Humidity Dependence of the Condensational Growth of α-Pinene Secondary Organic Aerosol Particles

Volatility Basis Set Distributions and Viscosity of Organic Aerosol Mixtures: Insights from Chemical Characterization Using Temperature-Programmed Desorption-Direct Analysis in Real-Time High-Resolution Mass Spectrometry

Quantitative assessment of gas-particle partitioning of individual components within complex atmospheric organic aerosol (OA) mixtures is critical for predicting and comprehending the formation and evolution of OA particles in the atmosphere. This investigation leverages previously documented data obtained through a temperature-programmed desorption-direct analysis in real-time, high-resolution mass spectrometry (TPD-DART-HRMS) platform. This methodology facilitates the bottom-up construction of volatility basis set (VBS) distributions for constituents found in three biogenic secondary organic aerosol (SOA) mixtures produced through the ozonolysis of α-pinene, limonene, and ocimene. The apparent enthalpies (ΔH*, kJ mol-1) and saturation mass concentrations (CT*, μg·m-3) of individual SOA components, determined as a function of temperature (T, K), facilitated an assessment of changes in VBS distributions and gas-particle partitioning with respect to T and atmospheric total organic mass loadings (tOM, μg·m-3). The VBS distributions reveal distinct differences in volatilities among monomers, dimers, and trimers, categorized into separate volatility bins. At the ambient temperature of T = 298 K, only monomers efficiently partition between gas and particle phases across a broad range of atmospherically relevant tOM values of 1-100 μg·m-3. Partitioning of dimers and trimers becomes notable only at T > 360 K and T > 420 K, respectively. The viscosity of SOA mixtures is assessed using a bottom-up calculation approach, incorporating the input of elemental formulas, ΔH*, CT*, and particle-phase mass fractions of the SOA components. Through this approach, we are able to accurately estimate the variations in SOA viscosity that result from the evaporation of its components. These variations are, in turn, influenced by atmospherically relevant changes in tOM and T. Comparison of the calculated SOA viscosity and diffusivity values with literature reported experimental results shows close agreement, thereby validating the employed calculation approach. These findings underscore the significant potential for TPD-DART-HRMS measurements in enabling the untargeted analysis of organic molecules within OA mixtures. This approach facilitates quantitative assessment of their gas-particle partitioning and allows for the estimation of their viscosity and condensed-phase diffusion, thereby contributing valuable insights to atmospheric models.

Phase State, Surface Tension, Water Activity, and Accommodation Coefficient of Water-Organic Clusters Near the Critical Size for Atmospheric New Particle Formation

Interactions between water and organic molecules in sub-4 nm clusters play a significant role in the formation and growth of secondary organic aerosol (SOA) particles. However, a complete understanding of the relevant water microphysics has not yet been achieved due to challenges in the experimental characterization of soft nuclei. Here, we use molecular dynamics simulations to study the phase-mixing states, surface tension, water activity, and water accommodation coefficient of organic-water clusters representative of freshly nucleated SOA particles. Our results reveal large deviations from the behavior expected based on continuum theories. In particular, the phase-mixing state has a strong dependence on cluster size; surface tension displays a minimum at a specific organic-water mass ratio (morg/mw ∼ 4.5 in this study) corresponding to a minimum inhibition of droplet nucleation associated with the Kelvin effect; and the water accommodation coefficient increases by a factor of 2 with nanocluster hygroscopic growth, in agreement with recent experimental studies. Overall, our results yield parametric relations for water microphysical properties in sub-4 nm clusters and provide insight into the role of water in the initial stages of SOA nucleation and growth.

Molecular Understanding of the Enhancement in Organic Aerosol Mass at High Relative Humidity

The mechanistic pathway by which high relative humidity (RH) affects gas-particle partitioning remains poorly understood, although many studies report increased secondary organic aerosol (SOA) yields at high RH. Here, we use real-time, molecular measurements of both the gas and particle phase to provide a mechanistic understanding of the effect of RH on the partitioning of biogenic oxidized organic molecules (from α-pinene and isoprene) at low temperatures (243 and 263 K) at the CLOUD chamber at CERN. We observe increases in SOA mass of 45 and 85% with increasing RH from 10-20 to 60-80% at 243 and 263 K, respectively, and attribute it to the increased partitioning of semi-volatile compounds. At 263 K, we measure an increase of a factor 2-4 in the concentration of C10H16O2-3, while the particle-phase concentrations of low-volatility species, such as C10H16O6-8, remain almost constant. This results in a substantial shift in the chemical composition and volatility distribution toward less oxygenated and more volatile species at higher RH (e.g., at 263 K, O/C ratio = 0.55 and 0.40, at RH = 10 and 80%, respectively). By modeling particle growth using an aerosol growth model, which accounts for kinetic limitations, we can explain the enhancement in the semi-volatile fraction through the complementary effect of decreased compound activity and increased bulk-phase diffusivity. Our results highlight the importance of particle water content as a diluting agent and a plasticizer for organic aerosol growth.

Toward a molecular understanding of the surface composition of atmospherically relevant organic particles

Many mass spectrometry methods using various ionization sources provide bulk composition of airborne particles, but little is known about the surface species that play a major role in determining their physicochemical properties that impact air quality, climate, and health. The present work shows that the composition of surface layers of atmospherically relevant submicron organic particles can be probed without the use of an external ionization source. Solid dicarboxylic acid particles are used as models, with glutaric acid being the most efficient at generating ions. Coating with small diacids or products from α-pinene ozonolysis demonstrates that ions are ejected from the surface, providing surface molecular characterization of organic particles on the fly. This unique approach provides a path forward for elucidating the role of the surface in determining chemical and physical properties of particles, including heterogeneous reactions, particle growth, water uptake, and interactions with biological systems.

Molecular characteristics and stable carbon isotope compositions of dicarboxylic acids and related compounds in wintertime aerosols of Northwest China

Dicarboxylic acids are one of the important water-soluble organic compounds in atmospheric aerosols, causing adverse effects to both climate and human health. More attention has therefore been paid to organic acids in aerosols. In this study, the molecular distribution and diurnal variations of wintertime dicarboxylic acids in a rural site of Guanzhong Plain, Northwest China, were explored. Oxalic acid (C₂, day: 438.9 ± 346.8 ng m⁻³, night: 398.8 ± 392.3 ng m⁻³) is the most abundant compound followed by methylglyoxal (mGly, day: 207.8 ± 281.1 ng m⁻³, night: 222.9 ± 231.0 ng m⁻³) and azelaic (C₉, day: 212.8 ± 269.1 ng m⁻³, night: 211.4 ± 136.7 ng m⁻³) acid. The ratios of C₉/C₆ and C₉/Ph indicating that atmospheric dicarboxylic acids in winter in the region mainly come from biomass burning. Furthermore, secondary inorganic ions (NO₃⁻, SO₄²⁻, and NH₄⁺), relative humidity, liquid water content, and in-situ pH of aerosols are highly linearly correlated with C₂, suggesting that liquid phase oxidation is an important pathway for the formation of dicarboxylic acids. The δ¹³C analysis of C₂ suggested that lighter carbon isotope compositions tend to be oxidized to form aqueous-phase secondary organic aerosols (aqSOA), leading to the decay of ¹³C in aqSOA products rather than aerosol aging. This study provides a theoretical basis for the mechanism of formation of dicarboxylic acid.

Australian Bushfires (2019–2020): Aerosol Optical Properties and Radiative Forcing

In the present study, we present the aerosol optical properties and radiative forcing (RF) of the tropospheric and stratospheric smoke layers, observed by the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) satellite, during the extraordinary Australian biomass burning (BB) event in 2019–2020. These BB layers were studied and analyzed within the longitude range 140° E–20° W and the latitude band 20°–60° S, as they were gradually transported from the Australian banks to the South American continent. These layers were found to be trapped within the Andes circulation, staying for longer time periods in the same longitude region. The BB aerosols reached altitudes even up to 22 km amsl., and regarding their optical properties, they were found to be nearly spherical (particle linear depolarization ratio (PLDR) < 0.10) in the troposphere; while, in the stratosphere, they were more depolarizing with PLDR values reaching up to 0.20. Fine and ultrafine smoke particles were dominant in the stratosphere, according to the observed Ångström exponent, related to the backscatter coefficients obtained by the pair of wavelengths 532 and 1064 nm (Åb up to 3), in contrast to the Åb values in the troposphere (Åb < 1) indicative of the presence of coarser particles. As the aerosols fend off the source, towards North America, a slightly descending trend was observed in the tropospheric Åb values, while the stratospheric ones were lightly increased. A maximum aerosol optical depth (AOD) value of 0.54 was recorded in the lower troposphere over the fire spots, while, in the stratosphere, AOD values up to 0.29 were observed. Sharp changes of carbon monoxide (CO) and ozone (O3) concentrations were also recorded by the Copernicus Atmosphere Monitoring Service (CAMS) in various atmospheric heights over the study region, associated with fire smoke emissions. The tropospheric smoke layers were found to have a negative mean radiative effect, ranging from −12.83 W/m2 at the top of the atmosphere (TOA), to −32.22 W/m2 on the surface (SRF), while the radiative effect of the stratospheric smoke was estimated between −7.36 at the TOA to −18.51 W/m2 at the SRF.

Partitioning of Organonitrates in the Production of Secondary Organic Aerosols from α-Pinene Photo-Oxidation

The chemical pathways for the production of secondary organic aerosols (SOA) are influenced by the concentration of nitrogen oxides (NO_x), including the production of organonitrates (ON). Herein, a series of experiments conducted in an environmental chamber investigated the production and partitioning of total organonitrates from α-pinene photo-oxidation from <1 to 24 ppb NO_x. Gas-phase and particle-phase organonitrates (gON and pON, respectively) were measured by laser-induced fluorescence (LIF). The composition of the particle phase and the particle mass concentration were simultaneously characterized by online aerosol mass spectrometry. The LIF and MS measurements of pON concentrations had a Pearson correlation coefficient of 0.91 from 0.3 to 1.1 μg m^-3. For 1-6 ppb NO_x, the yield of SOA particle mass concentration increased from 0.02 to 0.044 with NO_x concentration. For >6 ppb NO_x, the yield steadily dropped, reaching 0.034 at 24 ppb NO_x. By comparison, the yield of pON steadily increased from 0.002 to 0.022 across the range of investigated NO_x concentrations. The yield of gON likewise increased from 0.005 to 0.148. The gas-to-particle partitioning ratio (pON/(pON + gON)) depended strongly on the NO_x concentration, changing from 0.27 to 0.13 as the NO_x increased from <1 to 24 ppb. In the atmosphere, there is typically a cross-over point between clean and polluted conditions that strongly affects SOA production, and the results herein quantitatively identify 6 ppb NO_x as that point for α-pinene photo-oxidation under these study conditions, including the production and partitioning of organonitrates. The trends in SOA yield and partitioning ratio as a function of NOx occur because of the changes in pON volatility.

The pinene scaffold: its occurrence, chemistry, synthetic utility, and pharmacological importance

This review provides insight into the utility of pinene in the synthetic building block and as reagent in asymmetric synthesis.