Hindered Internal Rotation and Ortho-H2 Enrichment in Trans-Stilbene--H2/D2 Complexes

A supersonic free jet expansion has been used to prepare trans-stilbene--H2 and D2 complexes. The cooling in the jet collapses most of the ortho and para H2 and D2 rotational population to the lowest rotational levels of a given nuclear spin symmetry: j = 0 and j = 1. The laser-induced fluorescence excitation spectrum of stilbene--D2 shows a well-resolved doublet at the origin due to stilbene--D2( j = 0) and stilbene--D2( j = 1) complexes. The 4.9 cm-1 splitting of these transitions indicates that the D2 molecule is undergoing hindered internal rotation in the complex and that the barrier to internal rotation changes upon electronic excitation. The relative intensities of the stilbene--D2( j = 0) and stilbene--D2( j = 1) origins depend on the D2concentration in the jet. At low D2 flows the transitions arising from stilbene--D2( j = 1) are favored while at high D2 flows the ( j = 0)/(j = 1) transition intensities approach the 2:1 intensity ratio given by their nuclear spin statistical weights. By contrast, in stilbene--H2 we observe only a single transition at the origin which we assign to stilbene--H2( j = 1). We are able to place an upper bound on the stilbene--H2( j = 0) transition intensity of 5% of the stilbene--H2( j = 1) intensity. Dispersed fluorescence spectra are used to bracket the binding energies of the stilbene--H2/D2 complexes in both ground and excited states

Modeling Studies of the Effects of the Heterogeneous Reaction ClOOCl + HCl → Cl2 + HOOCl on Stratospheric Chlorine Activation and Ozone Depletion

The heterogeneous reaction ClOOCl + HCl → Cl2 + HOOCl was introduced into a chemical trajectory model of the stratosphere. Ten-day trajectories ending at ozonesonde stations at various northern latitudes were run to simulate the period January–March 1994. The reaction on sulfuric acid aerosol surfaces has a negligible effect on ozone chemistry if a sticking coefficient similar to that of ClONO2 + HCl is assumed. On polar stratospheric cloud (PSC) surfaces the chemical effects of the addition of this reaction depend on the fate of proposed product HOOCl: if this species photolyzes to produce either ClO + OH or Cl + HO2, then HCl is activated by the reaction with chlorine peroxide. This heterogeneous activation of chlorine by active chlorine can have a significant effect on Arctic ozone depletion rates in the days following an air parcel\u27s encounter with PSC surfaces. The ozone depletion rate usually increased but in some cases decreased, depending on the extent of PSC processing and on the initial [HCl]/[ClONO2] ratio. Averaged over 3 months, the column ozone loss rates between 350 and 675 K were accelerated by as much as 35% for a set of 10-day trajectories ending at an Arctic station. If, on the other hand, HOOCl decomposes at the surface into HCl and O2, the net effect of these reactions is to convert ClOOCl into Cl2. These species are functionally equivalent, and such a conversion does not perturb the model chemistry

State Mixing and Vibrational Predissociation in Large Molecule Van Der Waals Complexes: Trans‐Stilbene–X Complexes Where X=He, H2, Ne, and Ar

We report a detailed study of vibrational predissociation and intramolecular–intermolecular state mixing in the first excited singlet state of t r a n s‐stilbene van der Waals complexes with helium, hydrogen, neon, and argon. We present evidence that the helium atom in stilbene–He and the H2 molecule in stilbene–H2 possess very low frequency van der Waals bending levels involving delocalization of the complexed species over both phenyl rings. In stilbene–He, the mode‐selective, strong coupling of the out‐of‐plane phenyl ring modes with the pseudotranslation van der Waals modes leads to a dramatic, inhomogeneous broadening of the transitions to several times their breadth in in‐plane vibrations. The observed dispersed fluorescence spectra give product state distributions and internal clock lifetime estimates which can only be made consistent with direct lifetime measurements by assuming extensive state mixing of the intramolecular levels with the van der Waals levels in which the states accessed by the laser are actually only about 30% intramolecular in character. We conclude that in these complexes the processes of intramolecular–intermolecular state mixing (static IVR) and vibrational predissociation are not independent processes but are closely tied to one another. In fact, the vibrational product state distributions observed for the out‐of‐plane phenyl ring levels can best be interpreted as reflecting the percentage van der Waals character in the initially prepared state. In stilbene–H2 the mode selective coupling exhibits itself as a splitting of the out‐of‐plane transitions into a set of 5–6 closely spaced transitions separated by only about 1 cm− 1. The sequence of transitions is suggestive of an in‐plane potential for H2 motion which is nearly flat across the entire length of the stilbene molecule with a small barrier presented by the ethylenic carbons through which the H2 molecule can tunnel. Dispersed fluorescence spectra from these levels point to a two‐tiered coupling scheme with the bound van der Waals levels. In contrast, the out‐of‐plane phenyl transitions in stilbene–Ne and stilbene–Ar possess unusual shifts, but the transitions are narrow once again. In these cases the complexed atom appears to be largely localized over a single phenyl ring

Atmospheric Condensed-Phase Reactions of Glyoxal with Methylamine

[1] Glyoxal reacts with methylamine in drying cloud droplet/aerosol surrogates to form high molecular mass oligomers along with smaller amounts of 1,3-dimethylimidazole and light-absorbing compounds. The patterns observed by high-resolution time-of-flight aerosol mass spectrometry indicate that oligomers form from repeated imine units. The reactions are 1st order in each reactant: rate-limiting imine formation is followed by rapid dimer and oligomer formation. While excess methylamine evaporates from the droplet, half the glyoxal does not, due to self-oligomerization reactions that occur in the absence of methylamine. Glyoxal irreversibly traps volatile amine compounds in the aerosol phase, converting them into oligomers. This is the first reported mechanism for the formation of stable secondary organic aerosol (SOA) material from methylamine, a substance with only one carbon, and could produce as much as 11 Tg SOA yr−1 globally if glyoxal reacts exclusively by this pathway

Methylglyoxal Uptake Coefficients on Aqueous Aerosol Surfaces

In order to predict the amount of secondary organic aerosol formed by heterogeneous processing of methylglyoxal, uptake coefficients (γ) and estimates of uptake reversibility are needed. Here, uptake coefficients are extracted from chamber studies involving ammonium sulfate and glycine seed aerosol at high relative humidity (RH ≥ 72%). Methylglyoxal uptake coefficients on prereacted glycine aerosol particles had a strong dependence on RH, increasing from γ = 0.4 × 10–3 to 5.7 × 10–3 between 72 and 99% RH. Continuous methylglyoxal losses were also observed in the presence of aqueous ammonium sulfate at 95% RH (γAS,wet = 3.7 ± 0.8 × 10–3). Methylglyoxal uptake coefficients measured at ≥95% RH are larger than those reported for glyoxal on nonacidified, aqueous aerosol surfaces at 90% RH. Slight curvature in first-order uptake plots suggests that methylglyoxal uptake onto aqueous aerosol surfaces is not entirely irreversible after 20 min. Methylglyoxal uptake by cloud droplets was rapid and largely reversible, approaching equilibrium within the 1 min mixing time of the chamber. PTR-MS measurements showed that each cloud event extracted 3 to 8% of aerosol-phase methylglyoxal and returned it to the gas phase, likely by an oligomer hydrolysis mechanism

Brown Carbon Production by Aqueous-Phase Interactions of Glyoxal and SO2

Oxalic acid and sulfate salts are major components of aerosol particles. Here, we explore the potential for their respective precursor species, glyoxal and SO2, to form atmospheric brown carbon via aqueous-phase reactions in a series of bulk aqueous and flow chamber aerosol experiments. In bulk aqueous solutions, UV- and visible-light-absorbing products are observed at pH 3–4 and 5–6, respectively, with small but detectable yields of hydroxyquinone and polyketone products formed, especially at pH 6. Hydroxymethanesulfonate (HMS), C2, and C3 sulfonates are major products detected by electrospray ionization mass spectrometry (ESI-MS) at pH 5. Past studies have assumed that the reaction of formaldehyde and sulfite was the only atmospheric source of HMS. In flow chamber experiments involving sulfite aerosol and gas-phase glyoxal with only 1 min residence times, significant aerosol growth is observed. Rapid brown carbon formation is seen with aqueous aerosol particles at \u3e80% relative humidity (RH). Brown carbon formation slows at 50–60% RH and when the aerosol particles are acidified with sulfuric acid but stops entirely only under dry conditions. This chemistry may therefore contribute to brown carbon production in cloud-processed pollution plumes as oxidizing volatile organic compounds (VOCs) interact with SO2 and water

Brown Carbon from Photo-Oxidation of Glyoxal and SO2 in Aqueous Aerosol

Aqueous-phase dark reactions during the co-oxidation of glyoxal and S(IV) were recently identified as a potential source of brown carbon (BrC). Here, we explore the effects of sunlight and oxidants on aqueous solutions of glyoxal and S(IV), and on aqueous aerosol exposed to glyoxal and SO2. We find that BrC is able to form in sunlit, bulk-phase, sulfite-containing solutions, albeit more slowly than in the dark. In more atmospherically relevant chamber experiments where suspended aqueous aerosol particles are exposed to gas-phase glyoxal and SO2, the formation of detectable amounts of BrC requires an OH radical source and occurs most rapidly after a cloud event. From these observations we infer that this photobrowning is caused by radical-initiated reactions as evaporation concentrates aqueous-phase reactants and aerosol viscosity increases. Positive-mode electrospray ionization mass spectrometric analysis of aerosol-phase products reveals a large number of CxHyOz oligomers that are reduced rather than oxidized (relative to glyoxal), with the degree of reduction increasing in the presence of OH radicals. This again suggests a radical-initiated redox mechanism where photolytically produced aqueous radical species trigger S(IV)–O2 auto-oxidation chain reactions, and glyoxal-S(IV) redox reactions especially if aerosol-phase O2 is depleted. This process may contribute to daytime BrC production and aqueous-phase sulfur oxidation in the atmosphere. The BrC produced, however, is about an order of magnitude less light-absorbing than wood smoke BrC at 365 nm

Glyoxal’s impact on dry ammonium salts: fast and reversible surface aerosol browning

Alpha-dicarbonyl compounds are believed to form brown carbon in the atmosphere via reactions with ammonium sulfate (AS) in cloud droplets and aqueous aerosol particles. In this work, brown carbon formation in AS and other aerosol particles was quantified as a function of relative humidity (RH) during exposure to gas-phase glyoxal (GX) in chamber experiments. Under dry conditions (RH \u3c 5%), solid AS, AS/glycine, and methylammonium sulfate aerosol particles brown within minutes upon exposure to GX, while sodium sulfate particles do not. When GX concentrations decline, browning goes away, demonstrating that this dry browning process is reversible. Declines in aerosol albedo are found to be a function of [GX]2, and are consistent between AS and AS/glycine aerosol. Dry methylammonium sulfate aerosol browns 4´ more than dry AS aerosol, but deliquesced AS aerosol browns much less than dry AS aerosol. Optical measurements at 405, 450, and 530 nm provide an estimated Ångstrom absorbance coefficient of -16 ±4. This coefficient and the empirical relationship between GX and albedo are used to estimate an upper limit to global radiative forcing by brown carbon formed by 70 ppt GX reacting with AS (+7.6 ´10-5 W/m2). This quantity is \u3c 1% of the total radiative forcing by secondary brown carbon, but occurs almost entirely in the ultraviolet range