Climate & Space Department

We present initial results from the first community-wide effort to compare global plasma interaction model results for Mars. Seven modeling groups participated in this activity, using MHD, multi-fluid, and hybrid assumptions in their simulations. Moderate solar wind and solar EUV conditions were chosen, and the conditions were implemented in the models and run to steady state. Model output was compared in three ways to determine how pressure was partitioned and conserved in each model, the location and asymmetry of plasma boundaries and pathways for planetary ion escape, and the total escape flux of planetary oxygen ions. The two participating MHD models provided similar results, while the five sets of multi-fluid and hybrid results were different in many ways. All multi-fluid and hybrid results, however, showed two main channels for planetary ion escape, while the MHD models showed one. Most models showed a transition from an upstream region dominated by plasma dynamic pressure to a magnetosheath region dominated by thermal pressure to a low altitude region dominated by magnetic pressure. However, calculated escape rates for a single ion species varied by roughly an order of magnitude for similar input conditions, suggesting that the uncertainties in both the current and integrated escape over Martian history as determined by models are large. These uncertainties are in addition to those associated with the evolution of the Sun, the Martian dynamo, and the early atmosphere, highlighting the challenges we face in constructing Mars' past using models.

This paper reviews the use of accelerometer data for determining atmospheric density during the Mars Global Survey and Mars Odyssey missions and provides preliminary results from the Mars Reconnaissance Orbiter aerobraking operations. For MGS and ODY, accelerometer data were analyzed in both near real time and post flight to provide estimates of density, density scale height, latitudinal gradients, global longitudinal wave structure, and small scale (gravity) wave spectra. MGS (Odyssey) provided data during about 850 (300) passes at altitudes ranging from 100 to 160 km (95 to 150) and covering a latitude range of 60 o N to 90 o S (30N to 90N). A summary is given of the atmospheric phenomena encountered during the aerobraking phase of the missions and of some of the scientific results based on these data. MRO is expected to provide another 500 aerobraking passes over an altitude range from 95 to 170 km and from the south pole to the equator. These data provide powerful constraints on upper atmospheric models. Accelerometer and space craft requirements to enhance scientific return are discussed.

We present initial results from the first community-wide effort to compare global plasma interaction model results for Mars. Seven modeling groups participated in this activity, using MHD, multi-fluid, and hybrid assumptions in their simulations. Moderate solar wind and solar EUV conditions were chosen, and the conditions were implemented in the models and run to steady state. Model output was compared in three ways to determine how pressure was partitioned and conserved in each model, the location and asymmetry of plasma boundaries and pathways for planetary ion escape, and the total escape flux of planetary oxygen ions. The two participating MHD models provided similar results, while the five sets of multi-fluid and hybrid results were different in many ways. All multi-fluid and hybrid results, however, showed two main channels for planetary ion escape, while the MHD models showed one. Most models showed a transition from an upstream region dominated by plasma dynamic pressure to a magnetosheath region dominated by thermal pressure to a low altitude region dominated by magnetic pressure. However, calculated escape rates for a single ion species varied by roughly an order of magnitude for similar input conditions, suggesting that the uncertainties in both the current and integrated escape over Martian history as determined by models are large. These uncertainties are in addition to those associated with the evolution of the Sun, the Martian dynamo, and the early atmosphere, highlighting the challenges we face in constructing Mars' past using models.

Venus Express (VEX) has been monitoring key nightglow emissions and thermal features (O 2 IR nightglow, NO UV nightglow, and nightside temperatures) which contribute to a comprehensive understanding of the global dynamics and circulation patterns above ∼90 km. The nightglow emissions serve as effective tracers of Venus' middle and upper atmosphere global wind system due to their variable peak brightness and horizontal distributions. A statistical map has been created utilizing O 2 IR nightglow VEX observations, and a statistical map for NO UV is being developed. A nightside warm layer near 100 km has been observed by VEX and ground-based observations. The National Center for Atmospheric Research (NCAR) Venus Thermospheric General Circulation Model (VTGCM) has been updated and revised in order to address these key VEX observations and to provide diagnostic interpretation. The VTGCM is first used to capture the statistically averaged mean state of these three key observations. This correspondence implies a weak retrograde superrotating zonal flow (RSZ) from ∼80 km to 110 km and above 110 km the emergence of modest RSZ winds approaching 60 m s -1 above ∼130 km. Subsequently, VTGCM sensitivity tests are performed using two tuneable parameters (the nightside eddy diffusion coefficient and the wave drag term) to examine corresponding variability within the VTGCM. These tests identified a possible mechanism for the observed noncorrelation of the O 2 and NO emissions. The dynamical explanation requires the nightglow layers to be at least ∼15 km apart and the retrograde zonal wind to increase dramatically over 110 to 130 km.

In this study, we develop a best fit description of Titan's upper atmosphere between 500 km and 1500 km, using a one-dimensional (1-D) version of the three-dimensional (3-D) Titan Global Ionosphere-Thermosphere Model. For this modeling, we use constraints from several lower atmospheric Cassini-Huygens investigations and validate our simulation results against in situ Cassini Ion-Neutral Mass Spectrometer (INMS) measurements of N 2 , CH 4 , H 2 , 40 Ar, HCN, and the major stable isotopic ratios of 14 N/ 15 N in N 2. We focus our investigation on aspects of Titan's upper atmosphere that determine the amount of atmospheric escape required to match the INMS measurements: the amount of turbulence, the inclusion of chemistry, and the effects of including a self-consistent thermal balance. We systematically examine both hydrodynamic escape scenarios for methane and scenarios with significantly reduced atmospheric escape. Our results show that the optimum configuration of Titan's upper atmosphere is one with a methane homopause near 1000 km and atmospheric escape rates of 1.41-1.47 × 10 11 CH 4 m −2 s −1 and 1.08 × 10 14 H 2 m −2 s −1 (scaled relative to the surface). We also demonstrate that simulations consistent with hydrodynamic escape of methane systematically produce inferior fits to the multiple validation points presented here. Unfortunately, direct measurements of neutral atmospheric escape are not made by instruments on the Cassini orbiter and there are currently no published constraints on neutral methane escape from magnetospheric measurements. Thus, we must rely upon numerical models, usually one-dimensional (1-D), to infer atmospheric escape rates from Titan. These 1-D models are constrained to simultaneously match atmosphere composition below 500 km and Cassini Ion-Neutral Mass Spectrometer (INMS) data above 1000 km. Key parameters, such as eddy diffusion and topside escape rates, are then adjusted until the simulated densities and mixing ratios match INMS data. Using this approach, Strobel [2009], Yelle et al. [2008], and Cui et al. [2012] estimated that large escape rates of methane (between ∼2.0 and 3.0 × 10 13 CH 4 m −2 s −1 or ∼40 and 60 kg s −1 globally) are required to reproduce the INMS composition measurements. Strobel [2008, 2009] concluded that these inferred atmospheric escape rates of CH 4 were evidence of hydrodynamic escape. Conversely, several studies have suggested that hydrodynamic escape rates of CH 4 are not required to match INMS data. Tucker and Johnson [2009], and BELL ET AL.

The Modern-Era Retrospective Analysis for Research and Application (MERRA) is a reanalysis designed to produce an improved representation of the Earth’s hydrologic cycle. This study examines the representation of deep convective clouds in MERRA, comparing analyzed liquid and ice clouds with deep convective cloud objects observed by instruments on the Tropical Rainfall Measuring Mission satellite. Results show that MERRA contains deep convective cloud in 98.1% of the observed cases. MERRA-derived probability density functions (PDFs) of cloud properties have a similar form as the observed PDFs and exhibit a similar trend with changes in object size. Total water path, optical depth, and outgoing shortwave radiation (OSR) in MERRA are found to match the cloud object observations quite well; however, there appears to be a bias toward higher-than-observed cloud tops in the MERRA. The reanalysis fits the observations most closely for the largest class of convective systems, with performanc...

This study investigates the resonated diurnal impact of the surface wind forcing from several atmospheric wind products, including CCMP (Cross-Calibrated Multi-Platform), ERA Interim, NCEP-2 (NCEP-DOE AMIP-II Reanalysis), MERRA (Modern-Era Retrospective analysis for Research and Application), NARR (Northern American Regional Reanalysis) and NAM (North American Mesoscale Forecast System), on the wind-driven near-inertial ocean motions in the northern Gulf of Mexico by numerical simulation, and in comparison, with NDBC buoy surface observed winds. Our analyses show that the near-inertial wind power input, which can affect vertical mixing in the water column, is closely associated with the variability of land-sea breeze by various wind products. In comparison with buoy observations in the northern Gulf of Mexico, the MERRA has the worst land-sea breeze representation, while the NCEP2 and CCMP, which are chosen to represent the coarsest and medium-high horizontal resolution, have the best performance. We further show that by comparison of the buoy data, that the near-inertial wind power input derived from all the wind products is underestimated by 30-50% approximately.

We propose, and quantify from first principles, two novel HO x -regenerating unimolecular reactions in isoprene oxidation, which are estimated to yield in pristine tropical forest conditions about 0.7 HO 2 and 0.03 OH radicals per isoprene oxidized; it is further argued that the photolabile coproduct of HO 2 can be a major source of OH, with a yield of the order of 1. The newly proposed chemistry could provide a rationalization for the unexpectedly high OH concentrations often observed in forested environments, such as over the Amazon forest in the recent Gabriel campaign.

The potential energy surface for the O( 3 P) + C 2 H 4 reaction, which plays an important role in C 2 H 4 /O 2 flames and in hydrocarbon combustion in general, was theoretically reinvestigated using various quantum chemical methods, including G3, CBS-QB3, G2M(CC,MP2), and MRCI. The energy surfaces of both the lowest-lying triplet and singlet electronic states were constructed. The primary product distribution for the multiwell multichannel reaction was then determined by RRKM statistical rate theory and weak-collision master equation analysis using the exact stochastic simulation method. Intersystem crossing of the "hot" CH 2 CH 2 O triplet adduct to the singlet surface, shown to account for about half of the products, was estimated to proceed at a rate of ≈1.5 × 10 11 s -1 . In addition, the thermal rate coefficients k(O + C 2 H 4 ) in the T ) 200-2000 K range were computed using multistate transition state theory and fitted by a modified Arrhenius expression as k(T) ) 1.69 × 10 -16 × T 1.66 × exp(-331 K/T) . Our computed rates and product distributions agree well with the available experimental results. Product yields are found to show a monotonic dependence on temperature. The major products (with predicted yields at T ) 300 K/2000 K) are: CH 3 + CHO (48/37%), H + CH 2 CHO (40/19%), and CH 2 (X 3 B 1 ) + H 2 CO (5/29%), whereas H + CH 3 CO, H 2 + H 2 CCO, and CH 4 + CO are all minor (e5%).

A comparative theoretical study is presented on the formation and decomposition of R-hydroxy-alkylperoxyl radicals, Q(OH)OO • (Q ) RR′C:), important intermediates in the oxidation of several classes of oxygenated organic compounds in atmospheric chemistry, combustion, and liquid-phase autoxidation of hydrocarbons. Detailed potential energy surfaces (PESs) were computed for the HOCH 2 O 2 • h HO 2 • + CH 2 O reaction and its analogues for the alkyl-substituted RCH(OH)OO • and R 2 C(OH)OO • and the cyclic cyclo-C 6 H 10 (OH)OO • . The state-of-the-art ab initio methods G3 and CBS-QB3 and a nearly converged G2M//B3LYP-DFT variant were found to give quasi-identical results. On the basis of the G2M//B3LYP-DFT PES, the kinetics of the ≈15 kcal/mol endothermal R-hydroxy-alkylperoxyl decompositions and of the reverse HO 2 • + ketone/aldehyde reactions were evaluated using multiconformer transition state theory. The excellent agreement with the available experimental (kinetic) data validates our methodologies. Contrary to current views, HO 2 • is found to react as fast with ketones as with aldehydes. The high forward and reverse rates are shown to lead to a fast Q(OH)-OO • h HO 2 • + carbonyl quasi-equilibrium. The sizable [Q(OH)OO • ]/[carbonyl] ratios predicted for formaldehyde, acetone, and cyclo-hexanone at the low temperatures (below 220 K) of the earth's tropopause are shown to result in efficient removal of these carbonyls through fast subsequent Q(OH)OO • reactions with NO and HO 2 • . IMAGES model calculations indicate that at the tropical tropopause the HO 2 • -initiated oxidation of formaldehyde and acetone may account for 30% of the total removal of these major atmospheric carbonyls, thereby also substantially affecting the hydroxyl and hydroperoxyl radical budgets and contributing to the production of formic and acetic acids in the upper troposphere and lower stratosphere. On the other hand, an RRKM-master equation analysis shows that hot R-hydroxy-alkylperoxyls formed by the addition of O 2 to C 1 -, C 2 -, and C 3 -R-hydroxy-alkyl radicals will quasi-uniquely fragment to HO 2 • plus the carbonyl under all atmospheric conditions.

The product distribution of the reaction of acetic acid, CH 3 COOH, with hydroxyl radicals, OH, was studied experimentally and theoretically. Mass-spectrometric measurements at 290 K and 2 Torr of He of the CO 2 yield versus the loss of acetic acid yielded a branching fraction of 64 ( 14% for the abstraction of the acidic hydrogen as follows:

The potential energy surfaces of dithioformic acid and its functional isomer, dithiohydroxy carbene, in both neutral and ionized states have been investigated using ab initio molecular orbital calculations at both (U)-MP2 and (U)QCISD(T) levels with the 6-31G(d,p) and 6-311++G(d,p) basis sets. The unimolecular decomposition of HC(dS)SH giving H 2 + CS 2 and H 2 S + CdS, its rearrangement to HS-C-SH, and different simple bond cleavages have been examined. In both electronic states, the carbene is a stable isomer lying in a relatively deep potential well. Formation of H 2 S via a one-step decomposition of acid is found to be favored over that of CS 2 whose reaction path involves a carbene intermediate. Carbene radical cations also participate as crucial intermediates in unimolecular transformation of ionized acids. Calculations on different hydrogenbonded dimers of acid and carbene suggest that, in the dimer form, while the carbene is not stable, elimination of both H 2 and H 2 S is accelerated. The role of H 2 S and H 2 as possible catalysts in the acid decomposition has also been investigated. H 2 S induces an efficient catalytic effect in different hydrogen transfer pathways. The standard heats of formation are estimated to be ∆H°f ,298 [HC(dS)SH] ) 113 ( 8 kJ/mol and ∆H°f ,298 [HS-C-SH] ) 267 ( 12 kJ/mol based on CCSD(T)/6-311++G(3df,2p) calculations. Adiabatic ionization energies are predicted to be IE a (HS-C-SH) ) 8.2 ( 0.3 eV and IE a (HC(dS)SH)] ) 9.0 ( 0.3 eV, and proton affinities are PA[HC(dS)SH] ) 802 ( 12 kJ/mol and PA[HS-C-SH] ) 956 ( 12 kJ/mol.