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As smaller particle sizes are increasingly included in microplastic research, it is critical to chemically characterize microparticles to identify whether particles are indeed microplastics. To increase the accessibility of methods for characterizing microparticles via Raman spectroscopy, we created an application-based library of Raman spectroscopy parameters specific to microplastics based on color, morphology, and size. We also created two spectral libraries that are representative of microplastics found in environmental samples. Here, we present SLoPP, a spectral library of plastic particles, consisting of 148 reference spectra, including a diversity of polymer types, colors, and morphologies. To account for the effects of aging on microplastics and associated changes to Raman spectra, we present a spectral library of plastic particles aged in the environment (SLoPP-E). SLoPP-E includes 113 spectra, including a diversity of types, colors, and morphologies. The microplastics used to make SLoPP-E include environmental samples obtained across a range of matrices, geographies, and time. Our libraries increase the likelihood of spectral matching for a broad range of microplastics because our libraries include plastics containing a range of additives and pigments that are not generally included in commercial libraries. When used in combination with commercial libraries of over 24 000 spectra, 63% of the top 5 matches across all particles tested (product and environmental) are from SLoPP and SLoPP-E. These tools were developed to improve the accessibility of microplastics research in response to a growing and multidisciplinary field, as well as to enhance data quality and consistency.

Infrared action spectroscopy is utilized to characterize the gas-phase, hydrogen-bonded H(2)O-HO complex, a primary interaction in the hydration of the hydroxyl radical. The OH radical stretch of the H(2)O-HO complex is identified at 3490 cm(-1), shifted 78 cm(-1) to lower frequency of the OH monomer transition. The stability of the complex, D(0) < or = 5.14 kcal mol(-1), is derived from the highest observed OH product channel in the associated product state distribution. The assignment is supported by high level ab initio calculations of the spectral shift of the binary complex from free OH and its dissociation energy, D(e)(CBS-infinity) = 5.6 kcal mol(-1). A second weaker feature, appearing 15 cm(-1) to lower frequency at 3475 cm(-1), is attributed to a hot band, the OH radical stretch originating from an out-of-plane H(2)O bending state, based on two-dimensional calculations of frequencies and strengths of transitions involving the coupled vibrational modes.

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The reaction of nitric acid with the hydroxyl radical influences the residence time of HONO2 in the lower atmosphere. Prior studies [Brown SS, Burkholder JB, Talukdar RK, Ravishankara AR (2001) J Phys Chem A 105:1605-1614] have revealed unusual kinetic behavior for this reaction, including a negative temperature dependence, a complex pressure dependence, and an overall reaction rate strongly affected by isotopic substitution. This behavior suggested that the reaction occurs through an intermediate, theoretically predicted to be a hydrogen-bonded OH-HONO2 complex in a six-membered ring-like configuration. In this study, the intermediate is generated directly by the association of photolytically generated OH radicals with HONO2 and stabilized in a pulsed supersonic expansion. Infrared action spectroscopy is used to identify the intermediate by the OH radical stretch ( 1) and OH stretch of nitric acid ( 2) in the OH-HONO2 complex. Two vibrational features are attributed to OH-HONO2: a rotationally structured 1 band at 3516.8 cm ؊1 and an extensively broadened 2 feature at 3260 cm ؊1 , both shifted from their respective monomers. These same transitions are identified for OD-DONO 2. Assignments of the features are based on their vibrational frequencies, analysis of rotational band structure, and comparison with complementary high level ab initio calculations. In addition, the OH (v ‫؍‬ 0) product state distributions resulting from 1 and 2 excitation are used to determine the binding energy of OH-HONO2, D0 < 5.3 kcal⅐mol ؊1 , which is in good accord with ab initio predictions.atmospheric chemistry ͉ hydroxyl radical ͉ nitric acid ͉ reaction intermediate N itric acid is a chemically inactive and photochemically stable reservoir for reactive HO x and NO x species in the atmosphere. In the troposphere, HONO 2 is usually removed by dry deposition or rainout faster than it is photochemically converted back to NO x , making it a permanent sink for NO x (1, 2). In the upper troposphere and stratosphere, where there is no rain, HONO 2 can be removed by solar photolysis and reaction with OH (3, 4),This reaction results in the conversion of HONO 2 into reactive NO x species, because the unstable nitrate radical rapidly decomposes into NO or NO 2 .Kinetic studies have shown that the OH ϩ HONO 2 reaction is unusual in several respects (3, 5-11), particularly under the temperature and pressure conditions found in the lower atmosphere. Under these conditions, this reaction exhibits a negative temperature dependence, a pressure dependence, and a strong kinetic isotope effect. The indirect mechanism inferred from these studies consists of initial OH radical addition forming an energized OH-HONO 2 * intermediate, followed by redissociation or reaction to form products. The energized intermediate can also be temporarily stabilized by collisions with bath gas M, and then redissociate to reactants or decompose to products as shown in the following scheme:The unusual kinetic behavior arises because of enhanced formation and collisional st...