8 research outputs found
Global Sensitivity Analysis of the Regional Atmospheric Chemical Mechanism: An Application of Random Sampling-High Dimensional Model Representation to Urban Oxidation Chemistry
Chemical mechanisms play a crucial part for the air quality
modeling
and pollution control decision-making. Parameters in a chemical mechanism
have uncertainties, leading to the uncertainties of model predictions.
A recently developed global sensitivity analysis (SA) method based
on Random Sampling-High Dimensional Model Representation (RS-HDMR)
was applied to the Regional Atmospheric Chemical Mechanism (RACM)
within a zero-dimensional photochemical model to highlight the main
uncertainty sources of atmospheric hydroxyl (OH) and hydroperoxyl
(HO<sub>2</sub>) radicals. This global SA approach can be applied
as a routine in zero-dimensional photochemical modeling to comprehensively
assess model uncertainty and sensitivity under different conditions.
It also highlights the parameters to which the model is most sensitive
during periods when the model/measurement OH and HO<sub>2</sub> discrepancies
are greatest. Uncertainties in 584 model parameters were assigned
for measured constituents used to constrain the model, for photolysis
and kinetic rate coefficients, and for product yields of the reactions.
With simulations performed for the hourly field data of two typical
days, modeled and measured OH and HO<sub>2</sub> generally agree better
for polluted conditions than for cleaner conditions, except during
morning rush hour. Sensitivity analysis shows that the modeled OH
and HO<sub>2</sub> depend most critically on the reactions of xylenes
and isoprene with OH, NO<sub>2</sub> with OH, NO with HO<sub>2</sub>, and internal alkenes with O<sub>3</sub> and suggests that model/measurement
discrepancies in OH and HO<sub>2</sub> would benefit from a closer
examination of these reactions
Application of the Carbon Balance Method to Flare Emissions Characteristics
The destruction and removal efficiency (DRE) computation
of target
hydrocarbon species in the flaring process is derived using carbon
balance methodologies. This analysis approach is applied to data acquired
during the Texas Commission on Environmental Quality 2010 Flare Study.
Example DRE calculations are described and discussed. Carbon balance
is achieved to within 2% for the analysis of flare vent gases. Overall
method uncertainty is evaluated and examined together with apparent
variability in flare combustion performance. Using fast response direct
sampling measurements to characterize flare combustion parameters
is sufficiently accurate to produce performance curves on a large-scale
industrial flare operating at low vent gas flow rates
Direct Measurements Show Decreasing Methane Emissions from Natural Gas Local Distribution Systems in the United States
Fugitive losses from natural gas
distribution systems are a significant
source of anthropogenic methane. Here, we report on a national sampling
program to measure methane emissions from 13 urban distribution systems
across the U.S. Emission factors were derived from direct measurements
at 230 underground pipeline leaks and 229 metering and regulating
facilities using stratified random sampling. When these new emission
factors are combined with estimates for customer meters, maintenance,
and upsets, and current pipeline miles and numbers of facilities,
the total estimate is 393 Gg/yr with a 95% upper confidence limit
of 854 Gg/yr (0.10% to 0.22% of the methane delivered nationwide).
This fraction includes emissions from city gates to the customer meter,
but does not include other urban sources or those downstream of customer
meters. The upper confidence limit accounts for the skewed distribution
of measurements, where a few large emitters accounted for most of
the emissions. This emission estimate is 36% to 70% less than the
2011 EPA inventory, (based largely on 1990s emission data), and reflects
significant upgrades at metering and regulating stations, improvements
in leak detection and maintenance activities, as well as potential
effects from differences in methodologies between the two studies
Atmospheric Emission Characterization of Marcellus Shale Natural Gas Development Sites
Limited direct measurements of criteria
pollutants emissions and
precursors, as well as natural gas constituents, from Marcellus shale
gas development activities contribute to uncertainty about their atmospheric
impact. Real-time measurements were made with the Aerodyne Research
Inc. Mobile Laboratory to characterize emission rates of atmospheric
pollutants. Sites investigated include production well pads, a well
pad with a drill rig, a well completion, and compressor stations.
Tracer release ratio methods were used to estimate emission rates.
A first-order correction factor was developed to account for errors
introduced by fenceline tracer release. In contrast to observations
from other shale plays, elevated volatile organic compounds, other
than CH<sub>4</sub> and C<sub>2</sub>H<sub>6</sub>, were generally
not observed at the investigated sites. Elevated submicrometer particle
mass concentrations were also generally not observed. Emission rates
from compressor stations ranged from 0.006 to 0.162 tons per day (tpd)
for NO<sub><i>x</i></sub>, 0.029 to 0.426 tpd for CO, and
67.9 to 371 tpd for CO<sub>2</sub>. CH<sub>4</sub> and C<sub>2</sub>H<sub>6</sub> emission rates from compressor stations ranged from
0.411 to 4.936 tpd and 0.023 to 0.062 tpd, respectively. Although
limited in sample size, this study provides emission rate estimates
for some processes in a newly developed natural gas resource and contributes
valuable comparisons to other shale gas studies
Combustion and Destruction/Removal Efficiencies of In-Use Chemical Flares in the Greater Houston Area
Alkene emissions from the petrochemical industry contribute
significantly
to ozone production in the greater Houston area but are underestimated
in emission inventories. It is not well-known which processes (e.g.,
fugitive emissions, chemical flare emissions, etc.) are responsible
for these underreported emissions. We use fast time response and ground-based
mobile measurements of numerous trace gas species to characterize
alkene plumes from three identified chemical flares in the greater
Houston area. We calculate the combustion efficiency and destruction
and removal efficiency (DRE) values of these flares using the carbon
balance method. All three flares were operating at DRE values lower
than required by regulation. An examination of photochemistry in flare
exhaust plumes indicates that the impact of direct formaldehyde emissions
from flares on ozone formation is small as compared to the impact
of alkene emissions
Chemical Compositions of Black Carbon Particle Cores and Coatings via Soot Particle Aerosol Mass Spectrometry with Photoionization and Electron Ionization
Black
carbon is an important constituent of atmospheric aerosol
particle matter (PM) with significant effects on the global radiation
budget and on human health. The soot particle aerosol mass spectrometer
(SP-AMS) has been developed and deployed for real-time ambient measurements
of refractory carbon particles. In the SP-AMS, black carbon or metallic
particles are vaporized through absorption of 1064 nm light from a
CW Nd:YAG laser. This scheme allows for continuous āsoftā
vaporization of both core and coating materials. The main focus of
this work is to characterize the extent to which this vaporization
scheme provides enhanced chemical composition information about aerosol
particles. This information is difficult to extract from standard
SP-AMS mass spectra because they are complicated by extensive fragmentation
from the harsh 70 eV EI ionization scheme that is typically used in
these instruments. Thus, in this work synchotron-generated vacuum
ultraviolet (VUV) light in the 8ā14 eV range is used to measure
VUV-SP-AMS spectra with minimal fragmentation. VUV-SP-AMS spectra
of commercially available carbon black, fullerene black, and laboratory
generated flame soots were obtained. Small carbon cluster cations
(C<sup>+</sup>āC<sub>5</sub><sup>+</sup>) were found to dominate
the VUV-SP-AMS spectra of all the samples, indicating that the corresponding
neutral clusters are key products of the SP vaporization process.
Intercomparisons of carbon cluster ratios observed in VUV-SP-AMS and
SP-AMS spectra are used to confirm spectral features that could be
used to distinguish between different types of refractory carbon particles.
VUV-SP-AMS spectra of oxidized organic species adsorbed on absorbing
cores are also examined and found to display less thermally induced
decomposition and fragmentation than spectra obtained with thermal
vaporization at 200 Ā°C (the minimum temperature needed to quantitatively
vaporize ambient oxidized organic aerosol with a continuously heated
surface). The particle cores tested in these studies include black
carbon, silver, gold, and platinum nanoparticles. These results demonstrate
that SP vaporization is capable of providing enhanced organic chemical
composition information for a wide range of organic coating materials
and IR absorbing particle cores. The potential of using this technique
to study organic species of interest in seeded laboratory chamber
or flow reactor studies is discussed
Goetz_etal_2017_data
The zip file holds six tab delimited text files that contain 1 Hz mobile lab data and accompanying local background estimates from 2012 measurements in SW PA and NE PA and 2015 measurements in NE PA. See publication for more details. The provided local background data is derived from 20 minute 35th percentile smoothing. All time information is in military format. Date information is in mm/dd/yy. Concentration data is in units of ppbv. GPS data is in units of decimal degrees and in datum WGS84. Please contact authors if additional information is needed. All use of the provided data should be accompanied by the proper citations
Demonstration of an Ethane Spectrometer for Methane Source Identification
Methane
is an important greenhouse gas and tropospheric ozone precursor.
Simultaneous observation of ethane with methane can help identify
specific methane source types. Aerodyne Ethane-Mini spectrometers,
employing recently available mid-infrared distributed feedback tunable
diode lasers (DFB-TDL), provide 1 s ethane measurements with sub-ppb
precision. In this work, an Ethane-Mini spectrometer has been integrated
into two mobile sampling platforms, a ground vehicle and a small airplane,
and used to measure ethane/methane enhancement ratios downwind of
methane sources. Methane emissions with precisely known sources are
shown to have ethane/methane enhancement ratios that differ greatly
depending on the source type. Large differences between biogenic and
thermogenic sources are observed. Variation within thermogenic sources
are detected and tabulated. Methane emitters are classified by their
expected ethane content. Categories include the following: biogenic
(<0.2%), dry gas (1ā6%), wet gas (>6%), pipeline grade
natural
gas (<15%), and processed natural gas liquids (>30%). Regional
scale observations in the Dallas/Fort Worth area of Texas show two
distinct ethane/methane enhancement ratios bridged by a transitional
region. These results demonstrate the usefulness of continuous and
fast ethane measurements in experimental studies of methane emissions,
particularly in the oil and natural gas sector