Secondary organic aerosols (SOA), which are produced by the transformations of volatile organic compounds in the atmosphere, play a central role in air quality, public health, visibility and climate, but their formation and aging remain poorly characterized. This study evidences a new mechanism for SOA formation based on photosensitized particulate-phase chemistry. Experiments were performed with a horizontal aerosol flow reactor where the diameter growth of the particles was determined as a function of various parameters. In the absence of gas-phase oxidant, experiments in which ammonium sulfate seeds containing glyoxal were exposed to gas-phase limonene and UV light exhibited a photo-induced SOA growth. Further experiments showed that this growth was due to traces of imidazole-2-carboxaldehyde (IC) in the seeds, a condensation product of glyoxal acting as an efficient photosensitizer. Over a 19 min irradiation time, 50 nm seed particles containing this compound were observed to grow between 3.5 and 30 +/- 3% in the presence of either limonene, isoprene, alpha-pinene, beta-pinene, or toluene in concentrations between 1.8 and 352 ppmv. The other condensation products of glyoxal, imidazole (IM) and 2,2-bi1H-imidazole (BI), also acted as photosensitizer but with much less efficiency under the same conditions. In the atmosphere, glyoxal and potentially other gas precursors would thus produce efficient photosensitizers in aerosol and autophotocatalyze SOA growth.

In recent years, it has been proposed that gas phase glyoxal could significantly contribute to ambient organic aerosol (OA) mass through multiphase chemistry. Of particular interest is the reaction between glyoxal and ammonium cations producing light-absorbing compounds such as imidazole derivatives. It was recently shown that imidazole-2-carboxaldehyde (IC) can act as a photosensitizer, initiating aerosol growth in the presence of gaseous volatile organic compounds. Given the potential importance of this new photosensitized growth pathway for ambient OA, the related reaction mechanism was investigated at a molecular level. Bulk and flow tube experiments were performed to identify major products of the reaction of limonene with the triplet state of IC by direct (±)ESI-HRMS and UPLC/(±)HESI-HRMS analysis. Detection of recombination products of IC with limonene or with itself, in bulk and flow tube experiments, showed that IC is able to initiate a radical chemistry in the aerosol phase under realistic irradiation conditions. Furthermore, highly oxygenated limonene reaction products were detected, clearly explaining the observed OA growth. The chemistry of peroxy radicals derived from limonene upon addition of oxygen explains the formation of such low-volatile compounds without any traditional gas phase oxidant.

Imidacloprid (IMD) is the most widely used neonicotinoid insecticide found on environmental surfaces and in water. Analysis of surface-bound IMD photolysis products was performed using attenuated total reflectance Fourier transfer infrared (ATR-FTIR) analysis, electrospray ionization (ESI-MS), direct analysis in real time mass spectrometry (DART-MS), and transmission FTIR for gas-phase products. Photolysis quantum yields (ϕ) for loss of IMD were determined to be (1.6 ± 0.6) × 10 (1s) at 305 nm and (8.5 ± 2.1) × 10 (1s) at 254 nm. The major product is the imidacloprid urea derivative (IMD-UR, 84% yield), with smaller amounts of the desnitro-imidacloprid (DN-IMD, 16% yield) product, and gaseous nitrous oxide (NO). Theoretical calculations show that the first step of the main mechanism is the photodissociation of NO, which then recombines with the ground electronic state of IMD radical to form IMD-UR and NO in a thermally driven process. The photolytic lifetime of IMD at a solar zenith angle of 35° is calculated to be 16 h, indicating the significant reaction of IMD over the course of a day. Desnitro-imidacloprid has been identified by others as having increased binding to target receptors compared to IMD, suggesting that photolysis on environmental surfaces increases toxicity.

Abstract. The multiphase chemistry of glyoxal is a source of secondary organic aerosol (SOA), including its light-absorbing product imidazole-2-carboxaldehyde (IC). IC is a photosensitizer that can contribute to additional aerosol ageing and growth when its excited triplet state oxidizes hydrocarbons (reactive uptake) via H-transfer chemistry. We have conducted a series of photochemical coated-wall flow tube (CWFT) experiments using films of IC and citric acid (CA), an organic proxy and H donor in the condensed phase. The formation rate of gas-phase HO2 radicals (PHO2) was measured indirectly by converting gas-phase NO into NO2. We report on experiments that relied on measurements of NO2 formation, NO loss and HONO formation. PHO2 was found to be a linear function of (1) the [IC]  ×  [CA] concentration product and (2) the photon actinic flux. Additionally, (3) a more complex function of relative humidity (25 %  <  RH  <  63 %) and of (4) the O2 ∕ N2 ratio (15 %  <  O2 ∕ N2  <  56 %) was observed, most likely indicating competing effects of dilution, HO2 mobility and losses in the film. The maximum PHO2 was observed at 25–55 % RH and at ambient O2 ∕ N2. The HO2 radicals form in the condensed phase when excited IC triplet states are reduced by H transfer from a donor, CA in our system, and subsequently react with O2 to regenerate IC, leading to a catalytic cycle. OH does not appear to be formed as a primary product but is produced from the reaction of NO with HO2 in the gas phase. Further, seed aerosols containing IC and ammonium sulfate were exposed to gas-phase limonene and NOx in aerosol flow tube experiments, confirming significant PHO2 from aerosol surfaces. Our results indicate a potentially relevant contribution of triplet state photochemistry for gas-phase HO2 production, aerosol growth and ageing in the atmosphere.

The neonicotinoid nitenpyram (NPM) is a multifunctional nitroenamine [(R1N)(R2N)C=CHNO2] pesticide. As a nitroalkene, it is structurally similar to other emerging contaminants such as the pharmaceuticals ranitidine and nizatidine. Because ozone is a common atmospheric oxidant, such compounds may be oxidized on contact with air to form new products that have different toxicity compared to the parent compounds. Here we show that oxidation of thin solid films of NPM by gas-phase ozone produces unexpected products, the majority of which do not contain oxygen, despite the highly oxidizing reactant. A further surprising finding is the formation of gas-phase nitrous acid (HONO), a species known to be a major photolytic source of the highly reactive hydroxyl radical in air. The results of application of a kinetic multilayer model show that reaction was not restricted to the surface layers but, at sufficiently high ozone concentrations, occurred throughout the film. The rate constant derived for the O3−NPM reaction is 1 × 10−18cm3⋅s−1, and the diffusion coefficient of ozone in the thin film is 9 × 10−10cm2⋅s−1. These findings highlight the unique chemistry of multifunctional nitroenamines and demonstrate that known chemical mechanisms for individual moieties in such compounds cannot be extrapolated from simple alkenes. This is critical for guiding assessments of the environmental fates and impacts of pesticides and pharmaceuticals, and for providing guidance in designing better future alternatives.

The environmental fates of nitenpyram (NPM), a widely used neonicotinoid insecticide, are not well-known. A thin solid film of NPM deposited on a germanium attenuated total reflectance (ATR) crystal was exposed to radiation from a low-pressure mercury lamp at 254 nm, or from broadband low pressure mercury photolysis lamps centered at 350 or 313 nm. The loss during photolysis was followed in time using FTIR. The photolysis quantum yields (ϕ), defined as the number of NPM molecules lost per photon absorbed, were determined to be (9.4 ± 1.5) × 10 at 350 nm, (1.0 ± 0.3) × 10 at 313 nm, and (1.2 ± 0.4) × 10 at 254 nm (±2σ). Imines, one with a carbonyl group, were detected as surface-bound products and gaseous NO was generated in low (11%) yield. The UV-vis absorption spectra of NPM in water was different from that in acetonitrile, dichloromethane, and methanol, or in a thin solid film. The photolytic lifetime of solid NPM at a solar zenith angle at 35° is calculated to be 36 min, while that for NPM in water is 269 min, assuming that the quantum yield is the same as in the solid. Thus, there may be a significant sensitivity to the medium for photolytic degradation and the lifetime of NPM in the environment.

Neonicotinoids (NN), first introduced in 1991, are found on environmental surfaces where they undergo photolytic degradation. Photolysis studies of thin films of NN were performed using two approaches: (1) transmission FTIR, in which solid films of NN and the gas-phase products were analyzed simultaneously, and (2) attenuated-total-reflectance FTIR combined with transmission FTIR, in which solid films of NN and the gas-phase products were probed in the same experiment but not at the same time. Photolysis quantum yields using broadband irradiation centered at 313 nm were (2.2 ± 0.9) × 10 −3 for clothianidin (CLD), (3.9 ± 0.3) × 10 −3 for thiamethoxam (TMX), and (3.3 ± 0.5) × 10 −3 for dinotefuran (DNF), with all errors being ±1s. At 254 nm, which was used to gain insight into the wavelength dependence, quantum yields were in the range of (0.8−20) × 10 −3 for all NNs, including acetamiprid (ACM) and thiacloprid (TCD). Nitrous oxide (N 2 O), a potent greenhouse gas, was the only gas-phase product detected for the photolysis of nitroguanidines, with yields of ΔN 2 O/ΔNN > 0.5 in air at both 313 and 254 nm. The atmospheric lifetimes with respect to photolysis for CLD, TMX, and DNF, which absorb light in the actinic region, are estimated to be 15, 10, and 11 h, respectively, at a solar zenith angle of 35°and 12, 8, and 10 h at a solar zenith angle of 15°.

Reactions in the atmospheric aqueous phase are an important source of secondary organic aerosols (SOA). Within the present study, the reactions of triplet-state imidazole-2-carboxaldehyde (32-IC*) with methyl vinyl ketone (MVK, R1), methacrolein (MACR, R2), and methacrylic acid (MAA, R3), as well as the reaction of triplet-state 3,4-dimethoxybenzaldehyde (3DMB*) with the unsaturated compounds (MVK, R4), (MACR, R5), and (MAA, R6), in the aqueous phase were investigated using laser flash excitation–laser long path absorption and ultraperformance liquid chromatography coupled with high definition electrospray ionization spectrometry. The second-order reaction constants for 32-IC* were determined to be k 1 = (1.0 ± 0.1) × 109 L mol–1 s–1 at pH 4–5 and 9, k 2 = (1.4 ± 0.4) × 109 L mol–1 s–1 and (1.5 ± 0.1) × 109 L mol–1 s–1 at pH 4–5 and 9, and k 3 = (1.4 ± 0.4) × 109 L mol–1 s–1 and (1.1 ± 0.4) × 108 L mol–1 s–1 at pH 4–5 and 9, respectively. The main products of the [2 + 2] photocycloaddition reactions of 32-IC* with both monomer and dimer of MVK as well as MACR were characterized. Similarly, the [2 + 2] photocycloaddition of the carbonyl of the excited triplet state of 3,4-dimethoxybenzaldehyde (3DMB*) with MVK was observed. The second order rate constants for the reactions of 3DMB* were determined: k 4 = (1.5 ± 0.2) × 108 L mol–1 s–1, k 5 = (2.8 ± 0.5) × 108 L mol–1 s–1, and k 6 = (5.2 ± 1.2) × 106 L mol–1 s–1 at pH 9. The studied reactions show that different triplet photosensitizers react with strongly varying rate constants. Advanced CAPRAM process model studies show that active photosensitizers such as 3DMB* can quickly react with unsaturated organic compounds under deliquesced aerosol conditions modifying SOA, while the quenching with oxygen dominates the excited photosensitizer loss under cloud conditions.