610 research outputs found
Air–snowpack exchange of bromine, ozone and mercury in the springtime Arctic simulated by the 1-D model PHANTAS – Part 1: In-snow bromine activation and its impact on ozone
To provide a theoretical framework towards a better understanding of ozone
depletion events (ODEs) and atmospheric mercury depletion events (AMDEs) in
the polar boundary layer, we have developed a one-dimensional model that
simulates multiphase chemistry and transport of trace constituents from
porous snowpack and through the atmospheric boundary layer (ABL) as a unified
system. This paper constitutes Part 1 of the study, describing a general
configuration of the model and the results of simulations related to reactive
bromine release from the snowpack and ODEs during the Arctic spring. A common
set of aqueous-phase reactions describes chemistry both within the
liquid-like layer (LLL) on the grain surface of the snowpack and within
deliquesced "haze" aerosols mainly composed of sulfate in the atmosphere.
Gas-phase reactions are also represented by the same mechanism in the
atmosphere and in the snowpack interstitial air (SIA). Consequently, the
model attains the capacity of simulating interactions between chemistry and
mass transfer that become particularly intricate near the interface between
the atmosphere and the snowpack. In the SIA, reactive uptake on LLL-coated
snow grains and vertical mass transfer act simultaneously on gaseous HOBr, a
fraction of which enters from the atmosphere while another fraction is formed
via gas-phase chemistry in the SIA itself. A "bromine explosion", by which
HOBr formed in the ambient air is deposited and then converted
heterogeneously to Br<sub>2</sub>, is found to be a dominant process of reactive
bromine formation in the top 1 mm layer of the snowpack. Deeper in the
snowpack, HOBr formed within the SIA leads to an in-snow bromine explosion,
but a significant fraction of Br<sub>2</sub> is also produced via aqueous
radical chemistry in the LLL on the surface of the snow grains. These top-
and deeper-layer productions of Br<sub>2</sub> both contribute to the release of
Br<sub>2</sub> to the atmosphere, but the deeper-layer production is found to be
more important for the net outflux of reactive bromine. Although ozone is
removed via bromine chemistry, it is also among the key species that control
both the conventional and in-snow bromine explosions. On the other hand,
aqueous-phase radical chemistry initiated by photolytic OH formation in the
LLL is also a significant contributor to the in-snow source of Br<sub>2</sub>
and can operate without ozone, whereas the delivery of Br<sub>2</sub> to the
atmosphere becomes much smaller after ozone is depleted. Catalytic ozone loss
via bromine radical chemistry occurs more rapidly in the SIA than in the
ambient air, giving rise to apparent dry deposition velocities for ozone from
the air to the snow on the order of 10<sup>−3</sup> cm s<sup>−1</sup> during
daytime. Overall, however, the depletion of ozone in the system is caused
predominantly by ozone loss in the ambient air. Increasing depth of the
turbulent ABL under windy conditions will delay the buildup of reactive
bromine and the resultant loss of ozone, while leading to the higher column
amount of BrO in the atmosphere. During the Arctic spring, if moderately
saline and acidic snowpack is as prevalent as assumed in our model runs on
sea ice, the shallow, stable ABL under calm weather conditions may undergo
persistent ODEs without substantial contributions from blowing/drifting snow
and wind-pumping mechanisms, whereas the column densities of BrO in the ABL
will likely remain too low in the course of such events to be detected
unambiguously by satellite nadir measurements
Depletion of gaseous polycyclic aromatic hydrocarbons by a forest canopy
Rapid uptake of gaseous polycyclic aromatic hydrocarbons (PAHs) by a forest canopy was observed at Borden in Southern Ontario, Canada during bud break in early spring 2003. High volume air samples were taken on 12 individual days at three different heights (44.4, 29.1, and 16.7 m) on a scaffolding tower and on the forest floor below the canopy (1.5 m). Concentrations of PAHs were positively correlated to ambient temperature, resulting from relatively warm and polluted air masses passing over the Eastern United States and Toronto prior to arriving at the sampling site. An analysis of vertical profiles and gas/particle partitioning of the PAHs showed that gaseous PAHs established a concentration gradient with height, whereas levels of particulate PAHs were relatively uniform, implying that only the uptake of gaseous PAHs by the forest canopy was sufficiently rapid to be observed. Specifically, the gaseous concentrations of intermediate PAHs, such as phenanthrene, anthracene, and pyrene, during budburst and leaf emergence were reduced within and above the canopy. When a gradient was observed, the percentage of PAHs on particles increased at the elevations experiencing a decrease in gas phase concentrations. The uptake of intermediate PAHs by the canopy also led to significant differences in gaseous PAH composition with height. These results are the most direct evidence yet of the filter effect of forest canopies for gaseous PAHs in early spring. PAH deposition fluxes and dry gaseous deposition velocities to the forest canopy were estimated from the concentration gradients
The local magnetic field scaling of gyrokinetic turbulence and its impact on tokamak transport
Reactive uptake of ammonia to secondary organic aerosols: kinetics of organonitrogen formation
As a class of brown carbon, organonitrogen compounds originating from the
heterogeneous uptake of NH3 by secondary organic aerosol (SOA) have
received significant attention recently. In the current work, particulate
organonitrogen formation during the ozonolysis of α-pinene and the OH
oxidation of m-xylene in the presence of ammonia (34–125 ppb) was studied
in a smog chamber equipped with a high resolution time-of-flight aerosol mass
spectrometer and a quantum cascade laser instrument. A large diversity of
nitrogen-containing organic (NOC) fragments was observed which were
consistent with the reactions between ammonia and carbonyl-containing SOA.
Ammonia uptake coefficients onto SOA which led to organonitrogen compounds
were reported for the first time, and were in the range of ∼
10-3–10−2, decreasing significantly to -5 after
6 h of reaction. At the end of experiments (~ 6 h) the NOC mass
contributed 8.9 ± 1.7 and 31.5 ± 4.4 wt % to the total
α-pinene- and m-xylene-derived SOA, respectively, and
4–15 wt % of the total nitrogen in the system. Uptake coefficients were
also found to be positively correlated with particle acidity and negatively
correlated with NH3 concentration, indicating that heterogeneous
reactions were responsible for the observed NOC mass, possibly limited by
liquid phase diffusion. Under these conditions, the data also indicate that
the formation of NOC can compete kinetically with inorganic acid
neutralization. The formation of NOC in this study suggests that a
significant portion of the ambient particle associated N may be derived from
NH3 heterogeneous reactions with SOA. NOC from such a mechanism may be
an important and unaccounted for source of PM associated nitrogen. This
mechanism may also contribute to the medium or long-range transport and
wet/dry deposition of atmospheric nitrogen
Metamorphosis of plasma turbulence-shear flow dynamics through a transcritical bifurcation
The structural properties of an economical model for a confined plasma
turbulence governor are investigated through bifurcation and stability
analyses. A close relationship is demonstrated between the underlying
bifurcation framework of the model and typical behavior associated with low- to
high-confinement transitions such as shear flow stabilization of turbulence and
oscillatory collective action. In particular, the analysis evinces two types of
discontinuous transition that are qualitatively distinct. One involves
classical hysteresis, governed by viscous dissipation. The other is
intrinsically oscillatory and non-hysteretic, and thus provides a model for the
so-called dithering transitions that are frequently observed. This
metamorphosis, or transformation, of the system dynamics is an important late
side-effect of symmetry-breaking, which manifests as an unusual non-symmetric
transcritical bifurcation induced by a significant shear flow drive.Comment: 17 pages, revtex text, 9 figures comprised of 16 postscript files.
Submitted to Phys. Rev.
The effects of main-ion dilution on turbulence in low q95 C-Mod ohmic plasmas, and comparisons with nonlinear GYRO
Recent experiments on C-mod seeding nitrogen into ohmic plasmas with [subscript q]95 = 3.4 found that the seeding greatly reduced long-wavelength (ITG-scale) turbulence. The long-wavelength turbulence that was reduced by the nitrogen seeding was localized to the region of r/a≈0.85, where the turbulence is well above marginal stability (as evidenced by Q[subscript i]/Q[subscript GB]≫1). The nonlinear gyrokinetic code GYRO was used to simulate the expected turbulence in these plasmas, and the simulated turbulent density fluctuations and turbulent energy fluxes quantitatively agreed with the experimental measurements both before and after the nitrogen seeding. Unexpectedly, the intrinsic rotation of the plasma was also found to be affected by the nitrogen seeding, in a manner apparently unrelated to a change in the electron-ion collisionality that was proposed by other experiments.United States. Dept. of Energy. Office of Fusion Energy Sciences (Award E-FG02-94-ER54235
A fully autonomous ozone, aerosol and nighttime water vapor lidar: a synergistic approach to profiling the atmosphere in the Canadian oil sands region
Lidar technology has been rapidly advancing over the past several decades. It
can be used to measure a variety of atmospheric constituents at very high
temporal and spatial resolutions. While the number of lidars continues to
increase worldwide, there is generally a dependency on an operator,
particularly for high-powered lidar systems. Environment and Climate Change
Canada (ECCC) has recently developed a fully autonomous, mobile lidar system
called AMOLITE (Autonomous Mobile Ozone Lidar Instrument for Tropospheric
Experiments) to simultaneously measure the vertical profile of tropospheric
ozone, aerosol and water vapor (nighttime only) from near the ground to
altitudes reaching 10 to 15 km. This current system uses a dual-laser,
dual-lidar design housed in a single climate-controlled trailer. Ozone
profiles are measured by the differential absorption lidar (DIAL) technique
using a single 1 m Raman cell filled with CO2. The DIAL
wavelengths of 287 and 299 nm are generated as the second and third Stokes
lines resulting from stimulated Raman scattering of the cell pumped using the
fourth harmonic of a Nd:YAG laser (266 nm). The aerosol lidar transmits
three wavelengths simultaneously (355, 532 and 1064 nm) employing a detector
designed to measure the three backscatter channels, two nitrogen Raman
channels (387 and 607 nm) and one cross-polarization channel at 355 nm. In
addition, we added a water vapor channel arising from the Raman-shifted
355 nm output (407 nm) to provide nighttime water vapor profiles. AMOLITE
participated in a validation experiment alongside four other ozone DIAL
systems before being deployed to the ECCC Oski-ôtin ground site in the
Alberta oil sands region in November 2016. Ozone was found to increase
throughout the troposphere by as much as a factor of 2 from stratospheric
intrusions. The dry stratospheric air within the intrusion was measured to be
less than 0.2 g kg−1. A biomass burning event that impacted the region
over an 8-day period produced lidar ratios of 35 to 65 sr at 355 nm and 40
to 100 sr at 532. Over the same period the Ã…ngström exponent
decreased from 1.56±0.2 to 1.35±0.2 in the 2–4 km smoke region.</p
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Stability of a radiative mantle in ITER
We report results of a study to evaluate the efficacy of various impurities for heat dispersal by a radiative mantle and radiative divertor(including SOL). We have derived a stability criterion for the mantle radiation which favors low Z impurities and low ratios of edge to core thermal conductivities. Since on the other hand the relative strength of boundary line radiation to core bremsstrahlung favors high Z impurities, we find that for the ITER physics phase argon is the best gaseous impurity for mantle radiation. For the engineering phase of ITER, more detailed analysis is needed to select between krypton and argon
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Mortality estimation in fully stocked stands of young-growth Douglas-fir
Aerosol deposition to the boreal forest in the vicinity of the Alberta Oil Sands
Measurements of size-resolved aerosol concentration and
fluxes were made in a forest in the Athabasca Oil Sands Region (AOSR) of
Alberta, Canada, in August 2021 with the aim of investigating (a) particle
size distributions from different sources, (b) size-resolved particle
deposition velocities, and (c) the rate of vertical mixing in the canopy.
Particle size distributions were attributed to different sources determined
by wind direction. Air mixed with smokestack plumes from oil sands
processing facilities had higher number concentrations with peak number at
diameters near 70 nm. Aerosols from the direction of open-pit mine faces
showed number concentration peaks near 150 nm and volume distribution peaks
near 250 nm (with secondary peaks near 600 nm). Size-resolved deposition
fluxes were calculated which show good agreement with previous measurements
and a recent parameterization. There is a minimum deposition velocity of
vd=0.02 cm s−1 for particles of 80 nm diameter; however, there is
a large amount of variation in the measurements, and this value is not
significantly different from zero in the 68 % confidence interval.
Finally, gradient measurements of aerosol particles (with diameters
<1 µm) demonstrated nighttime decoupling of air within and
above the forest canopy, with median lag times at night of up to 40 min and
lag times between 2 and 5 min during the day. Aerosol mass fluxes (diameters <1 µm) determined using flux–gradient methods (with
different diffusion parameterizations) underestimate the flux magnitude
relative to eddy covariance flux measurements when averaged over the nearly
1-month measurement period. However, there is significant uncertainty in the
averages determined using the flux–gradient method.</p
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