Secondary Organic Aerosol Formation from Aromatic Alkene Ozonolysis: Influence of the Precursor Structure on Yield, Chemical Composition, and Mechanism

Insight into the crucial reason causing the difference in secondary organic aerosol yields of monocyclic aromatic hydrocarbons with different methyl substituent numbers

The secondary organic aerosol (SOA) yield of toluene photooxidation was reported to substantially higher than that of trimethylbenzene due to the effect of the number of methyl substituents. However, the intrinsic mechanism for this disparity is not clear enough. In this study, a highly-sensitive thermal-desorption photoinduced associative ionization mass spectrometer (TD-PAI-MS) was used to real-time characterize the molecular composition and its evolution of the SOA generated from the photooxidation of toluene and 1,2,3-trimethylbenzene (1,2,3-TMB) in a smog chamber. In the new particle formation (NPF) stage, toluene generated more variety of nucleation precursors, such as benzaldehyde (MW 106) and benzoic acid (MW 122), resulting in a much higher nucleation rate and SOA number concentration. In the SOA growth/aging stage, the key SOA components of toluene were mainly dialdehydes, e.g., 2-oxopropanedial (MW 86) and 4-oxopent-2-enedial (MW 112), which played an important role in the formation of highly oxidized species (HOS) through oligomerization or cyclization reactions. In contrast, due to the presence of more methyl groups, 1,2,3-TMB was inclined to produce ketones, e.g., 2,3-butanedione (MW 86) and 3-methyl-4-oxopent-2-enal (MW 112), which would be cleaved into high-volatility low molecular compounds, e.g., acetic acid, through fragmentation. Taken together, relative to 1,2,3-TMB, the higher nucleation rate during NPF and the significant oligomerization/functionalization process during SOA growth are thought to be the major reasons resulting in the higher SOA yield of toluene. This work provides a reference for the insight into the different SOA yields of monocyclic aromatic hydrocarbons (MAHs) through further revealing the SOA formation mechanism during toluene and 1,2,3-TMB photooxidation.

Addressing new chemicals of emerging concern (CECs) in an indoor office

Indoor pollutants change over time and place. Exposure to hazardous organics is associated with adverse health effects. This work sampled gaseous organics by Tenax TA tubes in two indoor rooms, i.e., an office set as samples, and the room of chassis dynamometer (RCD) set as backgrounds. Compounds are analyzed by a thermal desorption comprehensive two-dimensional gas chromatography-quadrupole mass spectrometer (TD-GC × GC-qMS). Four new chemicals of emerging concern (CECs) are screened in 469 organics quantified. We proposed a three-step pipeline for CECs screening utilizing GC × GC including 1) non-target scanning of organics with convincing molecular structures and quantification results, 2) statistical analysis between samples and backgrounds to extract useful information, and 3) pixel-based property estimation to evaluate the contamination potential of addressed chemicals. New CECs spotted in this work are all intermediate volatility organic compounds (IVOCs), containing mintketone, isolongifolene, β-funebrene, and (5α)-androstane. Mintketone and sesquiterpenes may be derived from the use of volatile chemical products (VCPs), while (5α)-androstane is probably human-emitted. The occurrence and contamination potential of the addressed new CECs are reported for the first time. Non-target scanning and the measurement of IVOCs are of vital importance to get a full glimpse of indoor organics.

Comprehensive chemical characterization of gaseous I/SVOC emissions from heavy-duty diesel vehicles using two-dimensional gas chromatography time-of-flight mass spectrometry

Intermediate-volatility and semi-volatile organic compounds (I/SVOCs) are key precursors of secondary organic aerosol (SOA). However, the comprehensive characterization of I/SVOCs has long been an analytical challenge. Here, we develop a novel method of speciating and quantifying I/SVOCs using two-dimensional gas chromatography time-of-flight mass spectrometry (GC × GC-ToF-MS) by constructing class-screening programs based on their characteristic fragments and mass spectrum patterns. Using this new approach, we then present a comprehensive analysis of gaseous I/SVOC emissions from heavy-duty diesel vehicles (HDDVs). Over three-thousand compounds are identified and classified into twenty-one categories. The dominant compound groups of I/SVCOs emitted by HDDVs are alkanes (including normal and branched alkanes, 37-66%), benzylic alcohols (7-20%), alkenes (3-11%), cycloalkanes (3-9%), and benzylic ketones (1-4%). Oxygenated I/SVOCs (O-I/SVOCs, e.g., benzylic alcohols and ketones) are first quantified and account for >20% of the total I/SVOC mass. Advanced aftertreatment devices largely reduce the total I/SVOC emissions but increase the proportion of O-I/SVOCs. With the speciation data, we successfully map the I/SVOCs into the two-dimensional volatility basis set space, which facilitates a better estimation of SOA. As aging time goes by, approximate 45% difference between the two scenarios after seven-day aging is observed, which confirms the significant impact of speciated I/SVOC emission data on SOA prediction.

Pathogenic Mechanisms of Secondary Organic Aerosols

Air pollution represents a major health problem and an economic burden. In recent years, advances in air pollution research has allowed particle fractionation and identification of secondary organic aerosol (SOA). SOA is formed from either biogenic or anthropogenic emissions, through a mass transfer from the gaseous mass to the particulate phase in the atmosphere. They can have deleterious impact on health and the mortality of individuals with chronic inflammatory diseases. The pleiotropic effects of SOA could involve different and interconnected pathogenic mechanisms ranging from oxidative stress, inflammation, and immune system dysfunction. The purpose of this review is to present recent findings about SOA pathogenic roles and potential underlying mechanisms focusing on the lungs; the latter being the primary exposed organ to atmospheric pollutants.

Atmospheric photochemistry and secondary aerosol formation of urban air in Lyon, France

Photochemical aging of volatile organic compounds (VOCs) in the atmosphere is an important source of secondary organic aerosol (SOA). To evaluate the formation potential of SOA at an urban site in Lyon (France), an outdoor experiment using a Potential Aerosol Mass (PAM) oxidation flow reactor (OFR) was conducted throughout entire days during January-February 2017. Diurnal variation of SOA formations and their correlation with OH radical exposure (OHexp), ambient pollutants (VOCs and particulate matters, PM), Relative Humidity (RH), and temperature were explored in this study. Ambient urban air was exposed to high concentration of OH radicals with OHexp in range of (0.2-1.2)×10¹² molecule/(cm³•sec), corresponding to several days to weeks of equivalent atmospheric photochemical aging. The results informed that urban air at Lyon has high potency to contribute to SOA, and these SOA productions were favored from OH radical photochemical oxidation rather than via ozonolysis. Maximum SOA formation (36 µg/m³) was obtained at OHexp of about 7.4 × 10¹¹molecule/(cm³•sec), equivalent to approximately 5 days of atmospheric oxidation. The correlation between SOA formation and ambient environment conditions (RH & temperature, VOCs and PM) was observed. It was the first time to estimate SOA formation potential from ambient air over a long period in urban environment of Lyon.

Heterogeneous Ozonolysis of Tetrahydrocannabinol: Implications for Thirdhand Cannabis Smoke

Thirdhand smoke (THS) deposits to surfaces following smoking events and is a source of chemical exposure to humans. However, the evolution of THS in indoor environments is not well understood. Cannabis THS is a chemically distinct and prevalent form of THS, which has not been studied. The heterogeneous reaction of Δ⁹-tetrahydrocannabinol (THC), a major component of cannabis smoke, with ozone was examined as a pure compound and within cannabis smoke. Oxidative decay via ozonolysis and product formation were monitored by liquid chromatography-tandem mass spectrometry. Epoxide, dicarbonyl, and secondary ozonide THC reaction products were detected from both pure THC and cannabis experiments, with the product ratios dependent on relative humidity. The observed reaction kinetics for loss of THC on glass and cotton surfaces are consistent with a relatively short loss lifetime, which will be strongly dependent on the film thickness, ozone mixing ratio, and ozone reactivity of the surface substrate. The low volatility of THC and its oxidation products suggest that their contributions to thirdhand cannabis smoke will be less significant than the role that nicotine plays in thirdhand tobacco smoke.

Transient species in the ozonolysis of tetramethylethene

The reaction of alkenes with ozone has great effect on atmospheric oxidation, its transient species can produce OH radicals and contribute to the formation of secondary organic aerosols (SOA). In the present study, the reaction of tetramethylethene (TME) with ozone was investigated using self-assembled low temperature matrix isolation system. The TME and ozone were co-deposited on a salt plate at 15 K, and then slowly warmed up the plate. The first transient species primary ozonide (POZ) was detected, indicating that the reaction followed Criegee mechanism. Then POZ began to decompose at 180 K. However, secondary ozonide (SOZ) was not observed according to Criegee mechanism. Probably, Criegee Intermediate (CI) did not react with inert carbonyl of acetone, but with remaining TME formed tetra-methyl epoxide (EPO).

Rapid unimolecular reaction of stabilized Criegee intermediates and implications for atmospheric chemistry

Elucidating atmospheric oxidation mechanisms is necessary for estimating the lifetimes of atmospheric species and understanding secondary organic aerosol formation and atmospheric oxidation capacity. We report an unexpectedly fast mechanistic pathway for the unimolecular reactions of large stabilized Criegee intermediates, which involves the formation of bicyclic structures from large Criegee intermediates containing an aldehyde group. The barrier heights of the mechanistic pathways are unexpectedly low – about 2–3 kcal/mol – and are at least 10 kcal/mol lower than those of hydrogen shift processes in large syn Criegee intermediates; and the calculated rate constants show that the mechanistic pathways are 10⁵-10⁹ times faster than those of the corresponding hydrogen shift processes. The present findings indicate that analogous low-energy pathways can now also be expected in other large Criegee intermediates and that oxidative capacity of some Criegee intermediates is smaller than would be predicted by existing models.

Criegee intermediates have received much attention in atmospheric chemistry because of their importance in ozonolysis mechanisms. Here, using quantum mechanical kinetics, the authors reveal an unexpectedly fast mechanistic pathway for unimolecular reactions of large stabilized Criegee intermediates.