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Monthly trends of methane emissions in Los Angeles from 2011 to 2015 inferred by CLARS-FTS observations

Description

This paper presents an analysis of methane emissions from the Los Angeles Basin at monthly timescales across a 4-year time period – from September 2011 to August 2015. Using observations

This paper presents an analysis of methane emissions from the Los Angeles Basin at monthly timescales across a 4-year time period – from September 2011 to August 2015. Using observations acquired by a ground-based near-infrared remote sensing instrument on Mount Wilson, California, combined with atmospheric CH[subscript 4]–CO[subscript 2] tracer–tracer correlations, we observed −18 to +22 % monthly variability in CH[subscript 4] : CO[subscript 2] from the annual mean in the Los Angeles Basin. Top-down estimates of methane emissions for the basin also exhibit significant monthly variability (−19 to +31 % from annual mean and a maximum month-to-month change of 47 %). During this period, methane emissions consistently peaked in the late summer/early fall and winter. The estimated annual methane emissions did not show a statistically significant trend over the 2011 to 2015 time period.

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  • 2016-10-26

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Global atmospheric carbon budget: results from an ensemble of atmospheric CO2 inversions

Description

Atmospheric CO[subscript 2] inversions estimate surface carbon fluxes from an optimal fit to atmospheric CO[subscript 2] measurements, usually including prior constraints on the flux estimates. Eleven sets of carbon flux

Atmospheric CO[subscript 2] inversions estimate surface carbon fluxes from an optimal fit to atmospheric CO[subscript 2] measurements, usually including prior constraints on the flux estimates. Eleven sets of carbon flux estimates are compared, generated by different inversions systems that vary in their inversions methods, choice of atmospheric data, transport model and prior information. The inversions were run for at least 5 yr in the period between 1990 and 2010. Mean fluxes for 2001–2004, seasonal cycles, interannual variability and trends are compared for the tropics and northern and southern extra-tropics, and separately for land and ocean. Some continental/basin-scale subdivisions are also considered where the atmospheric network is denser. Four-year mean fluxes are reasonably consistent across inversions at global/latitudinal scale, with a large total (land plus ocean) carbon uptake in the north (−3.4 Pg C yr[superscript −1] (±0.5 Pg C yr[superscript −1] standard deviation), with slightly more uptake over land than over ocean), a significant although more variable source over the tropics (1.6 ± 0.9 Pg C yr[superscript −1]) and a compensatory sink of similar magnitude in the south (−1.4 ± 0.5 Pg C yr[superscript −1]) corresponding mainly to an ocean sink. Largest differences across inversions occur in the balance between tropical land sources and southern land sinks. Interannual variability (IAV) in carbon fluxes is larger for land than ocean regions (standard deviation around 1.06 versus 0.33 Pg C yr[superscript −1] for the 1996–2007 period), with much higher consistency among the inversions for the land. While the tropical land explains most of the IAV (standard deviation ~ 0.65 Pg C yr[superscript −1]), the northern and southern land also contribute (standard deviation ~ 0.39 Pg C yr[superscript −1]). Most inversions tend to indicate an increase of the northern land carbon uptake from late 1990s to 2008 (around 0.1 Pg C yr[superscript −1], predominantly in North Asia. The mean seasonal cycle appears to be well constrained by the atmospheric data over the northern land (at the continental scale), but still highly dependent on the prior flux seasonality over the ocean. Finally we provide recommendations to interpret the regional fluxes, along with the uncertainty estimates.

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Date Created
  • 2013-10-24

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Sensitivity of simulated CO2 concentration to sub-annual variations in fossil fuel CO2 emissions

Description

Recent advances in fossil fuel CO[subscript 2] (FFCO[subscript 2]) emission inventories enable sensitivity tests of simulated atmospheric CO[subscript 2] concentrations to sub-annual variations in FFCO[subscript 2] emissions and what this

Recent advances in fossil fuel CO[subscript 2] (FFCO[subscript 2]) emission inventories enable sensitivity tests of simulated atmospheric CO[subscript 2] concentrations to sub-annual variations in FFCO[subscript 2] emissions and what this implies for the interpretation of observed CO[subscript 2]. Six experiments are conducted to investigate the potential impact of three cycles of FFCO[subscript 2] emission variability (diurnal, weekly and monthly) using a global tracer transport model. Results show an annual FFCO[subscript 2] rectification varying from −1.35 to +0.13 ppm from the combination of all three cycles. This rectification is driven by a large negative diurnal FFCO[subscript 2] rectification due to the covariation of diurnal FFCO[subscript 2] emissions and diurnal vertical mixing, as well as a smaller positive seasonal FFCO[subscript 2] rectification driven by the covariation of monthly FFCO[subscript 2] emissions and monthly atmospheric transport. The diurnal FFCO[subscript 2] emissions are responsible for a diurnal FFCO[subscript 2] concentration amplitude of up to 9.12 ppm at the grid cell scale. Similarly, the monthly FFCO[subscript 2] emissions are responsible for a simulated seasonal CO[subscript 2] amplitude of up to 6.11 ppm at the grid cell scale. The impact of the diurnal FFCO[subscript 2] emissions, when only sampled in the local afternoon, is also important, causing an increase of +1.13 ppmv at the grid cell scale. The simulated CO[subscript 2] concentration impacts from the diurnally and seasonally varying FFCO[subscript 2] emissions are centered over large source regions in the Northern Hemisphere, extending to downwind regions. This study demonstrates the influence of sub-annual variations in FFCO[subscript 2] emissions on simulated CO[subscript 2] concentration and suggests that inversion studies must take account of these variations in the affected regions.

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  • 2016-02-19

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Toward reduced transport errors in a high resolution urban CO2 inversion system

Description

We present a high-resolution atmospheric inversion system combining a Lagrangian Particle Dispersion Model (LPDM) and the Weather Research and Forecasting model (WRF), and test the impact of assimilating meteorological observation

We present a high-resolution atmospheric inversion system combining a Lagrangian Particle Dispersion Model (LPDM) and the Weather Research and Forecasting model (WRF), and test the impact of assimilating meteorological observation on transport accuracy. A Four Dimensional Data Assimilation (FDDA) technique continuously assimilates meteorological observations from various observing systems into the transport modeling system, and is coupled to the high resolution CO[subscript 2] emission product Hestia to simulate the atmospheric mole fractions of CO[subscript 2]. For the Indianapolis Flux Experiment (INFLUX) project, we evaluated the impact of assimilating different meteorological observation systems on the linearized adjoint solutions and the CO[subscript 2] inverse fluxes estimated using observed CO[subscript 2] mole fractions from 11 out of 12 communications towers over Indianapolis for the Sep.-Nov. 2013 period. While assimilating WMO surface measurements improved the simulated wind speed and direction, their impact on the planetary boundary layer (PBL) was limited. Simulated PBL wind statistics improved significantly when assimilating upper-air observations from the commercial airline program Aircraft Communications Addressing and Reporting System (ACARS) and continuous ground-based Doppler lidar wind observations. Wind direction mean absolute error (MAE) decreased from 26 to 14 degrees and the wind speed MAE decreased from 2.0 to 1.2 m s[subscript –1], while the bias remains small in all configurations (< 6 degrees and 0.2 m s[subscript –1]). Wind speed MAE and ME are larger in daytime than in nighttime. PBL depth MAE is reduced by ~10%, with little bias reduction. The inverse results indicate that the spatial distribution of CO[subscript 2] inverse fluxes were affected by the model performance while the overall flux estimates changed little across WRF simulations when aggregated over the entire domain. Our results show that PBL wind observations are a potent tool for increasing the precision of urban meteorological reanalyses, but that the impact on inverse flux estimates is dependent on the specific urban environment.

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Created

Date Created
  • 2017-05-23

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Los Angeles megacity: a high-resolution land–atmosphere modelling system for urban CO2 emissions

Description

Megacities are major sources of anthropogenic fossil fuel CO[subscript 2] (FFCO[subscript 2]) emissions. The spatial extents of these large urban systems cover areas of 10 000 km[superscript 2] or more with complex

Megacities are major sources of anthropogenic fossil fuel CO[subscript 2] (FFCO[subscript 2]) emissions. The spatial extents of these large urban systems cover areas of 10 000 km[superscript 2] or more with complex topography and changing landscapes. We present a high-resolution land–atmosphere modelling system for urban CO[subscript 2] emissions over the Los Angeles (LA) megacity area. The Weather Research and Forecasting (WRF)-Chem model was coupled to a very high-resolution FFCO[subscript 2] emission product, Hestia-LA, to simulate atmospheric CO[subscript 2] concentrations across the LA megacity at spatial resolutions as fine as  ∼  1 km. We evaluated multiple WRF configurations, selecting one that minimized errors in wind speed, wind direction, and boundary layer height as evaluated by its performance against meteorological data collected during the CalNex-LA campaign (May–June 2010). Our results show no significant difference between moderate-resolution (4 km) and high-resolution (1.3 km) simulations when evaluated against surface meteorological data, but the high-resolution configurations better resolved planetary boundary layer heights and vertical gradients in the horizontal mean winds. We coupled our WRF configuration with the Vulcan 2.2 (10 km resolution) and Hestia-LA (1.3 km resolution) fossil fuel CO[subscript 2] emission products to evaluate the impact of the spatial resolution of the CO[subscript 2] emission products and the meteorological transport model on the representation of spatiotemporal variability in simulated atmospheric CO[subscript 2] concentrations. We find that high spatial resolution in the fossil fuel CO[subscript 2] emissions is more important than in the atmospheric model to capture CO[subscript 2] concentration variability across the LA megacity. Finally, we present a novel approach that employs simultaneous correlations of the simulated atmospheric CO[subscript 2] fields to qualitatively evaluate the greenhouse gas measurement network over the LA megacity. Spatial correlations in the atmospheric CO[subscript 2] fields reflect the coverage of individual measurement sites when a statistically significant number of sites observe emissions from a specific source or location. We conclude that elevated atmospheric CO[subscript 2] concentrations over the LA megacity are composed of multiple fine-scale plumes rather than a single homogenous urban dome. Furthermore, we conclude that FFCO[subscript 2] emissions monitoring in the LA megacity requires FFCO[subscript 2] emissions modelling with  ∼  1 km resolution because coarser-resolution emissions modelling tends to overestimate the observational constraints on the emissions estimates.

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Date Created
  • 2016-07-22

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Toward consistency between trends in bottom-up CO2 emissions and top-down atmospheric measurements in the Los Angeles megacity

Description

Large urban emissions of greenhouse gases result in large atmospheric enhancements relative to background that are easily measured. Using CO[subscript 2] mole fractions and Δ[superscript 14]C and δ[superscript 13]C values

Large urban emissions of greenhouse gases result in large atmospheric enhancements relative to background that are easily measured. Using CO[subscript 2] mole fractions and Δ[superscript 14]C and δ[superscript 13]C values of CO[subscript 2] in the Los Angeles megacity observed in inland Pasadena (2006–2013) and coastal Palos Verdes peninsula (autumn 2009–2013), we have determined time series for CO[subscript 2] contributions from fossil fuel combustion (C[subscript ff]) for both sites and broken those down into contributions from petroleum and/or gasoline and natural gas burning for Pasadena. We find a 10 % reduction in Pasadena C[subscript ff] during the Great Recession of 2008–2010, which is consistent with the bottom-up inventory determined by the California Air Resources Board. The isotopic variations and total atmospheric CO[subscript 2] from our observations are used to infer seasonality of natural gas and petroleum combustion. The trend of CO[subscript 2] contributions to the atmosphere from natural gas combustion is out of phase with the seasonal cycle of total natural gas combustion seasonal patterns in bottom-up inventories but is consistent with the seasonality of natural gas usage by the area's electricity generating power plants. For petroleum, the inferred seasonality of CO[subscript 2] contributions from burning petroleum is delayed by several months relative to usage indicated by statewide gasoline taxes. Using the high-resolution Hestia-LA data product to compare C[subscript ff] from parts of the basin sampled by winds at different times of year, we find that variations in observed fossil fuel CO[subscript 2] reflect seasonal variations in wind direction. The seasonality of the local CO[subscript 2] excess from fossil fuel combustion along the coast, on Palos Verdes peninsula, is higher in autumn and winter than spring and summer, almost completely out of phase with that from Pasadena, also because of the annual variations of winds in the region. Variations in fossil fuel CO[subscript 2] signals are consistent with sampling the bottom-up Hestia-LA fossil CO[subscript 2] emissions product for sub-city source regions in the LA megacity domain when wind directions are considered.

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Date Created
  • 2016-03-22

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Constraints on emissions of carbon monoxide, methane, and a suite of hydrocarbons in the Colorado Front Range using observations of 14CO2

Description

Atmospheric radiocarbon ([superscript 14]C) represents an important observational constraint on emissions of fossil-fuel derived carbon into the atmosphere due to the absence of [superscript 14]C in fossil fuel reservoirs. The

Atmospheric radiocarbon ([superscript 14]C) represents an important observational constraint on emissions of fossil-fuel derived carbon into the atmosphere due to the absence of [superscript 14]C in fossil fuel reservoirs. The high sensitivity and precision that accelerator mass spectrometry (AMS) affords in atmospheric [superscript 14]C analysis has greatly increased the potential for using such measurements to evaluate bottom-up emissions inventories of fossil fuel CO[subscript 2] (CO[subscript 2]ff), as well as those for other co-emitted species. Here we use observations of [superscript 14]CO[subscript 2] and a series of primary hydrocarbons and combustion tracers from discrete air samples collected between June 2009 and September 2010 at the National Oceanic and Atmospheric Administration Boulder Atmospheric Observatory (BAO; Lat: 40.050° N, Lon: 105.004° W) to derive emission ratios of each species with respect to CO[subscript 2]ff. The BAO tower is situated at the boundary of the Denver metropolitan area to the south and a large industrial and agricultural region to the north and east, making it an ideal location to study the contrasting mix of emissions from the activities in each region. The species considered in this analysis are carbon monoxide (CO), methane (CH[subscript 4]), acetylene (C[subscript 2]H[subscript 2]), benzene (C[subscript 6]H[subscript 6]), and C[subscript 3]–C[subscript 5] alkanes. We estimate emissions for a subset of these species by using the Vulcan high resolution CO2ff emission data product as a reference. We find that CO is overestimated in the 2008 National Emissions Inventory (NEI08) by a factor of ~2. A close evaluation of the inventory suggests that the ratio of CO emitted per unit fuel burned from on-road gasoline vehicles is likely over-estimated by a factor of 2.5. Using a wind-directional analysis of the data, we find enhanced concentrations of CH[subscript 4], relative to CO[subscript 2]ff, in air influenced by emissions to the north and east of the BAO tower when compared to air influenced by emissions in the Denver metro region to the south. Along with enhanced CH[subscript 4], the strongest enhancements of the C[subscript 3]–C[subscript 5] alkanes are also found in the north and east wind sector, suggesting that both the alkane and CH[subscript 4] enhancements are sourced from oil and gas fields located to the northeast, though it was not possible to rule out the contribution of non oil and gas CH[subscript 4] sources.

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Date Created
  • 2013-11-15

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The Indianapolis Flux Experiment (INFLUX): A test-bed for developing urban greenhouse gas emission measurements

Description

The objective of the Indianapolis Flux Experiment (INFLUX) is to develop, evaluate and improve methods for measuring greenhouse gas (GHG) emissions from cities. INFLUX’s scientific objectives are to quantify CO[subscript

The objective of the Indianapolis Flux Experiment (INFLUX) is to develop, evaluate and improve methods for measuring greenhouse gas (GHG) emissions from cities. INFLUX’s scientific objectives are to quantify CO[subscript 2] and CH[subscript 4] emission rates at 1 km[subscript 2] resolution with a 10% or better accuracy and precision, to determine whole-city emissions with similar skill, and to achieve high (weekly or finer) temporal resolution at both spatial resolutions. The experiment employs atmospheric GHG measurements from both towers and aircraft, atmospheric transport observations and models, and activity-based inventory products to quantify urban GHG emissions. Multiple, independent methods for estimating urban emissions are a central facet of our experimental design. INFLUX was initiated in 2010 and measurements and analyses are ongoing. To date we have quantified urban atmospheric GHG enhancements using aircraft and towers with measurements collected over multiple years, and have estimated whole-city CO[subscript 2] and CH[subscript 4] emissions using aircraft and tower GHG measurements, and inventory methods. Significant differences exist across methods; these differences have not yet been resolved; research to reduce uncertainties and reconcile these differences is underway. Sectorally- and spatially-resolved flux estimates, and detection of changes of fluxes over time, are also active research topics. Major challenges include developing methods for distinguishing anthropogenic from biogenic CO[subscript 2] fluxes, improving our ability to interpret atmospheric GHG measurements close to urban GHG sources and across a broader range of atmospheric stability conditions, and quantifying uncertainties in inventory data products. INFLUX data and tools are intended to serve as an open resource and test bed for future investigations. Well-documented, public archival of data and methods is under development in support of this objective.

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Created

Date Created
  • 2017-05-23

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Assessment of uncertainties of an aircraft-based mass balance approach for quantifying urban greenhouse gas emissions

Description

Urban environments are the primary contributors to global anthropogenic carbon emissions. Because much of the growth in CO[subscript 2] emissions will originate from cities, there is a need to develop,

Urban environments are the primary contributors to global anthropogenic carbon emissions. Because much of the growth in CO[subscript 2] emissions will originate from cities, there is a need to develop, assess, and improve measurement and modeling strategies for quantifying and monitoring greenhouse gas emissions from large urban centers. In this study the uncertainties in an aircraft-based mass balance approach for quantifying carbon dioxide and methane emissions from an urban environment, focusing on Indianapolis, IN, USA, are described. The relatively level terrain of Indianapolis facilitated the application of mean wind fields in the mass balance approach. We investigate the uncertainties in our aircraft-based mass balance approach by (1) assessing the sensitivity of the measured flux to important measurement and analysis parameters including wind speed, background CO[subscript 2] and CH[subscript 4], boundary layer depth, and interpolation technique, and (2) determining the flux at two or more downwind distances from a point or area source (with relatively large source strengths such as solid waste facilities and a power generating station) in rapid succession, assuming that the emission flux is constant. When we quantify the precision in the approach by comparing the estimated emissions derived from measurements at two or more downwind distances from an area or point source, we find that the minimum and maximum repeatability were 12 and 52%, with an average of 31%. We suggest that improvements in the experimental design can be achieved by careful determination of the background concentration, monitoring the evolution of the boundary layer through the measurement period, and increasing the number of downwind horizontal transect measurements at multiple altitudes within the boundary layer.

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Date Created
  • 2014-09-02

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Reconciling the differences between a bottom-up and inverse-estimated FFCO2 emissions estimate in a large US urban area

Description

The INFLUX experiment has taken multiple approaches to estimate the carbon dioxide (CO[subscript 2]) flux in a domain centered on the city of Indianapolis, Indiana. One approach, Hestia, uses a

The INFLUX experiment has taken multiple approaches to estimate the carbon dioxide (CO[subscript 2]) flux in a domain centered on the city of Indianapolis, Indiana. One approach, Hestia, uses a bottom-up technique relying on a mixture of activity data, fuel statistics, direct flux measurement and modeling algorithms. A second uses a Bayesian atmospheric inverse approach constrained by atmospheric CO[subscript 2] measurements and the Hestia emissions estimate as a prior CO[subscript 2] flux. The difference in the central estimate of the two approaches comes to 0.94 MtC (an 18.7% difference) over the eight-month period between September 1, 2012 and April 30, 2013, a statistically significant difference at the 2-sigma level. Here we explore possible explanations for this apparent discrepancy in an attempt to reconcile the flux estimates. We focus on two broad categories: 1) biases in the largest of bottom-up flux contributions and 2) missing CO[subscript 2] sources. Though there is some evidence for small biases in the Hestia fossil fuel carbon dioxide (FFCO2) flux estimate as an explanation for the calculated difference, we find more support for missing CO[subscript 2] fluxes, with biological respiration the largest of these. Incorporation of these differences bring the Hestia bottom-up and the INFLUX inversion flux estimates into statistical agreement and are additionally consistent with wintertime measurements of atmospheric [superscript 14]CO[subscript 2]. We conclude that comparison of bottom-up and top-down approaches must consider all flux contributions and highlight the important contribution to urban carbon budgets of animal and biotic respiration. Incorporation of missing CO[subscript 2] fluxes reconciles the bottom-up and inverse-based approach in the INFLUX domain.

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Created

Date Created
  • 2017-08-03