674 research outputs found
A Semi-Blind Source Separation Method for Differential Optical Absorption Spectroscopy of Atmospheric Gas Mixtures
Differential optical absorption spectroscopy (DOAS) is a powerful tool for
detecting and quantifying trace gases in atmospheric chemistry
\cite{Platt_Stutz08}. DOAS spectra consist of a linear combination of complex
multi-peak multi-scale structures. Most DOAS analysis routines in use today are
based on least squares techniques, for example, the approach developed in the
1970s uses polynomial fits to remove a slowly varying background, and known
reference spectra to retrieve the identity and concentrations of reference
gases. An open problem is to identify unknown gases in the fitting residuals
for complex atmospheric mixtures.
In this work, we develop a novel three step semi-blind source separation
method. The first step uses a multi-resolution analysis to remove the
slow-varying and fast-varying components in the DOAS spectral data matrix .
The second step decomposes the preprocessed data in the first step
into a linear combination of the reference spectra plus a remainder, or
, where columns of matrix are known reference spectra,
and the matrix contains the unknown non-negative coefficients that are
proportional to concentration. The second step is realized by a convex
minimization problem ,
where the norm is a hybrid norm (Huber estimator) that helps to
maintain the non-negativity of . The third step performs a blind independent
component analysis of the remainder matrix to extract remnant gas
components. We first illustrate the proposed method in processing a set of DOAS
experimental data by a satisfactory blind extraction of an a-priori unknown
trace gas (ozone) from the remainder matrix. Numerical results also show that
the method can identify multiple trace gases from the residuals.Comment: submitted to Journal of Scientific Computin
State-of-the-Art Quantum Chemistry Meets Variable Reaction Coordinate Transition State Theory to Solve the Puzzling Case of the H2S + Cl System
The atmospheric reaction of HS with Cl has been reinvestigated to check
if, as previously suggested, only explicit dynamical computations can lead to
an accurate evaluation of the reaction rate because of strong recrossing
effects and the breakdown of the variational extension of transition state
theory. For this reason, the corresponding potential energy surface has been
thoroughly investigated, thus leading to an accurate characterization of all
stationary points, whose energetics has been computed at the state of the art.
To this end, coupled-cluster theory including up to quadruple excitations has
been employed, together with the extrapolation to the complete basis set limit
and also incorporating core-valence correlation, spin-orbit, and scalar
relativistic effects as well as diagonal Born-Oppenheimer corrections. This
highly accurate composite scheme has also been paralleled by less expensive yet
promising computational approaches. Moving to kinetics, variational transition
state theory and its variable reaction coordinate extension for barrierless
steps have been exploited, thus obtaining a reaction rate constant (8.16 x
10 cm molecule s at 300 K and 1 atm) in remarkable
agreement with the experimental counterpart. Therefore, contrary to previous
claims, there is no need to invoke any failure of the transition state theory,
provided that sufficiently accurate quantum-chemical computations are
performed. The investigation of the puzzling case of the HS + Cl system
allowed us to present a robust approach for disclosing the thermochemistry and
kinetics of reactions of atmospheric and astrophysical interest.Comment: 49 pages, 7 figures, published online in JCT
New insights into atmospherically relevant reaction systems using direct analysis in real-time mass spectrometry (DART-MS)
The application of direct analysis in real-time mass spectrometry (DART-MS),
which is finding increasing use in atmospheric chemistry, to two different
laboratory model systems for airborne particles is investigated: (1) submicron C3–C7 dicarboxylic acid (diacid) particles reacted with
gas-phase trimethylamine (TMA) or butylamine (BA) and (2) secondary organic
aerosol (SOA) particles from the ozonolysis of α-cedrene. The diacid
particles exhibit a clear odd–even pattern in their chemical reactivity
toward TMA and BA, with the odd-carbon diacid particles being substantially
more reactive than even ones. The ratio of base to diacid in reacted
particles, determined using known diacid–base mixtures, was compared to that
measured by high-resolution time-of-flight aerosol mass spectrometry
(HR-ToF-AMS), which vaporizes the whole particle. Results show that DART-MS
probes ∼ 30 nm of the surface layer, consistent with other
studies on different systems. For α-cedrene SOA particles, it is
shown that varying the temperature of the particle stream as it enters the
DART-MS ionization region can distinguish between specific components with
the same molecular mass but different vapor pressures. These results
demonstrate the utility of DART-MS for (1) examining reactivity of
heterogeneous model systems for atmospheric particles and (2) probing
components of SOA particles based on volatility
Estimated Exposure Risks from Carcinogenic Nitrosamines in Urban Airborne Particulate Matter
Organic nitrogen (ON) compounds are present in atmospheric particulate matter (PM), but compared to their inorganic, hydrocarbon and oxygenated counterparts, they are difficult to characterize due to their complex chemical composition. Nitrosamines are a class of ON compounds known to be highly carcinogenic, and include species formed from nicotine degradation, but there are no detailed estimates of their abundance in ambient air. We use a highly sensitive analytical method, which is capable of separating over 700 ON compounds, to determine daily variability in nicotine, and 8 non specific and 4 tobacco specific nitrosamines in ambient PM from central London over two periods in winter and summer. The average total nitrosamine concentration was 5.2 ng m-3, substantially exceeding a current public recommendation of 0.3 ng m-3 on a daily basis. The lifetime cancer risk from nitrosamines in urban PM exceeded the U.S. Environmental Protection Agency guideline of 1 excess cancer cases per 1 million population exposed after 1 hour of exposure to observed concentrations per day over the duration of an adult lifetime. A clear relationship between ambient nitrosamines and total PM2.5 was observed with 1.2 ng m-3 ± 2.6 ng m-3 (total nitrosamine) per 10 µg m-3 PM2.5
Chemical Kinetics and Photochemical Data for Use in Atmospheric Studies Evaluation Number 15
This is the fifteenth in a series of evaluated sets of rate constants and photochemical cross sections compiled by the NASA Panel for Data Evaluation. The data are used primarily to model stratospheric and upper tropospheric processes, with particular emphasis on the ozone layer and its possible perturbation by anthropogenic and natural phenomena. Copies of this evaluation are available in electronic form and may be printed from the following Internet URL: http://jpldataeval.jpl.nasa.gov/
Surface-Enhanced Nitrate Photolysis on Ice
Heterogeneous nitrates photolysis is the trigger for many chemical processes occurring in the polar boundary layer and is widely believed to occur in a quasi-liquid layer (QLL) at the surface of ice. The dipole forbidden character of the electronic transition relevant to boundary layer atmospheric chemistry and the small photolysis/photoproducts quantum yields in ice (and in water) may confer a significant enhancement and interfacial specificity to this important photochemical reaction at the surface of ice. Using amorphous solid water films at cryogenic temperatures as models for the disordered interstitial air/ice interface within the snowpack suppresses the diffusive uptake kinetics thereby prolonging the residence time of nitrate anions at the surface of ice. This approach allows their slow heterogeneous photolysis kinetics to be studied providing the first direct evidence that nitrates adsorbed onto the first molecular layer at the surface of ice are photolyzed more effectively than those dissolved within the bulk. Vibrational spectroscopy allows the ~3-fold enhancement in photolysis rates to be correlated with the nitrates’ distorted intramolecular geometry thereby hinting at the role played by the greater chemical heterogeneity in their solvation environment at the surface of ice than in the bulk. A simple 1D kinetic model suggests 1-that a 3(6)-fold enhancement in photolysis rate for nitrates adsorbed onto the ice surface could increase the photochemical NO[subscript 2] emissions from a 5(8) nm thick photochemically active interfacial layer by 30%(60)%, and 2-that 25%(40%) of the NO[subscript 2] photochemical emissions to the snowpack interstitial air are released from the top-most molecularly thin surface layer on ice. These findings may provide a new paradigm for heterogeneous (photo)chemistry at temperatures below those required for a QLL to form at the ice surface
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