Estimates of global terrestrial isoprene emissions using MEGAN (Model of Emissions of Gases and Aerosols from Nature)

Reactive gases and aerosols are produced by terrestrial ecosystems, processed within plant canopies, and can then be emitted into the above-canopy atmosphere. Estimates of the above-canopy fluxes are needed for quantitative earth system studies and assessments of past, present and future air quality and climate. The Model of Emissions of Gases and Aerosols from Nature (MEGAN) is described and used to quantify net terrestrial biosphere emission of isoprene into the atmosphere. MEGAN is designed for both global and regional emission modeling and has global coverage with ~1 km² spatial resolution. Field and laboratory investigations of the processes controlling isoprene emission are described and data available for model development and evaluation are summarized. The factors controlling isoprene emissions include biological, physical and chemical driving variables. MEGAN driving variables are derived from models and satellite and ground observations. Tropical broadleaf trees contribute almost half of the estimated global annual isoprene emission due to their relatively high emission factors and because they are often exposed to conditions that are conducive for isoprene emission. The remaining flux is primarily from shrubs which have a widespread distribution. The annual global isoprene emission estimated with MEGAN ranges from about 500 to 750 Tg isoprene (440 to 660 Tg carbon) depending on the driving variables which include temperature, solar radiation, Leaf Area Index, and plant functional type. The global annual isoprene emission estimated using the standard driving variables is ~600 Tg isoprene. Differences in driving variables result in emission estimates that differ by more than a factor of three for specific times and locations. It is difficult to evaluate isoprene emission estimates using the concentration distributions simulated using chemistry and transport models, due to the substantial uncertainties in other model components, but at least some global models produce reasonable results when using isoprene emission distributions similar to MEGAN estimates. In addition, comparison with isoprene emissions estimated from satellite formaldehyde observations indicates reasonable agreement. The sensitivity of isoprene emissions to earth system changes (e.g., climate and land-use) demonstrates the potential for large future changes in emissions. Using temperature distributions simulated by global climate models for year 2100, MEGAN estimates that isoprene emissions increase by more than a factor of two. This is considerably greater than previous estimates and additional observations are needed to evaluate and improve the methods used to predict future isoprene emissions

Emissions of Volatile Organic Compounds Inferred From Airborne Flux Measurements over a Megacity

Toluene and benzene are used for assessing the ability to measure disjunct eddy covariance (DEC) fluxes of Volatile Organic Compounds (VOC) using Proton Transfer Reaction Mass Spectrometry (PTR-MS) on aircraft. Statistically significant correlation between vertical wind speed and mixing ratios suggests that airborne VOC eddy covariance (EC) flux measurements using PTR-MS are feasible. City-median midday toluene and benzene fluxes are calculated to be on the order of 14.1±4.0 mg/m<sup>2</sup>/h and 4.7±2.3 mg/m<sup>2</sup>/h, respectively. For comparison the adjusted CAM2004 emission inventory estimates toluene fluxes of 10 mg/m<sup>2</sup>/h along the footprint of the flight-track. Wavelet analysis of instantaneous toluene and benzene measurements during city overpasses is tested as a tool to assess surface emission heterogeneity. High toluene to benzene flux ratios above an industrial district (e.g. 10–15 g/g) including the International airport (e.g. 3–5 g/g) and a mean flux (concentration) ratio of 3.2±0.5 g/g (3.9±0.3 g/g) across Mexico City indicate that evaporative fuel and industrial emissions play an important role for the prevalence of aromatic compounds. Based on a tracer model, which was constrained by BTEX (BTEX– Benzene/Toluene/Ethylbenzene/m, p, o-Xylenes) compound concentration ratios, the fuel marker methyl-tertiary-butyl-ether (MTBE) and the biomass burning marker acetonitrile (CH<sub>3</sub>CN), we show that a combination of industrial, evaporative fuel, and exhaust emissions account for >87% of all BTEX sources. Our observations suggest that biomass burning emissions play a minor role for the abundance of BTEX compounds in the MCMA (2–13%)

Greenhouse gas and air pollutant emissions from power barges (powerships)

Power barges or powerships that operate on natural gas (NG) are an increasingly appealing easy-to-use solution to electricity deficits in Africa, Asia, the Middle East, and the Caribbean. Global generating capacity has increased from 0.1 to 2.6 GW, and 4.4 GW is under construction. South Africa has licensed three powerships to provide 1.2 GW generating capacity with foreign liquefied NG (LNG) over 20 years. To understand the importance of this source, we estimate lifecycle emissions of GHGs and air pollutants for South Africa and extend this to the global fleet. Annual lifecycle GHG emissions for 1.2 GW generating capacity total 2.6–3.8 Tg carbon dioxide equivalents (CO2e) using 100 year global warming potentials (GWPs). This increases to 4.0–7.1 Tg CO_{2} e using 20 year GWPs, due to the potency of fugitive methane (CH4). Adoption of air pollutant emission control technology will need to be enforced to achieve compliance with national standards for fine particles (PM) and nitrogen oxides (NO_{x}). A global fleet of 7.0 GW generating capacity reliant on domestic NG could emit 12 Tg CO_{2}, 2.2–8.6 Tg CH_{4}, 4.3 Gg NO_{x}, and 2.6 Gg PM. Additional NOx and SO2 emissions would result from imported LNG, as LNG tankers burn dirty fuel oil, though SO_{2} emissions may be curtailed with recent stricter limits on the fuel sulfur content. These powerships could have important regional impacts, but emission estimates are uncertain. Characteristic emission factors, detailed operating conditions, and NG composition data are urgently needed to address uncertainties in emissions for air quality and climate modelling of this emergent source

Burning of Traditional Biofuels and the Global Methane Budget

Understanding biomass burning is important for understanding atmospheric carbon budgets. The two main types of biomass burning are wildfires, and the burning of traditional biofuels. Both types of biomass burning produce methane, among other gases. Satellites in space help quantify area burned from wildfires. However, it is much more difficult to quantify the burning of traditional biofuels, such as animal waste, fuelwood, charcoal, crop material, and peat. Therefore, limited information is available regarding these process, and how they contribute to human caused climate change. Here, we have developed a global inventory of methane emissions from the burning of traditional biofuels over a 15 year time span. Using information from the UN Energy Statistics Database, a global upper estimate of annual methane emissions from this type of biomass burning was calculated for the years 2001-2015. The equation had three components: the final energy consumption of each of the considered sources, in each country, for each year; the relative smoldering or flaming of that material when burned; and the emission factor for methane gas specific to each material. Results show it is evident that on a global scale, methane emissions from the burning of traditional biofuels are increasing over this time span. In fact, the upper estimate calculated suggests a 17.5% increase in global methane emissions from 2001-2015 from this type of biomass burning. Charcoal production and consumption are leading these increases, specifically in tropical regions. By 2015, the estimate suggests 17.5 Tg of methane is being emitted from the burning of traditional biofuels. Thus, methane emissions from burning traditional biofuels are not negligible when evaluating the global methane budget. This information is crucial when assessing methane emissions from other sources such as microbial sources and fossil fuels

Estimation of mercury emissions from forest fires, lakes, regional and local sources using measurements in Milwaukee and an inverse method

Gaseous elemental mercury is a global pollutant that can lead to serious health concerns via deposition to the biosphere and bio-accumulation in the food chain. Hourly measurements between June 2004 and May 2005 in an urban site (Milwaukee, WI) show elevated levels of mercury in the atmosphere with numerous short-lived peaks as well as longer-lived episodes. The measurements are analyzed with an inverse model to obtain information about mercury emissions. The model is based on high resolution meteorological simulations (WRF), hourly back-trajectories (WRF-FLEXPART) and a chemical transport model (CAMx). The hybrid formulation combining back-trajectories and Eulerian simulations is used to identify potential source regions as well as the impacts of forest fires and lake surface emissions. Uncertainty bounds are estimated using a bootstrap method on the inversions. Comparison with the US Environmental Protection Agency's National Emission Inventory (NEI) and Toxic Release Inventory (TRI) shows that emissions from coal-fired power plants are properly characterized, but emissions from local urban sources, waste incineration and metal processing could be significantly under-estimated. Emissions from the lake surface and from forest fires were found to have significant impacts on mercury levels in Milwaukee, and to be underestimated by a factor of two or more

Emission factors for open and domestic biomass burning for use in atmospheric models

Biomass burning (BB) is the second largest source of trace gases and the largest source of primary fine carbonaceous particles in the global troposphere. Many recent BB studies have provided new emission factor (EF) measurements. This is especially true for non-methane organic compounds (NMOC), which influence secondary organic aerosol (SOA) and ozone formation. New EF should improve regional to global BB emissions estimates and therefore, the input for atmospheric models. In this work we present an up-to-date, comprehensive tabulation of EF for known pyrogenic species based on measurements made in smoke that has cooled to ambient temperature, but not yet undergone significant photochemical processing. All EFs are converted to one standard form (g compound emitted per kg dry biomass burned) using the carbon mass balance method and they are categorized into 14 fuel or vegetation types. Biomass burning terminology is defined to promote consistency. We compile a large number of measurements of biomass consumption per unit area for important fire types and summarize several recent estimates of global biomass consumption by the major types of biomass burning. Post emission processes are discussed to provide a context for the emission factor concept within overall atmospheric chemistry and also highlight the potential for rapid changes relative to the scale of some models or remote sensing products. Recent work shows that individual biomass fires emit significantly more gas-phase NMOC than previously thought and that including additional NMOC can improve photochemical model performance. A detailed global estimate suggests that BB emits at least 400 Tg yr^(−1) of gas-phase NMOC, which is almost 3 times larger than most previous estimates. Selected recent results (e.g. measurements of HONO and the BB tracers HCN and CH_3CN) are highlighted and key areas requiring future research are briefly discussed

Isolation of isoprene degrading bacteria from soils, development of isoA gene probes and identification of the active isoprene degrading soil community using DNA-stable isotope probing

Emissions of biogenic volatile organic compounds (bVOCs), are an important element in the global carbon cycle, accounting for a significant proportion of fixed carbon. They contribute directly and indirectly to global warming and climate change and have a major effect on atmospheric chemistry. Plants emit isoprene to the atmosphere in similar quantities to emissions of methane from all sources and each account for approximately one third of total VOCs. Although methanotrophs, capable of growth on methane, have been intensively studied, we know little of isoprene biodegradation. Here we report the isolation of two isoprene-degrading strains from the terrestrial environment and describe the design and testing of PCR primers targeting isoA, the gene encoding the active-site component of the conserved isoprene monooxygenase, which are capable of retrieving isoA sequences from isoprene-enriched environmental samples. Stable isotope probing experiments, using biosynthesized 13C-labelled isoprene, identified the active isoprene-degrading bacteria in soil. This study identifies novel isoprene-degrading strains using both culture-dependent and, for the first time, culture-independent methods and provides the tools and foundations for continued investigation of the biogeography and molecular ecology of isoprene-degrading bacteria. This article is protected by copyright. All rights reserved

A decadal satellite analysis of the origins and impacts of smoke in Colorado

We analyze the record of aerosol optical depth (AOD) measured by the MODerate resolution Imaging Spectroradiometer (MODIS) aboard the Terra satellite in combination with surface PM[subscript 2.5] to investigate the impact of fires on aerosol loading and air quality over Colorado from 2000 to 2012, and to evaluate the contribution of local versus transported smoke. Fire smoke contributed significantly to the AOD levels observed over Colorado. During the worst fire seasons of 2002 and 2012, average MODIS AOD over the Colorado Front Range corridor were 20–50% larger than the other 11 yr studied. Surface PM[subscript 2.5] was also unusually elevated during fire events and concentrations were in many occasions above the daily National Ambient Air Quality Standard (35 μg m[superscript −3]) and even reached locally unhealthy levels (> 100 μg m[superscript −3]) over populated areas during the 2012 High Park fire and the 2002 Hayman fire. Over the 13 yr examined, long-range transport of smoke from northwestern US and even California (> 1500 km distance) occurred often and affected AOD and surface PM[subscript 2.5]. During most of the transport events, MODIS AOD and surface PM[subscript 2.5] were reasonable correlated (r[superscript 2] = 0.2–0.9), indicating that smoke subsided into the Colorado boundary layer and reached surface levels. However, that is not always the case since at least one event of AOD enhancement was disconnected from the surface (r[superscript 2]<0.01 and low PM[subscript 2.5] levels). Observed plume heights from the Multi-angle Imaging SpectroRadiometer (MISR) satellite instrument and vertical aerosol profiles measured by the space-based Cloud-Aerosol LIdar with Orthogonal Polarization (CALIOP) showed a complex vertical distribution of smoke emitted by the High Park fire in 2012. Smoke was detected from a range of 1.5 to 7.5 km altitude at the fire origin and from ground levels to 12.3 km altitude far away from the source. The variability of smoke altitude as well as the local meteorology were key in determining the aerosol loading and air quality over the Colorado Front Range region. Our results underline the importance of accurate characterization of the vertical distribution of smoke for estimating the air quality degradation associated with fire activity and its link to human health.United States. National Park Service (Grant H2370 094000/J2350103006

Aerosol microphysical impact on summertime convective precipitation in the Rocky Mountain region

We present an aerosol-cloud-precipitation modeling study of convective clouds using the Weather Research and Forecasting model fully coupled with Chemistry (WRF-Chem) version 3.1.1. Comparison of the model output with measurements from a research site in the Rocky Mountains in Colorado revealed that the fraction of organics in the model is underpredicted. This is most likely due to missing processes in the aerosol module in the model version used, such as new particle formation and growth of secondary organic aerosols. When boundary conditions and domain-wide initial conditions of aerosol loading are changed in the model (factors of 0.1, 0.2, and 10 of initial aerosol mass of SO4-2, NH4+, and NO3-), the domain-wide precipitation changes by about 5%. Analysis of the model results reveals that the Rocky Mountain region and Front Range environment is not conducive for convective invigoration to play a major role, in increasing precipitation, as seen in some other studies. When localized organic aerosol emission are increased to mimic new particle formation, the resulting increased aerosol loading leads to increases in domain-wide precipitation, opposite to what is seen in the model simulations with changed boundary and initial conditions