The Role of Sulfur Dioxide in Stratospheric Aerosol Formation Evaluated Using In-Situ Measurements in the Tropical Lower Stratosphere.

Stratospheric aerosols (SAs) are a variable component of the Earth's albedo that may be intentionally enhanced in the future to offset greenhouse gases (geoengineering). The role of tropospheric-sourced sulfur dioxide (SO2) in maintaining background SAs has been debated for decades without in-situ measurements of SO2 at the tropical tropopause to inform this issue. Here we clarify the role of SO2 in maintaining SAs by using new in-situ SO2 measurements to evaluate climate models and satellite retrievals. We then use the observed tropical tropopause SO2 mixing ratios to estimate the global flux of SO2 across the tropical tropopause. These analyses show that the tropopause background SO2 is about 5 times smaller than reported by the average satellite observations that have been used recently to test atmospheric models. This shifts the view of SO2 as a dominant source of SAs to a near-negligible one, possibly revealing a significant gap in the SA budget.

Growth in stratospheric chlorine from short-lived chemicals not controlled by the Montreal Protocol.

We have developed a chemical mechanism describing the tropospheric degradation of chlorine containing very short-lived substances (VSLS). The scheme was included in a global atmospheric model and used to quantify the stratospheric injection of chlorine from anthropogenic VSLS ( ClyVSLS) between 2005 and 2013. By constraining the model with surface measurements of chloroform (CHCl3), dichloromethane (CH2Cl2), tetrachloroethene (C2Cl4), trichloroethene (C2HCl3), and 1,2-dichloroethane (CH2ClCH2Cl), we infer a 2013 ClyVSLS mixing ratio of 123 parts per trillion (ppt). Stratospheric injection of source gases dominates this supply, accounting for ∼83% of the total. The remainder comes from VSLS-derived organic products, phosgene (COCl2, 7%) and formyl chloride (CHClO, 2%), and also hydrogen chloride (HCl, 8%). Stratospheric ClyVSLS increased by ∼52% between 2005 and 2013, with a mean growth rate of 3.7 ppt Cl/yr. This increase is due to recent and ongoing growth in anthropogenic CH2Cl2-the most abundant chlorinated VSLS not controlled by the Montreal Protocol.

Recent Northern Hemisphere stratospheric HCl increase due to atmospheric circulation changes.

The abundance of chlorine in the Earth's atmosphere increased considerably during the 1970s to 1990s, following large emissions of anthropogenic long-lived chlorine-containing source gases, notably the chlorofluorocarbons. The chemical inertness of chlorofluorocarbons allows their transport and mixing throughout the troposphere on a global scale, before they reach the stratosphere where they release chlorine atoms that cause ozone depletion. The large ozone loss over Antarctica was the key observation that stimulated the definition and signing in 1987 of the Montreal Protocol, an international treaty establishing a schedule to reduce the production of the major chlorine- and bromine-containing halocarbons. Owing to its implementation, the near-surface total chlorine concentration showed a maximum in 1993, followed by a decrease of half a per cent to one per cent per year, in line with expectations. Remote-sensing data have revealed a peak in stratospheric chlorine after 1996, then a decrease of close to one per cent per year, in agreement with the surface observations of the chlorine source gases and model calculations. Here we present ground-based and satellite data that show a recent and significant increase, at the 2σ level, in hydrogen chloride (HCl), the main stratospheric chlorine reservoir, starting around 2007 in the lower stratosphere of the Northern Hemisphere, in contrast with the ongoing monotonic decrease of near-surface source gases. Using model simulations, we attribute this trend anomaly to a slowdown in the Northern Hemisphere atmospheric circulation, occurring over several consecutive years, transporting more aged air to the lower stratosphere, and characterized by a larger relative conversion of source gases to HCl. This short-term dynamical variability will also affect other stratospheric tracers and needs to be accounted for when studying the evolution of the stratospheric ozone layer.

Fourier transform emission spectroscopy of YH and YD: observation of new A1Δ and B1Π electronic states.

The emission spectra of YH and YD molecules have been investigated in the 3600-12,000 cm(-1) region using a Fourier transform spectrometer. Molecules were formed in an yttrium hollow cathode lamp operated with a continuous flow of a mixture of Ne and Ar gases, and YH and YD were observed together in the same spectra. A group of bands observed near 1 µm have been identified as 0-0 and 1-1 bands of the A(1)Δ-X(1)Σ(+) and B(1)Π-X(1)Σ(+) transitions of YH and the 0-0 bands of the same two transitions for YD. The A(1)Δ and B(1)Π states of YH are separated by only about 12 cm(-1) and are involved in strong interactions. A perturbation analysis has been performed using the PGOPHER program to fit the two interacting electronic states and spectroscopic parameters for the A(1)Δ and B(1)Π states, including the interaction matrix elements, have been obtained for the first time.

High resolution laser excitation spectroscopy of the B2E-X2A1 transitions of calcium and strontium monoborohydride.

High resolution spectra of the B2E-X2A1 transitions of CaBH4 and SrBH4 have been recorded using laser excitation spectroscopy in a laser ablation/molecular jet source. Because of rotational cooling in the molecular jet and nuclear spin statistics, transitions arising from only the K'=1<--K"=0, K'=2<--K"=1, and K'=0<--K"=1 subbands have been observed. For each molecule, an analysis of the data using 2E and 2A1 symmetric top Hamiltonians yielded rotational, spin-orbit, and spin-rotation parameters for the observed states. For both molecules the rotational constants compare well with those calculated for a tridentate borohydride structure. A large reduction in the spin-orbit splitting and in the metal-ligand separation for each molecule indicates an increase in the amount of d atomic orbital character in the first excited 2E states of the monoborohydrides as compared to the monomethyl derivatives. For each molecule no evidence of internal rotation of the BH4- ligand was found. A change in the magnitude and sign of the spin-rotation constant epsilon1 confirms an energy reordering of the first excited 2E and 2A1 states in both CaBH4 and SrBH4 as compared to CaCH3 and SrCH3. The data also suggest that the B2E1/2 rotational energy levels of CaBH4 may be perturbed by a vibronic component of the A2A1 state.

High-resolution laser excitation spectroscopy of the A2E-X2A1 transition of SrCH3.

High-resolution laser excitation spectroscopy has been used to record the A (2)E-X (2)A(1) electronic transition of SrCH(3) in a laser ablation/molecular jet source. Transitions arising from the K(')=1<--K(")=0, K(')=0<--K(")=1, and K(')=2<--K(")=1 subbands have been observed and assigned. The data were modeled with (2)E and (2)A(1) symmetric top Hamiltonian matrices in a Hund's case (a) basis, using a least squares fitting program. Rotational and fine structure parameters for the A (2)E state were determined. A comparison of the spin-orbit energy separation in the A (2)E state to other strontium containing free radicals showed that the Jahn-Teller effect is negligible. The spin-rotation constants for the A (2)E state were calculated using the pure precession model and were found to be in good agreement with the experimentally determined parameters. These calculations suggest that the A (2)E state of SrCH(3) is not entirely of p orbital character. The rotational constants were used to estimate the structural parameters of SrCH(3) in the A (2)E state. The strontium-carbon bond length was found to decrease by approximately 0.006 A, and the hydrogen-carbon-hydrogen bond angle opened by approximately 0.8 degrees compared to the X (2)A(1) state, similar to the geometry changes observed for CaCH(3).

High-resolution spectroscopic investigation of the ~B2A1-- ~X2A1 transitions of CaCH3 and SrCH3.

High-resolution spectra of the ~B(2)A(1)-- ~X(2)A(1) transitions of CaCH(3) and SrCH(3) have been recorded in a molecular jet/laser ablation source using laser excitation spectroscopy. Transitions arising from the K = 0 and 1 sub-bands have been observed for both molecules. An analysis of the data using a (2)A(1) symmetric top Hamiltonian has determined rotational and spin-rotation constants for the ~B(2)A(1) state of each molecule. From the rotational constants, structures have been estimated for both CaCH(3) and SrCH(3). The spin-rotation constant, epsilon(bc) = (epsilon(bb) + epsilon(cc))/2, in the ~B(2)A(1) state for both molecules is in reasonable agreement with the value calculated using the pure precession approximation. For CaCH(3), the K' = 1 levels of the ~B(2)A(1) state exhibit a perturbation that interchanges the energy ordering of the spin-rotation components.

Fourier transform infrared emission spectra of MgH and MgD.

High resolution Fourier transform infrared emission spectra of MgH and MgD have been recorded. The molecules were generated in an emission source that combines an electrical discharge with a high temperature furnace. Several vibration-rotation bands were observed for all six isotopomers in the X (2)Sigma(+) ground electronic state: v=1-->0 to 4-->3 for (24)MgH, v=1-->0 to 3-->2 for (25)MgH and (26)MgH, v=1-->0 to 5-->4 for (24)MgD, v=1-->0 to 4-->3 for (25)MgD and (26)MgD. The new data were combined with the previous ground state data, obtained from diode laser vibration-rotation measurements and pure rotation spectra, and spectroscopic constants were determined for the v=0 to 4 levels of (24)MgH and the v=0 to 5 levels of (24)MgD. In addition, Dunham constants and Born-Oppenheimer breakdown correction parameters were obtained in a combined fit of the six isotopomers. The equilibrium vibrational constants (omega(e)) for (24)MgH and (24)MgD were found to be 1492.776(7) cm(-1) and 1077.298(5) cm(-1), respectively, while the equilibrium rotational constants (B(e)) are 5.825 523(8) cm(-1) and 3.034 344(4) cm(-1). The associated equilibrium bond distances (r(e)) were determined to be 1.729 721(1) A for (24)MgH and 1.729 157(1) A for (24)MgD.

Fourier Transform Emission Spectroscopy of CuCl.

The electronic spectra of CuCl were observed in the 18 000 cm(-1) to 25 000 cm(-1) spectral region using a Bruker IFS 120 HR Fourier transform spectrometer (FTS) and with the FTS associated with the McMath-Pierce Solar Telescope at Kitt Peak. On the basis of ab initio calculations, the labels for the electronic states were revised, and the a(3)Sigma(+)(1)-X(1)Sigma(+) 0-0 band, the b(3)Pi(0)-X(1)Sigma(+) 0-0, 1-0, and 0-1 bands, the b(3)Pi(1)-X(1)Sigma(+) 0-0, 1-0, and 0-1 bands, the A(1)Pi-X(1)Sigma(+) 0-0, 1-0, and 0-2 bands, and the B(1)Sigma(+)-X(1)Sigma(+) 0-0 and 1-0 bands were measured. Improved spectroscopic constants were obtained for the excited and ground states. Copyright 2001 Academic Press.

Water on the sun: line assignments based on variational calculations.

The infrared spectrum of hot water observed in a sunspot has been assigned. The high temperature of the sunspot (3200 K) gave rise to a highly congested pure rotational spectrum in the 10-micrometer region that involved energy levels at least halfway to dissociation. Traditional spectroscopy, based on perturbation theory, is inadequate for this problem. Instead, accurate variational solutions of the vibration-rotation Schrödinger equation were used to make assignments, revealing unexpected features, including rotational difference bands and fewer degeneracies than anticipated. These results indicate that a shift away from perturbation theory to first principles calculations is necessary in order to assign spectra of hot polyatomic molecules such as water.

Far-infrared emission spectra of selected gas-phase PAHs: spectroscopic fingerprints.

The emission spectra of the gaseous polycyclic aromatic hydrocarbons (PAHs) naphthalene, chrysene, and pyrene were recorded in the far-infrared (far-IR) region. The vibrational bands that lie in the far IR are unique for each PAH molecule and allow discrimination among the three PAH molecules. The far-IR PAH spectra, therefore, may prove useful in the assignment of unidentified spectral features from astronomical objects.

High-resolution Fourier-transform emission spectroscopy of the A(1)II-X(1)Σ(+) system of AIH.

The emission spectrum of the A(1)II-X(1)Σ(+) system of AIH, excited in a hollow-cathode discharge lamp, has been observed at high resolution with a Fourier-transform spectrometer. The rotational lines in the 0-0 and the 1-1 bands have been measured with a precision of ±0.001 cm(-1). The present measurements provide a considerable improvement overthe previous data of Zeeman and Ritter [Can. J. Phys. 32, 555 (1954)]. The present data, combined with the previous high-resolution measurements of the 1-0 vibration-rotation band by White et al. [J. Chem. Phys. 99, 8371 (1993)] and the J = 1-0 pure rotational line of Goto and Saito [Astrophys. J. 452, L147 (1995)] have been used to determine improved molecular constants for the A(1)!! state.

Laboratory astrophysics and molecular astronomy of pure carbon molecules.

The pure carbon molecules Cn are currently of great experimental and theoretical interest. Our work in this area begins with detection of the SiC molecule, which is isovalent with C2. New infrared electronic transitions of C2 and C3 were discovered by emission spectroscopy of hydrocarbon dicharges. The C3 and C5 molecules were found by infrared vibration-rotation spectroscopy of the prototypical obscured carbon star, IRC+10216. C7 and C9 were searched for in the same source, but not found. The laboratory infrared emission spectrum of C60 was recorded to aid in a search for C60 in extraterrestrial sources.

Gas-phase inorganic chemistry: monovalent derivatives of calcium and strontium.

The chemistry and spectroscopy of monovalent derivatives of calcium and strontium are described. Laser-driven chemical reactions of calcium and strontium vapors with a variety of small molecules have provided many new free radicals. In general, these species are analogous to the stable molecules obtained in traditional inorganic or organometallic chemistry, but some new families of molecules have been discovered.

Detection of C5 in the Circumstellar Shell of IRC+10216.

The C(5) molecule has been identified in the infrared spectrum of the prototypical obscured carbon star, IRC+10216. In addition to their astrophysical importance, pure carbon chain molecules such as C(5) are of interest in the chemistry of flames and propellants.

Detection of C3 in the Circumstellar Shell of IRC+10216.

Vibration-rotation lines of C(3) have been identified in the circumstellar spectrum of the obscured carbon star IRC+10216. This molecule is of interest in both the chemistry of flames, where it may be involved in the formation of soot, and in astrophysics, where it is a potential building block for carbonaceous grains. This high-resolution infrared detection of the pure carbon chain molecule C(3) allows the estimation of the equilibrium C-C bond length, 1.297 angstroms. Possible astrophysical formation and destruction mechanisms for C(3) are reviewed, including the relationship between C(3) and carbon clusters.