at the Biosphere 2 Laboratory in Arizona to investigate the response of isoprene emission to elevated [CO2] and its interaction with water deficits. We focused on responses due to seasonal variation and variation in the mean climate from one year to the next. In fall and in spring, isoprene emission rate showed a similar inhibition by elevated [CO2], despite an 8-10°C seasonal difference in mean air temperature. The overall response of isoprene emission to drought was also similar for observations conducted during the spring or fall, and during the fall of two different years with an approximate 5°C difference in mean air temperature. In general, leaf-level isoprene emission rates, measured at constant temperature and photon-flux density, decreased slightly, or remained constant during drought, whereas ecosystem-level isoprene emission rates increased. The uncoupling of ecosystem- and leaf-scale responses is not due to differential dependence on leaf area index (LAI) as LAI increased only slightly, or decreased, during the drought treatments at the same time that ecosystem isoprene emission rate increased greatly. Nor does the difference in isoprene emission rate between leaves and ecosystems appear to be due solely to increases in canopy surface temperature during the drought, though some increase in temperature was observed. It is possible that still further factors, such as increased penetration of PPFD into the canopy as a result of changes in leaf angle, reduced sink strength of the soil for atmospheric isoprene, and decreases in the mean Ci of leaves, combined with the small increases in canopy surface temperature, increased the ecosystem isoprene emission rate. Whatever the causes of the differences in the leaf and ecosystem responses, we conclude that the overall shape of the leaf and ecosystem responses to drought was constant irrespective of season or year.

Increasing leaf temperature reduces the suppression of isoprene emission by elevated CO2 concentration

Including algorithms to account for the suppression of isoprene emission by elevated CO2 concentration affects estimates of global isoprene emission for future climate change scenarios. In this study, leaf-level measurements of isoprene emission were made to determine the short-term interactive effect of leaf temperature and CO2 concentration. For both greenhouse plants and plants grown under field conditions, the suppression of isoprene emission was reduced by increasing leaf temperature. For each of the four different tree species investigated, aspen (Populus tremuloides Michx.), cottonwood (Populus deltoides W. Bartram ex Marshall), red oak (Quercus rubra L.), and tundra dwarf willow (Salix pulchra Cham.), the suppression of isoprene by elevated CO2 was eliminated at increased temperature, and the maximum temperature where suppression was observed ranged from 25 to 35°C. Hypotheses proposed to explain the short-term suppression of isoprene emission by increased CO2 concentration were tested against this observation. Hypotheses related to cofactors in the methylerythritol phosphate (MEP) pathway were consistent with reduced suppression at elevated leaf temperature. Also, reduced solubility of CO2 with increased temperature can explain the reduced suppression for the phosphoenolpyruvate (PEP) carboxylase competition hypothesis. Some global models of isoprene emission include the short-term suppression effect, and should be modified to include the observed interaction. If these results are consistent at longer timescales, there are implications for predicting future global isoprene emission budgets and the reduced suppression at increased temperature could explain some of the variable responses observed in long-term CO2 exposure experiments.

Multi-Scale Simulations of Climate-Change Influence on Chicago Heat Island

Over the past half century, burgeoning urban areas such as Chicago have experienced elevated anthropogenic-induced alteration of local climates within urbanized regions. As a result, urban heat island (UHI) effect in these areas has intensified. Global climate change can further modulate UHI’s negative effects on human welfare and energy conservation. Various numerical models exist to understand, monitor, and predict UHI and its ramifications, but none can resolve all the relevant physical phenomena that span a wide range of scales. To this end, we have applied a comprehensive multi-scale approach to study UHI of Chicago.The coupling of global, mesoscale, and micro-scale models has allowed for dynamical downscaling from global to regional to city and finally to neighborhood scales. The output of the Community Climate System Model (CCSM5), a general circulation model (GCM), provides future climate scenario, and its coupling with Weather Research and Forecasting (WRF) model enables studies on mesoscale behavior at urban scales. The output from the WRF model at 0.333 km resolution is used to drive a micro-scale model, ENVI-met. Through this coupling the bane of obtaining reliable initial and boundary conditions for the micro-scale model from limited available observational records has been aptly remedied. It was found that the performance of ENVI-met improves when WRF output, rather than observational data, is supplied for initial conditions. The success of the downscaling procedure allowed reasonable application of micro-scale model to future climate scenario provided by CCSM5 and WRF models. The fine (2 m) resolution of ENVI-met enables the study of two key effects of UHI at micro-scale: decreased pedestrian comfort and increased building-scale energy consumption. ENVI-met model’s explicit treatment of key processes that underpin urban microclimate makes it captivating for pedestrian comfort analysis. Building energy, however, is not modeled by ENVI-met so we have developed a simplified building energy model to estimate future cooling needs.© 2014 ASME

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 (βt) 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.