Alex Guenther

Improving our fundamental understanding of the role of aerosol−cloud interactions in the climate system

The effect of an increase in atmospheric aerosol concentrations on the distribution and radiative properties of Earth’s clouds is the most uncertain component of the overall global radiative forcing from preindustrial time. General circulation models (GCMs) are the tool for predicting future climate, but the treatment of aerosols, clouds, and aerosol−cloud radiative effects carries large uncertainties that directly affect GCM predictions, such as climate sensitivity. Predictions are hampered by the large range of scales of interaction between various components that need to be captured. Observation systems (remote sensing, in situ) are increasingly being used to constrain predictions, but significant challenges exist, to some extent because of the large range of scales and the fact that the various measuring systems tend to address different scales. Fine-scale models represent clouds, aerosols, and aerosol−cloud interactions with high fidelity but do not include interactions with the larger scale and are therefore limited from a climatic point of view. We suggest strategies for improving estimates of aerosol−cloud relationships in climate models, for new remote sensing and in situ measurements, and for quantifying and reducing model uncertainty.

Measurement of Biogenic Sulfur Emissions from Soils and Vegetation: Application of Dynamic Enclosure Methods with Natusch Filter and GC/FPD Analysis

Emission rates of reduced sulfur gases from vegetation and soils were measured in various regions of the United States during the summer of 1985. The predominant sulfur gases emitted were hydrogen sulfide, carbonyl sulfide and dimethylsulfide. Typically, vegetative (forests, crops, etc.) emission fluxes varied between approximately 10 and 60 ng S m-2 min-1. Biogenic sulfur fluxes from mollisol and histisol soils averaged 15 and 217 ng S m-2 min-1, respectively. Salt water marsh fluxes with a geometric mean of 293 ng S m-2 min-1 were the highest measured. These biogenic sulfur fluxes are somewhat lower than those measured during the SURE study at some of the same sites. The natural sulfur emission fluxes reported herein together with those data included in the two accompanying manuscripts provide the basis for developing a national inventory of reduced sulfur emissions from soils, crops and trees. When combined these data also will provide a foundation for deriving uncertainty limits associated with these flux estimates.

Recent advances in understanding secondary organic aerosol: Implications for global climate forcing

Anthropogenic emissions and land use changes have modified atmospheric aerosol concentrations and size distributions over time. Understanding preindustrial conditions and changes in organic aerosol due to anthropogenic activities is important because these features (1) influence estimates of aerosol radiative forcing and (2) can confound estimates of the historical response of climate to increases in greenhouse gases. Secondary organic aerosol (SOA), formed in the atmosphere by oxidation of organic gases, represents a major fraction of global submicron-sized atmospheric organic aerosol. Over the past decade, significant advances in understanding SOA properties and formation mechanisms have occurred through measurements, yet current climate models typically do not comprehensively include all important processes. This review summarizes some of the important developments during the past decade in understanding SOA formation. We highlight the importance of some processes that influence the growth of SOA particles to sizes relevant for clouds and radiative forcing, including formation of extremely low volatility organics in the gas phase, acid-catalyzed multiphase chemistry of isoprene epoxydiols, particle-phase oligomerization, and physical properties such as volatility and viscosity. Several SOA processes highlighted in this review are complex and interdependent and have nonlinear effects on the properties, formation, and evolution of SOA. Current global models neglect this complexity and nonlinearity and thus are less likely to accurately predict the climate forcing of SOA and project future climate sensitivity to greenhouse gases. Efforts are also needed to rank the most influential processes and nonlinear process-related interactions, so that these processes can be accurately represented in atmospheric chemistry-climate models.

The Green Ocean Amazon Experiment (GoAmazon2014/5) Observes Pollution Affecting Gases, Aerosols, Clouds, and Rainfall over the Rain Forest

AbstractThe Observations and Modeling of the Green Ocean Amazon 2014–2015 (GoAmazon2014/5) experiment took place around the urban region of Manaus in central Amazonia across 2 years. The urban pollution plume was used to study the susceptibility of gases, aerosols, clouds, and rainfall to human activities in a tropical environment. Many aspects of air quality, weather, terrestrial ecosystems, and climate work differently in the tropics than in the more thoroughly studied temperate regions of Earth. GoAmazon2014/5, a cooperative project of Brazil, Germany, and the United States, employed an unparalleled suite of measurements at nine ground sites and on board two aircraft to investigate the flow of background air into Manaus, the emissions into the air over the city, and the advection of the pollution downwind of the city. Herein, to visualize this train of processes and its effects, observations aboard a low-flying aircraft are presented. Comparative measurements within and adjacent to the plume followed t...

Isoprene photochemistry over the Amazon rainforest

Significance For isolated regions of the planet, organic peroxy radicals produced as intermediates of atmospheric photochemistry have been expected to follow HO2 rather than NO pathways. Observational evidence, however, has been lacking. An accurate understanding of the relative roles of the two pathways is needed for quantitative predictions of the concentrations of particulate matter, oxidation capacity, and consequent environmental and climate impacts. The results herein, based on measurements, find that the ratio of the reaction rate of isoprene peroxy radicals with HO2 to that with NO is about unity for background conditions of Amazonia. The implication is that sufficient NO emissions are maintained by natural processes of the forest such that both HO2 and NO pathways are important, even in this nominally low-NO region. Isoprene photooxidation is a major driver of atmospheric chemistry over forested regions. Isoprene reacts with hydroxyl radicals (OH) and molecular oxygen to produce isoprene peroxy radicals (ISOPOO). These radicals can react with hydroperoxyl radicals (HO2) to dominantly produce hydroxyhydroperoxides (ISOPOOH). They can also react with nitric oxide (NO) to largely produce methyl vinyl ketone (MVK) and methacrolein (MACR). Unimolecular isomerization and bimolecular reactions with organic peroxy radicals are also possible. There is uncertainty about the relative importance of each of these pathways in the atmosphere and possible changes because of anthropogenic pollution. Herein, measurements of ISOPOOH and MVK + MACR concentrations are reported over the central region of the Amazon basin during the wet season. The research site, downwind of an urban region, intercepted both background and polluted air masses during the GoAmazon2014/5 Experiment. Under background conditions, the confidence interval for the ratio of the ISOPOOH concentration to that of MVK + MACR spanned 0.4–0.6. This result implies a ratio of the reaction rate of ISOPOO with HO2 to that with NO of approximately unity. A value of unity is significantly smaller than simulated at present by global chemical transport models for this important, nominally low-NO, forested region of Earth. Under polluted conditions, when the concentrations of reactive nitrogen compounds were high (>1 ppb), ISOPOOH concentrations dropped below the instrumental detection limit (<60 ppt). This abrupt shift in isoprene photooxidation, sparked by human activities, speaks to ongoing and possible future changes in the photochemistry active over the Amazon rainforest.

BioEarth: Envisioning and developing a new regional earth system model to inform natural and agricultural resource management

As managers of agricultural and natural resources are confronted with uncertainties in global change impacts, the complexities associated with the interconnected cycling of nitrogen, carbon, and water present daunting management challenges. Existing models provide detailed information on specific sub-systems (e.g., land, air, water, and economics). An increasing awareness of the unintended consequences of management decisions resulting from interconnectedness of these sub-systems, however, necessitates coupled regional earth system models (EaSMs). Decision makers’ needs and priorities can be integrated into the model design and development processes to enhance decision-making relevance and “usability” of EaSMs. BioEarth is a research initiative currently under development with a focus on the U.S. Pacific Northwest region that explores the coupling of multiple stand-alone EaSMs to generate usable information for resource decision-making. Direct engagement between model developers and non-academic stakeholders involved in resource and environmental management decisions throughout the model development process is a critical component of this effort. BioEarth utilizes a bottom-up approach for its land surface model that preserves fine spatial-scale sensitivities and lateral hydrologic connectivity, which makes it unique among many regional EaSMs. This paper describes the BioEarth initiative and highlights opportunities and challenges associated with coupling multiple stand-alone models to generate usable information for agricultural and natural resource decision-making.

Building wake dispersion at an Arctic industrial site: Field tracer observations and plume model evaluations

AtlftOspheric Environment Vol. 24A, No. 9, pp. 2329- 2347, 1990. Printed in -Great Britain. Pergamon Press pie BUILDING WAKE DISPERSION AT AN ARCTIC INDUSTRIAL SITE: FIELD TRACER OBSERVATIONS AND PLUME MODEL EVALUATIONS ALEX GUENTHER, BRIAN LAMB* and EUGENE ALLWINE Laboratory for Atmospheric R~search, Department of Civil & Environmental Engineering, Washington State University, Pullman, WA 99164-2910, U.S.A. (First received 8 June 1989 and in final form 21 February 1990) Abstract-Ten multi-hour atmospheric dispersion SF 6 tracer experiments were conducted during October and November of 1987 near a large oil gathering facility in the Prudhoe Bay, Alaska, oilfield reservation. The purpose of this study was to investigate dispersion under arctic conditions and in situations where building- generated airflow disturbances dominate downwind distributions of ground level pollutant concentrations. This was accomplished with a network of micrometeorological instruments, portable syringe tracer samplers, continuous tracer analyzers, and infrared visualization of near source plume behavior. Atmospheric stability and wind speed profiles at this arctic site are influenced by the smooth (surface roughness=0.03 cm), snow covered tundra surface which receives negligible levels of solar isolation in winter. The dispersion of pollutants emitted from sources within the oil gathering facility, however, is dominated by the influence of nearby buildings when high winds generate elevated ground level concentrations. An order of magnitude increase in maximum ground level concentration was observed as wind speeds increased from 5 to 8 ms - 1 and another order of magnitude increase was observed as winds increased from 8 to 16ms - 1 . Variation in maximum concentrations was also observed with changes in wind direction. Vertical plume diffusion (a,) near the buildings was a factor of 2-3 greater than that observed in open terrain and was dependent on both wind speed and the projected building width and location of nearby buildings. Wind tunnel tracer distributions for east winds agree with field observations but also indicate that a significant increase in plume downwash occurs with other wind directions. Concentration distributions were calculated using several versions of the Industrial Source Complex (ISC) model. Model estimates of ground level concentrations were within a factor of three depending on wind direction. The model predictions are extremely sensitive to the ratio of plume height to vertical plume diffusion which is significantly influenced by a complex aerodynamic wake in the field. Key word index : Arctic, dispersion, tracer, downwash, buildings, plume rise, ISC. 1. INTRODUCTION The development and evaluation of air quality modeling techniques have greatly benefited from transport and diffusion field studies. These include early experiments using smoke visualization techni- ques (Richardson, 1920); the classical large scale, open terrain diffusion experiments such as Project Prairie Grass (Haugen, 1959) which provide the empirical basis for the Pasquill-Gifford (PG) curves used in current regulatory air quality models; and more recent investigations of sites and conditions which deviate from the relatively simple dispersion environment of earlier experiments. In this paper we present the results of a field tracer investigation of plume trans- port and diffusion near the clustered and intercon- nected buildings of an arctic industrial complex. The natural gas compressor turbine which was the focus of this field study is typical of the major NOx sources in the Prudhoe Bay, Alaska, oilfields. Accurate simu- *To whom correspondence should be addressed. lation of pollutant dispersion from existing sources will aid in determining the present air quality within this tundra ecosystem and is needed for predicting the impacts of future development within potential oil- fields along the Alaska coast. We have previously analyzed open terrain micro- meteorological and tracer data collected at Prudhoe Bay to provide a description of the Arctic atmospheric boundary layer (Guenther and Lamb, 1990). Near the oil gathering facilities, building-generated disturban- ces to the flow field around the exhaust stacks signific- antly change dispersion patterns by altering mean airflow streamlines and by increasing turbulence lev- els. Maximum concentrations can increase signific- antly as high winds generate downwash conditions around the low turbine and heater stacks typical of an arctic industrial complex. Field observations of plume behavior from a 35 MHP turbine are described and compared to our analysis of a wind tunnel simulation of the same source and surrounding buildings (Guen- ther et al., 1989). Building downwash, diffusion, and plume rise algorithms in existing Gaussian plume models and cavity wake models are evaluated in this

New Particle Formation and Growth in an Isoprene-Dominated Ozark Forest: From Sub-5 nm to CCN-Active Sizes

Particle Investigations at a Northern Ozarks Tower: NOx, Oxidant, Isoprene Research (PINOT NOIR) were conducted in a Missouri forest dominated by isoprene emissions from May to October 2012. This study presents results of new particle formation (NPF) and the growth of new particles to cloud condensation nuclei (CCN)-active sizes (∼100 nm) observed during this field campaign. The measured sub-5 nm particles were up to ∼20,000 cm−3 during a typical NPF event. Nucleation rates J1 were relatively high (11.0 ± 10.6 cm−3 s−1), and one order of magnitude higher than formation rates of 5 nm particles (J5). Sub-5 nm particle formation events were observed during 64% of measurement days, with a high preference in biogenic volatile organic compounds (BVOCs)- and SO2-poor northwesterly (90%) air masses than in BVOCs-rich southerly air masses (13%). About 80% of sub-5 nm particle events led to the further growth. While high temperatures and high aerosol loadings in the southerly air masses were not favorable for nucleation, high BVOCs in the southerly air masses facilitated the growth of new particles to CCN-active sizes. In overall, 0.4–9.4% of the sub-5 nm particles grew to CCN-active sizes within each single NPF event. During a regional NPF event period that took place consecutively over several days, concentrations of CCN size particles increased by a factor of 4.7 in average. This enhanced production of CCN particles from new particles was commonly observed during all 13 regional NPF events during the campaign period. Copyright 2014 American Association for Aerosol Research

Atmospheric dispersion in the arctic: Winter-time boundary-layer measurements

The winter-time arctic atmospheric boundary layer was investigated with micrometeorological and SF6 tracer measurements collected in Prudhoe Bay, Alaska. The flat, snow-covered tundra surface at this site generates a very small (0.03 cm) surface roughness. The relatively warm maritime air mass originating over the nearby, partially frozen Beaufort Sea is cooled at the tundra surface resulting in strong (4 to 30 °C · (100 m)-1) temperature inversions with light winds and a persistent weak (1 to 2 °C · (100 m)-1) surface inversion with wind speeds up to 17 m s-1. The absence of any diurnal atmospheric stability pattern during the study was due to the very limited solar insolation. Vertical profiles were measured with a multi-level mast from 1 to 17 m and with a Doppler acoustic sounder from 60 to 450 m. With high wind speeds, stable layers below 17 m and above 300 m were typically separated by a layer of neutral stability. Turbulence statistics and spectra calculated at a height of 33 m are similar to measurements reported for non-arctic, open terrain sites and indicate that the production of turbulence is primarily due to wind shear. The distribution of wind direction recorded at 1 Hz was frequently non-Gaussian for 1-hr periods but was always Gaussian for 5-min periods. We also observed non-Gaussian hourly averaged crosswind concentration profiles and assume that they can be modeled by calculating sequential short-term concentrations, using the 5-min standard deviation of horizontal wind direction fluctuations (Σθ) to estimate a horizontal dispersion coefficient (Σy), and constructing hourly concentrations by averaging the short-term results. Non-Gaussian hourly crosswind distributions are not unique to the arctic and can be observed at most field sites. A weak correlation between horizontal (Σv) and vertical (Σw) turbulence observed for both 1-hr and 5-min periods indicates that a single stability classification method is not sufficient to determine both vertical and horizontal dispersion at this site. An estimate of the vertical dispersion coefficient, Σz, could be based on ΣΦ or a stability classification parameter which includes vertical thermal and wind shear effects (e.g., Monin-Obukhov length, L).

Sensitivity analysis of simulated SOA loadings using a variance-based statistical approach: SENSITIVITY ANALYSIS OF SOA

We investigate the sensitivity of secondary organic aerosol (SOA) loadings simulated by a regional chemical transport model to seven selected model parameters using a modified volatility basis-set (VBS) approach: four involving emissions of anthropogenic and biogenic volatile organic compounds, anthropogenic semivolatile and intermediate volatility organics (SIVOCs), and NOx; two involving dry deposition of SOA precursor gases, and one involving particle-phase transformation of SOA to low volatility. We adopt a quasi-Monte Carlo sampling approach to effectively sample the high-dimensional parameter space, and perform a 250 member ensemble of simulations using a regional model, accounting for some of the latest advances in SOA treatments based on our recent work. We then conduct a variance-based sensitivity analysis using the generalized linear model method to study the responses of simulated SOA loadings to the model parameters. Analysis of SOA variance from all 250 simulations shows that the volatility transformation parameter, which controls whether or not SOA that starts as semivolatile is rapidly transformed to nonvolatile SOA by particle-phase processes such as oligomerization and/or accretion, is the dominant contributor to variance of simulated surface-level daytime SOA (65% domain average contribution). We also split the simulations into two subsets of 125 each, depending on whether the volatility transformation is turned on/off. For each subset, the SOA variances are dominated by the parameters involving biogenic VOC and anthropogenic SIVOC emissions. Furthermore, biogenic VOC emissions have a larger contribution to SOA variance when the SOA transformation to nonvolatile is on, while anthropogenic SIVOC emissions have a larger contribution when the transformation is off. NOx contributes less than 4.3% to SOA variance, and this low contribution is mainly attributed to dominance of intermediate to high NOx conditions throughout the simulated domain. However, we note that SOA yields have a more complex nonlinear dependence on NOx levels, which needs to be addressed by more integrated model-measurement approaches focused on gaining a better process-level understanding of anthropogenic-biogenic interactions. The two parameters related to dry deposition of SOA precursor gases also have very low contributions to SOA variance. This study highlights the large sensitivity of SOA loadings to the particle-phase processes such as oligomerization that rapidly cause large decrease in the volatility of SOA, which is neglected in most previous models.