22 Search Results

We evaluate the capability of chemical ionization mass spectrometry (CIMS) using benzene cations as reagent ions (benzene CIMS) for detecting atmospheric trace gases. We characterize the ionization pathways and product ion distributions for 27 analytes spanning diverse chemical classes. To interpret the complex ion chemistry involving two reagent ions (C₆H$$^{+}_{6}$$ and (C₆H₆)$$^{+}_{2}$$) and multiple ionization pathways (charge transfer, proton transfer, adduct formation, and hydride abstraction), we introduce a thermodynamics-based framework that classifies analytes into three categories based on their ionization energy (IE), relative to those of benzene monomer (9.24 eV) and dimer (8.69 eV). Each class exhibits distinct ionization mechanismsmore » and product ions. Analytes with IE smaller than 8.69 eV (low IE) undergo charge transfer with both reagent ions; analytes with IE between 8.69 and 9.24 eV (mid IE) undergo charge transfer with C₆H$$^{+}_{6}$$ and potential adduct formation with (C₆H₆)$$^{+}_{2}$$; analytes with IE larger than 9.24 eV (high IE) could undergo adduct formation, proton transfer, or hydride abstraction. Analytes within each class also show similar sensitivity, enabling sensitivity estimation for compounds lacking calibration standards. In addition to volatile organic compounds (VOCs), benzene CIMS detects nitric oxide (NO) with a detection limit of 5 pptv for 1 min integration time, exceeding the performance of most commercial NOx analyzers. Field deployments in Chicago and St. Louis demonstrate good agreement with reference NO measurements. Isoprene measurements show good agreement with a co-located gas chromatography–photoionization detector (GC-PID) in St. Louis, but exhibit substantial positive bias in Chicago, likely due to interferences from anthropogenic VOCs in the polluted urban environment. These results highlight the potential of benzene CIMS for concurrent measurements of NO, VOCs, and their oxidation products using a single instrument, while also underscoring challenges in complex atmospheric conditions.« less

Aerosol properties were characterized at a rural site southwest of Houston from May to September 2022 during the intensive operation periods (IOPs) of the Tracking Aerosol Convection Interactions ExpeRiment (TRACER). Backward trajectory analysis reveals three major air mass types: marine air mass from the Gulf, urban air mass influenced by urban emissions, and regional air mass. Marine aerosols typically show a bimodal size distribution and have the lowest particle number and mass concentrations of PM₁ (particulate matter with an aerodynamic diameter of less than 1 µm), while aerosols from air masses strongly influenced by urban emissions exhibit the highest concentrations.more » Organic aerosol (OA) accounts for more than 50 % of PM₁ for urban and regional air masses, whereas sulfate is comparable to OA in marine air masses. Positive matrix factorization (PMF) analysis of aerosol mass spectra identifies 6 OA factors: hydrocarbon-like OA (HOA), OA from the oxidation of monoterpenes (MT-SOA), OA from the reactive uptake of isoprene epoxydiols by acidic sulfate particles (isoprene-SOA), oxygenated OA arising from shipping emissions (shipping-OOA), and two oxygenated OA factors with high O : C ratios (OOA1 and OOA2). OOA2 has the highest O : C ratio and exhibits elevated mass concentration in the afternoon. Similar diurnal variation of highly oxidized OA factors was commonly observed in the Houston area during previous studies and attributed to the SOA formation by photochemistry and mixing from aloft. Here, using air mass backward trajectories and a 1-D box model, we show the diurnal trend of OOA2 mass concentration is instead driven by changes in air mass arriving at the rural site. The air mass changes are likely caused by the shift between land breezes and sea/bay breezes. Within the same air mass type (e.g., either urban or marine air mass), OOA2 mass concentration is largely independent of wind direction and shows essentially no diurnal variation, suggesting OOA2 is related to aged OA with minimal influence by local emissions. This study helps identify the major sources of OA in the Houston region and highlights the impacts of both atmospheric chemistry and meteorology on aerosol properties in the coastal–rural environment.« less

The behavior of the dynamical electron correlation energy is remarkably complex at short internuclear distances: Δ E _DEC (Δ R ) = E _DEC (Δ R ) − E _DEC ( R = ∞) with Δ R = R − R _e .

Abstract The current study of the small lithium molecules Li₂^+,0,−and Li₃^+,0,−focuses on the nature of the bonding in these molecules as well as their structures and energetics (bond energies, ionization energies, and electron affinities). Valence CASSCF (2s,2p) calculations incorporate nondynamical electron correlation in the calculations, while the corresponding multireference configuration interaction and coupled cluster calculations incorporate dynamical electron correlation. Treatment of nondynamical correlation is critical for properly describing the Li_2,3^+,0,−molecules as well as the Li⁻anion with dynamical correlation, in general, only fine‐tuning the predictions. All lithium molecules and ions are bound, with the Li₃⁺and Li₂⁺ions being the most strongly bound,more » followed by Li₃⁻, Li₂, Li₂⁻and Li₃. The minimum energy structures of Li₃^+,0,−are, respectively, an equilateral triangle, an isosceles triangle, and a linear structure. The results of SCGVB calculations are analyzed to obtain insights into the nature of the bonding in these molecules. An important finding of this work is that interstitial orbitals, a concept first put forward by McAdon and Goddard in 1985, play an essential role in the bonding of all lithium molecules considered here except for Li₂. The interstitial orbitals found in the Li₃^+,0molecules likely give rise to the non‐nuclear attractors/maxima observed in these molecules.« less

The atmospheric sulfur cycle plays a key role in air quality, climate, and ecosystems, such as pollution, radiative forcing, new particle formation, and acid rain. In this study, we compare the spatially and temporally resolved measurements from the NASA Atmospheric Tomography (ATom) mission with simulations from five AeroCom III models for four sulfur species (dimethyl sulfide (DMS), sulfur dioxide (SO₂), particulate methanesulfonate (MSA), and particulate sulfate (SO₄)). We focus on remote regions over the Pacific, Atlantic, and Southern oceans from near the surface to ~12 km altitude range covering all four seasons. In general, the differences among model results canmore » be greater than 1 order of magnitude. Comparing with observations, model-simulated SO₂ is generally low, whereas SO₄ is generally high. Simulated DMS concentrations near the sea surface exceed observed levels by a factor of 5 in most cases, suggesting potential overestimation of DMS emissions in all models. With GEOS model simulations of tagging emission from anthropogenic, biomass burning, volcanic, and oceanic sources, we find that anthropogenic emissions are the dominant source of sulfate aerosol (40 %–60 % of the total amount) in the ATom measurements at almost all altitudes, followed by volcanic emissions (18 %–32 %) and oceanic sources (16 %–32 %). Similar source contributions can also be derived at broad ocean basins and on monthly scales, indicating the representativeness of ATom measurements for global ocean. Our work presents the first assessment of AeroCom sulfur study using ATom measurements, providing directions for improving sulfate simulations, which remain the largest uncertainty in radiative forcing estimates in aerosol climate models.« less

We extended our studies of the effect of dynamical electron correlation on the covalent bonds in the AH and AF series (A = B–F) to the recoupled pair bonds in the excited $$a$$⁴Σ^- states of the CH and CF molecules. Dynamical correlation is energetically less important in the $$a$$⁴Σ^- states than in the corresponding X²Π states for both molecules, which is reflected in smaller changes in bond energies ($$D$$_e). Changes in the equilibrium bond distance ($$R$$_e) and vibrational frequency ($$ω$$_e), on the other hand, are influenced by the changes in the slope and curvature of the dynamical electron correlation energymore » as a function of the internuclear distance $$R$$, $$E$$_DEC($$R$$). In the CH($$a$$⁴Σ^-) state, these changes are much smaller than in the CH(X²Π) state, but in the CF($$a$$⁴Σ^-) state, they are larger, reflecting a significant difference in the shapes of $$E$$_DEC($$R$$) curves. At large $$R$$, the shape of $$E$$_DEC($$R$$) curves for covalent and recoupled pair bonds is similar although different in magnitude. For the CH($$a$$⁴Σ^-) state, $$E$$_DEC($$R$$) has a minimum at $$R$$ = $$R$$_e + 0.72 Å as the orbitals associated with the formation of the recoupled pair bond switch places. $$E$$_DEC(R) for the CF($$a$$⁴Σ^-) state decreases continuously throughout the bound region of the potential energy curve because the dynamical correlation energy associated with the electrons in the lone pair orbitals is increasing. In conclusion, these results support our earlier conclusion that we still have much to learn about the nature of dynamical electron correlation in molecules.« less

Dynamical electron correlation has a major impact on the computed values of molecular properties and the energetics of molecular processes. This study focused on the effect of dynamical electron correlation on the spectroscopic constants ($$R$$_e, $$ω$$_e, $$D$$_e), and potential energy curves, Δ$$E$$($$R$$), of the covalently bound AH and AF molecules, A = B–F. The changes in the spectroscopic constants (Δ$$R$$_e, Δ$$ω$$_e, Δ$$D$$_e) caused by dynamical correlation are erratic and, at times, even surprising. These changes can be understood based on the dependence of the dynamical electron correlation energies of the AH and AF molecules as a function of the bondmore » distance, i.e., Δ$$E$$_DEC($$R$$). At large $$R$$, the magnitude of Δ$$E$$_DEC($$R$$) increases nearly exponentially with decreasing $$R$$, but this increase slows as $$R$$ continues to decrease and, in many cases, even reverses at very short $$R$$. The changes in Δ$$E$$_DEC($$R$$) in the region around $$R$$_e were as unexpected as they were surprising, e.g., distinct minima and maxima were found in the curves of Δ$$E$$_DEC($$R$$) for the most polar molecules. The variations in Δ$$E$$_DEC($$R$$) for $$R$$ ≲ $$R$$_e are directly correlated with major changes in the electronic structure of the molecules as revealed by a detailed analysis of the spin-coupled generalized valence bond wave function. In conclusion, the results reported here indicate that we have much to learn about the nature of dynamical electron correlation and its effect on chemical bonds and molecular properties and processes.« less

Hydroxymethanesulfonate (HMS) is produced in the aqueous-phase reaction of formaldehyde (HCHO) and sulfur dioxide (SO₂) and has been proposed as a significant contributor to midlatitude wintertime pollution events. In this work, we report HMS detection within submicrometer atmospheric aerosols during frequent late summer, regional fog events in an Arctic oil field. The number fraction of individual particles containing HMS increased during fog periods, consistent with aqueous-phase formation. The single-particle mass spectra showed the primary particle signature (oil field emissions), plus secondary oxidized organics and sulfate, consistent with aqueous-phase processing. HMS mass concentrations ranged from below the ion chromatography limit ofmore » detection (0.3 ng/m³) to 1.6 ng/m³, with sulfate concentrations of 37-222 ng/m³. HCHO and SO₂ measurements suggest that the fog HMS production rate is ~10 times higher in the oil fields than in the upwind Beaufort Sea. Aqueous-phase reactions of local oil field emissions during frequent summertime regional fog events likely have downwind impacts on Arctic aerosol composition. The potential for fog-based HMS production was estimated to be an order of magnitude higher in Fairbanks and Anchorage, AK, than in the oil fields and may explain the missing organosulfate source contributing to Fairbanks air quality.« less