High-NOx Photooxidation of n-Dodecane: Temperature Dependence of SOA Formation

Intermediate volatile organic compounds in Canadian residential air in winter: Implication to indoor air quality

Intermediate volatile organic compounds (IVOCs) have recently been characterized for their contributions to the formation of secondary organic aerosol in atmospheric air. However, IVOCs in air in various indoor environments have not been characterized yet. In this study, we characterized and measured IVOCs, volatile organic compounds (VOCs) and semi-volatile organic compounds (SVOCs), in residential indoor air in Ottawa, Canada. IVOCs, including n-alkanes, branched-chain alkanes (b-alkanes), unspecified complex mixtures (UCM) IVOCs, and oxygenated IVOCs (such as fatty acids), were found to have a large impact on indoor air quality. The results indicate that the indoor IVOCs behave differently from those in the outdoor environment. IVOCs in the studied residential air ranged from 14.4 to 69.0 μg/m³, with a geometric mean of 31.3 μg/m³, accounting for approximately 20% of the total organic compounds (IVOCs, VOCs and SVOCs) in indoor air. The total b-alkanes and UCM-IVOCs were found to have statistically significant positive correlations with indoor temperature but have no correlations with airborne particulate matter less than 2.5 μm (PM_2.5) as well as ozone (O₃) concentration. However, indoor oxygenated IVOCs behaved differently from b-alkanes and UCM-IVOCs, with a statistically significant positive correlation with indoor relative humidity but no correlation with other indoor environmental conditions.

Long-chain alkanes in the atmosphere: A review

As a representative species of intermediate volatile organic compounds (IVOCs), long-chain alkanes are considered to be important precursors of secondary organic aerosols (SOA) in the atmosphere. This work reviews the previous studies on long-chain alkanes in the atmosphere: (1) the detection methods and filed observations of long-chain alkanes in both gas and particle phases are summarized briefly; (2) the laboratory studies of long chain alkanes are reviewed, the kinetic data, reaction mechanism, SOA yields, and physicochemical properties of SOA are included in detail; (3) the research progress related to model simulations of long-chain alkanes are also discussed. In addition, based on available research results, several perspective contents are proposed that can be used as a guideline for future research plans.

Effect of chemical structure on optical properties of secondary organic aerosols derived from C12 alkanes

With the development of the economy, anthropogenic emissions in the atmospheric environment increases, and air pollution has caused wide public concern. Vehicle exhaust is an important emission source in the atmosphere, and alkanes are the representative components in it. In this study, the optical properties of secondary organic aerosol (SOA) derived from several C₁₂ alkanes (2-methylundecane, hexylcyclohexane, and cyclododecane) in the absence of NO_X were determined. Absorption (imaginary part of the refractive index (RI), k) at 532 nm was negligible for all the derived SOA, and the scattering (real part of RI, n) of the SOA at 532 nm followed the order of cyclododecane SOA < hexylcyclohexane SOA < 2-methylundecane SOA, at both room- (25 °C) and low- (5 °C) temperature. The chemical compositions of the SOA formed were analyzed with an electrospray ionization time-of-flight mass spectrometer (ESI-TOF-MS). The mass spectra showed that the oligomers were generated in the reactions. It was shown that the different reaction pathways (due to various alkane structures) leaded to the difference in SOA chemical composition, which changed the RI values. The low-temperature condition promoted the progress of the oligomerization reaction so that the final RI values also changed. This work suggested that when estimating the radiative forcing of SOA using regional or global models, the structure of the precursors and reaction conditions should be taken into account.

Differences in fine particle chemical composition on clear and cloudy days

Clouds are prevalent and alter PM_2.5 mass and chemical composition. Cloud-affected satellite retrievals are often removed from data products, hindering estimates of tropospheric chemical composition during cloudy times. We examine surface fine particulate matter (PM_2.5) chemical constituent concentrations in the Interagency Monitoring of PROtected Visual Environments network during Cloudy and Clear Sky times defined using Moderate Resolution Imaging Spectroradiometer (MODIS) cloud flags from 2010-2014 with a focus on differences in particle hygroscopicity and aerosol liquid water (ALW). Cloudy and Clear Sky periods exhibit significant differences in PM_2.5 and chemical composition that vary regionally and seasonally. In the eastern US, relative humidity alone cannot explain differences in ALW, suggesting emissions and in situ chemistry exert determining impacts. An implicit clear sky bias may hinder efforts to quantitatively to understand and improve model representation of aerosol-cloud interactions.

Kinetic Study of the Temperature Dependence of OH-Initiated Oxidation of n-Dodecane

Reaction rate constants for the reaction of n-dodecane with hydroxyl radicals were measured as a function of temperature between 283 and 303 K, using the relative rate method in the CESAM chamber (French acronym for "experimental multiphasic atmospheric simulation chamber"). The rate constants obtained at 283, 293, and 303 K are (1.27 ± 0.31) × 10^-11, (1.33 ± 0.34) × 10^-11, and (1.27 ± 0.40) × 10^-11 cm³ molecule^-1 s^-1, respectively. Rate constants measured were in excellent agreement with the few available data in the literature over the studied temperature range (283-340 K). Rate constants estimated by the structure-activity relationship and transition state theory methods agreed with our experimental data within 14%. From these data combined with previous literature measurement, the following Arrhenius expression, k_DDC+OH = (9.77 ± 6.19) × 10^-11 × exp[(-595 ± 5580)/T] cm³ molecule^-1 s^-1, was found to be valid over a temperature range (283-340 K) of the tropospheric interest.

Direct observation of the intermediate radical in the photodissociation of 1,3-cyclohexane dinitrite

A two-step photodissociation mechanism was proposed in the literature for dinitrites in the absence of direct evidence of the intermediate species. In this work, photodissociation dynamics of cis and trans 1,3-cyclohexane dinitrites are investigated by laser-induced fluorescence (LIF) spectroscopy and theoretical calculation methods. Observation of the fluorescence spectra of the 3-nitrosooxy cyclohexoxy radical provides direct experimental evidence that the intermediate species exists. The results indicate that photodissociation of dinitrites indeed follows a two-step mechanism, i.e. one of the O-NO bonds of the molecule breaks first upon 355 nm laser photolysis and generates an alkoxy radical (RO) plus NO; the alkoxy radical further dissociates in the secondary dissociation step and produces small fragments such as vinoxy etc.

Comparison of Measurement-Based Methodologies to Apportion Secondary Organic Carbon (SOC) in PM2.5: A Review of Recent Studies

Secondary organic aerosol (SOA) is known to account for a major fraction of airborne particulate matter, with significant impacts on air quality and climate at the global scale. Despite the substantial amount of research studies achieved during these last decades, the source apportionment of the SOA fraction remains difficult due to the complexity of the physicochemical processes involved. The selection and use of appropriate approaches are a major challenge for the atmospheric science community. Several methodologies are nowadays available to perform quantitative and/or predictive assessments of the SOA amount and composition. This review summarizes the current knowledge on the most commonly used approaches to evaluate secondary organic carbon (SOC) contents: elemental carbon (EC) tracer method, chemical mass balance (CMB), SOA tracer method, radiocarbon (14C) measurement and positive matrix factorization (PMF). The principles, limitations, challenges and good practices of each of these methodologies are discussed in the present article. Based on a comprehensive—although not exhaustive—review of research papers published during the last decade (2006–2016), SOC estimates obtained using these methodologies are also summarized for different regions across the world. Conclusions of some studies which are directly comparing the performances of different methodologies are then specifically discussed. An overall picture of SOC contributions and concentrations obtained worldwide for urban sites under similar conditions (i.e., geographical and seasonal ones) is also proposed here. Finally, further needs to improve SOC apportionment methodologies are also identified and discussed.