31 research outputs found
The use of disjunct eddy sampling methods for the determination of ecosystem level fluxes of trace gases
The concept of disjunct eddy sampling (DES)
for use in measuring ecosystem-level micrometeorological
fluxes is re-examined. The governing equations are discussed
as well as other practical considerations and guidelines concerning
this sampling method as it is applied to either the
disjunct eddy covariance (DEC) or disjunct eddy accumulation
(DEA) techniques. A disjunct eddy sampling system
was constructed that could either be combined with relatively
slow sensors (response time of 2 to 40 s) to measure
fluxes using DEC, or could also be used to accumulate samples
in stable reservoirs for later laboratory analysis (DEA
technique). Both the DEC and DEA modes of this sampler
were tested against conventional eddy covariance (EC) for
fluxes of either CO2 (DEC) or isoprene (DEA). Good agreement
in both modes was observed relative to the EC systems.
However, the uncertainty in a single DEA flux measurement
was considerable (40%) due to both the reduced statistical
sampling and the analytical precision of the concentration
difference measurements. We have also re-investigated
the effects of nonzero mean vertical wind velocity on accumulation
techniques as it relates to our DEA measurements.
Despite the higher uncertainty, disjunct eddy sampling can
provide an alternative technique to eddy covariance for determining
ecosystem-level fluxes for species where fast sensors
do not currently exist
A steady-state continuous flow chamber for the study of daytime and nighttime chemistry under atmospherically relevant NO levels
Experiments performed in laboratory chambers have contributed significantly to the understanding of the fundamental kinetics and mechanisms of the chemical reactions occurring in the atmosphere. Two chemical regimes, classified as high-NO vs. zero-NO conditions, have been extensively studied in previous chamber experiments. Results derived from these two chemical scenarios are widely parameterized in chemical transport models to represent key atmospheric processes in urban and pristine environments. As the anthropogenic NO_x emissions in the United States have decreased remarkably in the past few decades, the classic high-NO and zero-NO conditions are no longer applicable to many regions that are constantly impacted by both polluted and background air masses. We present here the development and characterization of the NCAR Atmospheric Simulation Chamber, which is operated in steady-state continuous flow mode for the study of atmospheric chemistry under intermediate NO conditions. This particular chemical regime is characterized by constant sub-ppb levels of NO and can be created in the chamber by precise control of the inflow NO concentration and the ratio of chamber mixing to residence timescales. Over the range of conditions achievable in the chamber, the lifetime of peroxy radicals (RO_2), a key intermediate from the atmospheric degradation of volatile organic compounds (VOCs), can be extended to several minutes, and a diverse array of reaction pathways, including unimolecular pathways and bimolecular reactions with NO and HO_2, can thus be explored. Characterization experiments under photolytic and dark conditions were performed and, in conjunction with model predictions, provide a basis for interpretation of prevailing atmospheric processes in environments with intertwined biogenic and anthropogenic activities. We demonstrate the proof of concept of the steady-state continuous flow chamber operation through measurements of major first-generation products, methacrolein (MACR) and methyl vinyl ketone (MVK), from OH- and NO_3- initiated oxidation of isoprene
A steady-state continuous flow chamber for the study of daytime and nighttime chemistry under atmospherically relevant NO levels
Experiments performed in laboratory chambers have contributed significantly to the understanding of the fundamental kinetics and mechanisms of the chemical reactions occurring in the atmosphere. Two chemical regimes, classified as high-NO vs. zero-NO conditions, have been extensively studied in previous chamber experiments. Results derived from these two chemical scenarios are widely parameterized in chemical transport models to represent key atmospheric processes in urban and pristine environments. As the anthropogenic NO_x emissions in the United States have decreased remarkably in the past few decades, the classic high-NO and zero-NO conditions are no longer applicable to many regions that are constantly impacted by both polluted and background air masses. We present here the development and characterization of the NCAR Atmospheric Simulation Chamber, which is operated in steady-state continuous flow mode for the study of atmospheric chemistry under intermediate NO conditions. This particular chemical regime is characterized by constant sub-ppb levels of NO and can be created in the chamber by precise control of the inflow NO concentration and the ratio of chamber mixing to residence timescales. Over the range of conditions achievable in the chamber, the lifetime of peroxy radicals (RO_2), a key intermediate from the atmospheric degradation of volatile organic compounds (VOCs), can be extended to several minutes, and a diverse array of reaction pathways, including unimolecular pathways and bimolecular reactions with NO and HO_2, can thus be explored. Characterization experiments under photolytic and dark conditions were performed and, in conjunction with model predictions, provide a basis for interpretation of prevailing atmospheric processes in environments with intertwined biogenic and anthropogenic activities. We demonstrate the proof of concept of the steady-state continuous flow chamber operation through measurements of major first-generation products, methacrolein (MACR) and methyl vinyl ketone (MVK), from OH- and NO_3- initiated oxidation of isoprene
Airborne Flux Measurements of BVOCs above Californian Oak Forests: Experimental Investigation of Surface and Entrainment Fluxes, OH Densities, and Damköhler Numbers
The article of record as published may be located at http://dx.doi.org/10.1175/JAS-D-13-054.1Airborne flux measurements of isoprene were performed over the Californian oak belts surrounding the
Central Valley. The authors demonstrate for the first time 1) the feasibility of airborne eddy covariance
measurements of reactive biogenic volatile organic compounds; 2) the effect of chemistry on the vertical
transport of reactive species, such as isoprene; and 3) the applicability of wavelet analysis to estimate regional
fluxes of biogenic volatile organic compounds. These flux measurements demonstrate that instrumentation
operating at slower response times (e.g., 1–5 s) can still be used to determine eddy covariance fluxes in the
mixed layer above land, where typical length scales of 0.5–3km were observed. Flux divergence of isoprene
measured in the planetary boundary layer (PBL) is indicative of OH densities in the range of 4–7 3 106
molecules per cubic centimeter and allows extrapolation of airborne fluxes to the surface with Damk€ohler
numbers (ratio between the mixing time scale and the chemical time scale) in the range of 0.3–0.9. Most of the
isoprene is oxidized in the PBL with entrainment fluxes of about 10% compared to the corresponding surface
fluxes. Entrainment velocities of 1–10 cm s21 were measured. The authors present implications for parameterizing
PBL schemes of reactive species in regional and global models
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Airborne flux measurements of bvocs above californian oak forests: Experimental investigation of surface and entrainment fluxes, OH densities, and damköhler numbers
Airborne flux measurements of isoprene were performed over the Californian oak belts surrounding the Central Valley. The authors demonstrate for the first time 1) the feasibility of airborne eddy covariance measurements of reactive biogenic volatile organic compounds; 2) the effect of chemistry on the vertical transport of reactive species, such as isoprene; and 3) the applicability of wavelet analysis to estimate regional fluxes of biogenic volatile organic compounds. These flux measurements demonstrate that instrumentation operating at slower response times (e.g., 1-5 s) can still be used to determine eddy covariance fluxes in the mixed layer above land, where typical length scales of 0.5-3km were observed. Flux divergence of isoprene measured in the planetary boundary layer (PBL) is indicative of OH densities in the range of 4-7 3 106 molecules per cubic centimeter and allows extrapolation of airborne fluxes to the surface with Damköhler numbers (ratio between the mixing time scale and the chemical time scale) in the range of 0.3-0.9. Most of the isoprene is oxidized in the PBL with entrainment fluxes of about 10% compared to the corresponding surface fluxes. Entrainment velocities of 1-10 cm s21 were measured. The authors present implications for parameterizing PBL schemes of reactive species in regional and global models. © 2013 American Meteorological Society
AÂ steady-state continuous flow chamber for the study of daytime and nighttime chemistry under atmospherically relevant NO levels
Experiments performed in laboratory chambers have contributed significantly
to the understanding of the fundamental kinetics and mechanisms of the
chemical reactions occurring in the atmosphere. Two chemical regimes,
classified as high-NO vs. zero-NO conditions, have been extensively
studied in previous chamber experiments. Results derived from these two
chemical scenarios are widely parameterized in chemical transport models to
represent key atmospheric processes in urban and pristine environments. As
the anthropogenic NOx emissions in the United States have decreased
remarkably in the past few decades, the classic high-NO and zero-NO
conditions are no longer applicable to many regions that are constantly
impacted by both polluted and background air masses. We present here the
development and characterization of the NCAR Atmospheric Simulation Chamber,
which is operated in steady-state continuous flow mode for the study of
atmospheric chemistry under intermediate NO conditions. This particular
chemical regime is characterized by constant sub-ppb levels of NO and can be
created in the chamber by precise control of the inflow NO concentration and
the ratio of chamber mixing to residence timescales. Over the range of
conditions achievable in the chamber, the lifetime of peroxy radicals
(RO2), a key intermediate from the atmospheric degradation of
volatile organic compounds (VOCs), can be extended to several minutes, and
a diverse array of reaction pathways, including unimolecular pathways and
bimolecular reactions with NO and HO2, can thus be explored.
Characterization experiments under photolytic and dark conditions were
performed and, in conjunction with model predictions, provide a basis for
interpretation of prevailing atmospheric processes in environments with
intertwined biogenic and anthropogenic activities. We demonstrate the proof
of concept of the steady-state continuous flow chamber operation through
measurements of major first-generation products, methacrolein (MACR) and
methyl vinyl ketone (MVK), from OH- and NO3-initiated oxidation of
isoprene
Recommended from our members
The use of disjunct eddy sampling methods for the determination of ecosystem level fluxes of trace gases
The concept of disjunct eddy sampling (DES) for use in measuring ecosystem-level micrometeorological fluxes is re-examined. The governing equations are discussed as well as other practical considerations and guidelines concerning this sampling method as it is applied to either the disjunct eddy covariance (DEC) or disjunct eddy accumulation (DEA) techniques. A disjunct eddy sampling system was constructed that could either be combined with relatively slow sensors (response time of 2 to 40 s) to measure fluxes using DEC, or could also be used to accumulate samples in stable reservoirs for later laboratory analysis (DEA technique). Both the DEC and DEA modes of this sampler were tested against conventional eddy covariance (EC) for fluxes of either CO2 (DEC) or isoprene (DEA). Good agreement in both modes was observed relative to the EC systems. However, the uncertainty in a single DEA flux measurement was considerable (∼40%) due to both the reduced statistical sampling and the analytical precision of the concentration difference measurements. We have also re-investigated the effects of nonzero mean vertical wind velocity on accumulation techniques as it relates to our DEA measurements. Despite the higher uncertainty, disjunct eddy sampling can provide an alternative technique to eddy covariance for determining ecosystem-level fluxes for species where fast sensors do not currently exist