Closing the ultrafine particle number concentration budget at road-to-ambient scale: Implications for particle dynamics

<p>Freshly emitted vehicle exhaust particles are diluted quickly as they mix into ambient air, but the contribution of evaporation, coagulation, and/or nucleation of new particles to the number concentration has been the subject of some debate. We analyzed one-second time resolution size distribution data from an early morning field campaign, data collected at a time at which dilution has a smaller (but still dominant; ∼70−80%) impact on particle concentrations. Because the plume is diluted over an hour, and a distance of 1500 m, we can constrain the processes with higher accuracy. We find that concentrations in the smaller size bins (5.6–23.7 nm) peak further downwind than the reference particles (42.1–562 nm), and decay significantly faster than larger particles particularly in the area 100−400 m downwind. Comparisons of the cumulative contributions of van der Waals enhanced coagulation, dry deposition, and dilution and the observed decay curves, imply that for up to the first 50–100 m there is nucleation and/or growth of particles smaller than 5.6 nm. In contrast, in the ∼100–400 m region, some of the smaller particles evaporate. In the further downwind areas (>400 m) the particles all appear to decay at rates consistent with the sum of dilution, coagulation, and deposition. We also find that a dry deposition parameterization at the low end of those available in the literature is most consistent with the observational data.</p> <p>© 2016 American Association for Aerosol Research</p

Real Refractive Indices and Formation Yields of Secondary Organic Aerosol Generated from Photooxidation of Limonene and α-Pinene: The Effect of the HC/NO_<i>x</i> Ratio

The refractive index is an important property affecting aerosol optical properties, which in turn help determine the aerosol direct effect and satellite retrieval results. Here, we investigate the real refractive indices (<i>m</i>_r) of secondary organic aerosols (SOA) generated from the photooxidation of limonene and α-pinene with different HC/NO_<i>x</i> ratios. Refractive indices were obtained from polar nephelometer data using parallel and perpendicular polarized 532 nm light combined with measured size distributions, and retrievals were performed using a genetic algorithm and Mie–Lorenz scattering theory. The absolute error associated with the <i>m</i>_r retrieval is ±0.03, and reliable retrievals are possible for mass concentrations above 5–20 μg/m³ depending on particle size. The limonene SOA data suggest the most important factor controlling the refractive index is the HC/NO_<i>x</i> ratio; the refractive index is much less sensitive to the aerosol age or mass concentration. The refractive index ranges from about 1.34 to 1.56 for limonene and from 1.36 to 1.52 for α-pinene, and generally decreases as the HC/NO_<i>x</i> ratio increases. Especially for limonene, the particle diameter is also inversely related to the HC/NO_<i>x</i> ratio; the final size mode increases from 220 to 330 nm as the HC/NO_<i>x</i> ratio decreases from 33 to 6. In an effort to explore the ability of models from the literature to explain the observed refractive indices, a recent limonene oxidation mechanism was combined with SOA partitioning and a structure–property relationship for estimating refractive indices of condensing species. The resulting refractive indices fell in a much narrower range (1.475 ± 0.02) of <i>m</i>_r than observed experimentally. We hypothesize the experimentally observed high <i>m</i>_r values are due to oligomerization and the low values to water uptake, small soluble molecules such as glyoxal and other factors, each of which is not included in the oxidation mechanism. Aerosol formation yields were measured over the mass concentration range from 6 to ∼150 μg/m³, over which they increased steadily, and were higher for high HC/NO_<i>x</i> ratio experiments

Probing the Source of Hydrogen Peroxide Associated with Coarse Mode Aerosol Particles in Southern California

Coarse mode aerosols were collected at three sites in the Los Angeles area, two in Riverside, CA, one upwind and the other downwind of a major freeway, and also on the campus of the University of California, Los Angeles (UCLA). Coarse mode aerosol mass, H2O2, and H2O2 normalized to aerosol mass averaged 46 ± 22 μg/m3, 17 ± 8 ng/m3, and 0.48 ± 0.32 ng/μg at the upwind Riverside site and 97 ± 27 μg/m3, 34 ± 14 ng/m3, and 0.37 ± 0.18 ng/μg at the downwind Riverside site, respectively. H2O2, which appears to be generated by the particles (Arellanes, C.; Paulson, S. E.; Fine, P. M.; Sioutas, C. Environ. Sci. Technol. 2006, 40, 4859−4866), was uncorrelated with particle mass, but was strongly correlated with soluble iron, zinc, and copper (r = 0.47−0.67, p = 0.00−0.01). H2O2 levels were not affected by the addition of dithiothreitol, a marker for quinone redox activity. H2O2 levels were sensitive to the pH of the particle extraction solutions, increasing as the pH was decreased. The initial rate of H2O2 generation by coarse mode aerosols was 7.8 (±5.7) × 10−8 M min−1, similar to initial rates of hydroxyl radical generation from dissolved Fe2+, Cu2+, and Zn2+ solutions. The results support the notion that the majority of coarse mode H2O2 generation is mediated by a small set of transition metals

OH Radical Yields from the Ozone Reaction with Cycloalkenes

OH radical formation yields from the reaction of ozone with several cycloalkenes were measured using small amounts of fast-reacting aromatics and aliphatic ethers to trace OH formation. Measured OH yields are much higher than for acyclic analogues. The yields are 0.62 ± 0.15, 0.54 ± 0.13, 0.36 ± 0.08, and 0.91 ± 0.20 for cyclopentene, cyclohexene, cycloheptene, and 1-methylcyclohexene, respectively. Density functional theory calculations at the B3LYP/6-31G(d,p) level are presented to aid in understanding the trends observed. Theory indicates that the OH production from cycloalkenes is largely controlled by the transition states for the cycloreversion of the primary ozonide

Dependence of Real Refractive Indices on O:C, H:C and Mass Fragments of Secondary Organic Aerosol Generated from Ozonolysis and Photooxidation of Limonene and α-Pinene

<div><p>The refractive index is a fundamental property controlling aerosol optical properties. Secondary organic aerosols have variable refractive indices, presumably reflecting variations in their chemical composition. Here, we investigate the real refractive indices (m_r) and chemical composition of secondary organic aerosols (SOA) generated from the oxidation of α-pinene and limonene with ozone and NO_x/sunlight at different HC/NO_x ratios. Refractive indices were retrieved from polar nephelometer measurements using parallel and perpendicular polarized 532-nm light. Particle chemical composition was monitored with a high-resolution time-of-flight aerosol mass spectrometer (HR-Tof-AMS). For photochemically generated SOA, the values of refractive indices are consistent with prior results, and ranged from about 1.34 to 1.55 for limonene and from 1.44 to 1.47 for α-pinene, generally increasing as the particles grew. While AMS fragments are strongly correlated to the refractive index for each type of SOA, the relationships are in most cases quite different for different SOA types. Consistent with its wide range of refractive index, limonene SOA shows larger variations compared to α-pinene SOA for most parameters measured with the AMS, including H:C, O:C, f₄₃ (<i>m/z</i> 43/organic), f_C4H7⁺, and others. Refractive indices for α-pinene ozonolysis SOA also fell in narrow ranges; 1.43–1.45 and 1.46–1.53 for particles generated at 19–22 and 23–29°C, respectively, with corresponding small changes of f₄₃ and H:C ratio and other parameters. Overall, H:C ratio, m/z 43 and 55 (C₂H₃O⁺, C₄H₇⁺) were the best correlated with refractive index for all aerosol types investigated. The relationships between m_r and most fragments support the notion that increasing condensation of less oxygenated semivolatile species (with a possible role for a concomitant decrease in low refractive index water) is responsible for the increasing m_rs observed as the experiments progress. However, the possibility that oligomerization reactions play a role cannot be ruled out.</p> </div

Iron and Copper Alter the Oxidative Potential of Secondary Organic Aerosol: Insights from Online Measurements and Model Development

The oxidative potential (OP) of particulate matter has been widely suggested as a key metric for describing atmospheric particle toxicity. Secondary organic aerosol (SOA) and redox-active transition metals, such as iron and copper, are key drivers of particle OP. However, their relative contributions to OP, as well as the influence of metal–organic interactions and particulate chemistry on OP, remains uncertain. In this work, we simultaneously deploy two novel online instruments for the first time, providing robust quantification of particle OP. We utilize online AA (OPAA) and 2,7-dichlorofluoroscein (ROSDCFH) methods to investigate the influence of Fe(II) and Cu(II) on the OP of secondary organic aerosol (SOA). In addition, we quantify the OH production (OPOH) from these particle mixtures. We observe a range of synergistic and antagonistic interactions when Fe(II) and Cu(II) are mixed with representative biogenic (β-pinene) and anthropogenic (naphthalene) SOA. A newly developed kinetic model revealed key reactions among SOA components, transition metals, and ascorbate, influencing OPAA. Model predictions agree well with OPAA measurements, highlighting metal–ascorbate and −naphthoquinone–ascorbate reactions as important drivers of OPAA. The simultaneous application of multiple OP assays and a kinetic model provides new insights into the influence of metal and SOA interactions on particle OP

Photolysis of Heptanal

Photolysis of heptanal is investigated from an experimental and theoretical point of view. Photoexcited heptanal is believed to undergo rapid intersystem crossing to the triplet manifold and from there undergoes internal H-abstraction to form biradical intermediates. The favored γ-H abstraction pathway can cyclize or cleave to 1-pentene and hydroxyethene, which tautomerizes to acetaldehyde. Yields of 1-pentene and acetaldehyde were measured at 62 ± 7% and 63 ± 7%, respectively, relative to photolyzed heptanal. Additionally, small quantities of hexanal and hexanol were observed. On the basis of combined experimental and theoretical evidence, the remaining heptanal photolysis proceeds to form an estimated 10% HCO + hexyl radical and 30% cyclic alcohols, particularly 2-propyl cyclobutanol and 2-ethyl cyclopentanol

Terephthalate Probe for Hydroxyl Radicals: Yield of 2-Hydroxyterephthalic Acid and Transition Metal Interference

Hydroxyl radicals (.OH) are key players in chemistry in surface waters, clouds, and aerosols. Additionally, .OH may contribute to the inflammation underlying adverse health outcomes associated with particulate matter exposure. Terephthalate is a particularly sensitive probe for hydroxyl radicals, with a detection limit as low as 2 nM. However, there is uncertainty in .OH quantification using this method, and potential for interferences from some transition metals. Terephthalate reacts with .OH to form a fluorescent product, 2-hydroxyterephthalic acid (hTA), with a moderate dependence on pH and temperature. However, there is disagreement in the literature on the yield of the fluorescent product (YhTA), which introduces a large uncertainty in the quantification of OH. Additionally, TA and similar organic probes are known to complex Cu(II) at high concentrations; thus, if this reaction is important at lower concentrations, Cu(II) could reduce apparent hTA formation, and reduce activity of Cu(II) in target samples. Using a pH 3.5 dark ferrous Fenton system to generate .OH radicals, we find that YhTA = 31.5 ± 7%. This is about double the recent literature value measured, but in excellent agreement with earlier measurements. Additionally, we find that interactions between Cu(II) and hTA are small enough to be ignored at Cu(II) concentrations below ∼50 µM.</p

Characterizing Hydroxyl Radical Formation from the Light-Driven Fe(II)–Peracetic Acid Reaction, a Key Process for Aerosol-Cloud Chemistry

The reaction of peracetic acid (PAA) and Fe(II) has recently gained attention due to its utility in wastewater treatment and its role in cloud chemistry. Aerosol-cloud interactions, partly mediated by aqueous hydroxyl radical (OH) chemistry, represent one of the largest uncertainties in the climate system. Ambiguities remain regarding the sources of OH in the cloud droplets. Our research group recently proposed that the dark and light-driven reaction of Fe(II) with peracids may be a key contributor to OH formation, producing a large burst of OH when aerosol particles take up water as they grow to become cloud droplets, in which reactants are consumed within 2 min. In this work, we quantify the OH production from the reaction of Fe(II) and PAA across a range of physical and chemical conditions. We show a strong dependence of OH formation on ultraviolet (UV) wavelength, with maximum OH formation at λ = 304 ± 5 nm, and demonstrate that the OH burst phenomenon is unique to Fe(II) and peracids. Using kinetics modeling and density functional theory calculations, we suggest the reaction proceeds through the formation of an [Fe(II)–(PAA)2(H2O)2] complex, followed by the formation of a Fe(IV) complex, which can also be photoactivated to produce additional OH. Determining the characteristics of OH production from this reaction advances our knowledge of the sources of OH in cloudwater and provides a framework to optimize this reaction for OH output for wastewater treatment purposes