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

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

HULIS Enhancement of Hydroxyl Radical Formation from Fe(II): Kinetics of Fulvic Acid–Fe(II) Complexes in the Presence of Lung Antioxidants

Oxidative stress mediated by reactive oxygen species (ROS) is a hypothesized mechanism for particulate-matter related health effects. Fe­(II) is a key player in ROS formation in surrogate lung fluid (SLF) containing antioxidants. Humic-like substances (HULIS) in particulate matter such as biomass burning aerosol chelate Fe­(II), but the effect on ROS formation in the presence of lung antioxidants is not known. We use Suwanee River Fulvic Acid (SRFA) as a surrogate for HULIS and investigate its effect on OH formation from Fe­(II). For the first time, a chemical kinetics model was developed to explain behavior of Fe­(II) and SRFA in SLF. Model and experimental results are used to find best-fit rate coefficients for key reactions. Modeling results indicate SRFA enhances Fe-mediated reduction of O₂ to O₂^– and destruction of H₂O₂ to OH to 5.1 ± 1.5 and (4.3 ± 1.4) × 10³ M^–1 s^–1 respectively. Best-fit rates for Citrate–Fe­(II) mediated O₂ to O₂^– and H₂O₂ to OH were 3.0 ± 0.7 and (4.2 ± 1.7) × 10³ M^–1 s^–1 respectively. The kinetics model agrees with both the experimental results and thermodynamic model calculations of chemical speciation for 0 and 5 μg/mL SRFA, but both models are less successful at predicting further enhancements to OH formation at higher SRFA Concentrations