Gas-phase autoxidation of organics can generate highly oxygenated organic molecules (HOMs) and thus increase secondary organic aerosol production and enable new-particle formation.
Here we present a ...new implementation of the volatility basis set (VBS) that explicitly resolves peroxy radical (RO2) products formed via autoxidation.
The model includes a strong temperature dependence for autoxidation as well as explicit termination of RO2, including reactions with NO, HO2, and other RO2.
The RO2 cross-reactions can produce dimers (ROOR).
We explore the temperature and NOx dependence of this chemistry, showing that temperature strongly influences the intrinsic volatility distribution and that NO can suppress autoxidation under conditions typically found in the atmosphere.
We present global direct radiative effect (DRE) calculations of carbonaceous aerosols emitted from biomass/biofuel burning addressing the interplay between two poorly constrained contributions to ...DRE: mixing state of black carbon (lensing) and light absorption by organic aerosol (OA) due to the presence of brown carbon (BrC). We use the parameterization of Saleh et al. (2014) which captures the variability in biomass/biofuel OA absorption. The global mean effect of OA absorption is +0.22 W/m2 and +0.12 W/m2 for externally and internally mixed cases, while the effect of lensing is +0.39 W/m2 and +0.29 W/m2 for nonabsorbing and absorbing OA cases, signifying the nonlinear interplay between OA absorption and lensing. These two effects can be overestimated if not treated simultaneously in radiative transfer calculations. The combined effect of OA absorption and lensing increases the global mean DRE of biomass/biofuel aerosols from −0.46 W/m2 to +0.05 W/m2 and appears to reduce the gap between existing model‐based and observationally constrained DRE estimates. We observed a strong sensitivity to these parameters in key regions, where DRE shifts from strongly negative (< −1 W/m2) to strongly positive (> +1 W/m2) when accounting for lensing and OA absorption.
Key Points
Brown carbon absorption and lensing are important in aerosol radiative effect
These effects can be overestimated if not treated simultaneously in radiative transfer calculations
Accounting for them reduces the uncertainty in aerosol radiative effect
The pressure dependence of carbonyl oxide (Criegee intermediate) stabilization can be measured via H2SO4 detection using chemical ionization mass spectrometry. By selectively scavenging OH radicals ...in a flow reactor containing an alkene, O3, and SO2, we measure an H2SO4 ratio related to the Criegee intermediate stabilization, and by performing experiments at multiple pressures, we constrain the pressure dependence of the stabilization. Here, we present results from a set of monoterpenes as well as isoprene, along with previously published results from tetramethylethylene and a sequence of symmetrical trans alkenes. We are able to reproduce the observations with a physically sensible set of parameters related to standard pressure falloff functions, providing both a consistent picture of the reaction dynamics and a method to describe the pressure stabilization following ozonolysis of all alkenes under a wide range of atmospheric conditions.
A list of authors and their affiliations appears at the end of the paper New-particle formation is a major contributor to urban smog
, but how it occurs in cities is often puzzling
. If the growth ...rates of urban particles are similar to those found in cleaner environments (1-10 nanometres per hour), then existing understanding suggests that new urban particles should be rapidly scavenged by the high concentration of pre-existing particles. Here we show, through experiments performed under atmospheric conditions in the CLOUD chamber at CERN, that below about +5 degrees Celsius, nitric acid and ammonia vapours can condense onto freshly nucleated particles as small as a few nanometres in diameter. Moreover, when it is cold enough (below -15 degrees Celsius), nitric acid and ammonia can nucleate directly through an acid-base stabilization mechanism to form ammonium nitrate particles. Given that these vapours are often one thousand times more abundant than sulfuric acid, the resulting particle growth rates can be extremely high, reaching well above 100 nanometres per hour. However, these high growth rates require the gas-particle ammonium nitrate system to be out of equilibrium in order to sustain gas-phase supersaturations. In view of the strong temperature dependence that we measure for the gas-phase supersaturations, we expect such transient conditions to occur in inhomogeneous urban settings, especially in wintertime, driven by vertical mixing and by strong local sources such as traffic. Even though rapid growth from nitric acid and ammonia condensation may last for only a few minutes, it is nonetheless fast enough to shepherd freshly nucleated particles through the smallest size range where they are most vulnerable to scavenging loss, thus greatly increasing their survival probability. We also expect nitric acid and ammonia nucleation and rapid growth to be important in the relatively clean and cold upper free troposphere, where ammonia can be convected from the continental boundary layer and nitric acid is abundant from electrical storms
.
About half of present-day cloud condensation nuclei originate from atmospheric nucleation, frequently appearing as a burst of new particles near midday. Atmospheric observations show that the growth ...rate of new particles often accelerates when the diameter of the particles is between one and ten nanometres. In this critical size range, new particles are most likely to be lost by coagulation with pre-existing particles, thereby failing to form new cloud condensation nuclei that are typically 50 to 100 nanometres across. Sulfuric acid vapour is often involved in nucleation but is too scarce to explain most subsequent growth, leaving organic vapours as the most plausible alternative, at least in the planetary boundary layer. Although recent studies predict that low-volatility organic vapours contribute during initial growth, direct evidence has been lacking. The accelerating growth may result from increased photolytic production of condensable organic species in the afternoon, and the presence of a possible Kelvin (curvature) effect, which inhibits organic vapour condensation on the smallest particles (the nano-Köhler theory), has so far remained ambiguous. Here we present experiments performed in a large chamber under atmospheric conditions that investigate the role of organic vapours in the initial growth of nucleated organic particles in the absence of inorganic acids and bases such as sulfuric acid or ammonia and amines, respectively. Using data from the same set of experiments, it has been shown that organic vapours alone can drive nucleation. We focus on the growth of nucleated particles and find that the organic vapours that drive initial growth have extremely low volatilities (saturation concentration less than 10(-4.5) micrograms per cubic metre). As the particles increase in size and the Kelvin barrier falls, subsequent growth is primarily due to more abundant organic vapours of slightly higher volatility (saturation concentrations of 10(-4.5) to 10(-0.5) micrograms per cubic metre). We present a particle growth model that quantitatively reproduces our measurements. Furthermore, we implement a parameterization of the first steps of growth in a global aerosol model and find that concentrations of atmospheric cloud concentration nuclei can change substantially in response, that is, by up to 50 per cent in comparison with previously assumed growth rate parameterizations.
Different populations of aerosol are constantly mixed throughout the atmosphere. Large-scale models often assume no particle-particle mixing or fast mixing among aerosol populations, so that they ...stay externally mixed or instantaneously form internal mixtures. We apply the kinetic multilayer model of gas-particle interactions (KM-GAP) to simulate the evaporation of semi-volatile species from one particle population and partitioning into another population with various phase states and nonideal mixing conditions. We find that the particle-particle mixing timescale (τ
mix
) is prolonged when the semi-volatile species transport to a population in which it is miscible, as more mass must be transported. Extremes of volatility prolong the τ
mix
, as low-volatility species evaporate slowly, while high-volatility species condense slowly. When the bulk diffusivities of the two populations are greater than 10
−15
cm
2
s
−1
, semi-volatile species mix rapidly; otherwise, the τ
mix
can be prolonged beyond 1 h. We apply KM-GAP to particle-particle mixing experiments of H-toluene SOA into D-toluene SOA and limonene SOA, showing that τ
mix
is prolonged when toluene SOA is highly viscous, while initial partitioning of gas phase semi-volatile species from toluene SOA into limonene SOA is rapid because of the low viscosity of limonene SOA. Simulations of mixing toluene SOA and β-caryophyllene SOA indicate that the apparent discrepancy of limited mixing under conditions where both are predicted to have low viscosity are explained by limited miscibility of the semi-volatile components. Our study demonstrates that particle-particle mixing timescales are affected by a complex interplay among volatility, diffusion limitations, and non-ideal miscibility.
Atmospheric aerosols exert a substantial influence on climate, ecosystems, visibility, and human health. Although secondary organic aerosols (SOA) dominate fine-particle mass, they comprise myriad ...compounds with uncertain sources, chemistry, and interactions. SOA formation involves absorption of vapors into particles, either because gas-phase chemistry produces low-volatility or semivolatile products that partition into particles or because morevolatile organics enter particles and react to form lower-volatility products. Thus, SOA formation involves both production of low-volatility compounds and their diffusion into particles. Most chemical transport models assume a single well-mixed phase of condensing organics and an instantaneous equilibrium between bulk gas and particle phases; however, direct observations constraining diffusion of semivolatile organics into particles containing SOA are scarce. Here we perform unique mixing experiments between SOA populations including semivolatile constituents using quantitative, single-particle mass spectrometry to probe any mass-transfer limitations in particles containing SOA. We show that, for several hours, particles containing SOA from toluene oxidation resist exchange of semivolatile constituents at low relative humidity (RH) but start to lose that resistance above 20% RH. Above 40% RH, the exchange of material remains constant up to 90% RH. We also show that dry particles containing SOA from α-pinene ozonolysis do not appear to resist exchange of semivolatile compounds. Our interpretation is that in-particle diffusion is not rate-limiting to mass transfer in these systems above 40% RH. To the extent that these systems are representative of ambient SOA, we conclude that diffusion limitations are likely not common under typical ambient boundary layer conditions.
The authors examines the saturation vapor pressures and transition enthalpies of low-volatility organic molecules of atmospheric relevance from dicarboxylic acids to complex mixtures.
Nucleation of aerosol particles from trace atmospheric vapours is thought to provide up to half of global cloud condensation nuclei. Aerosols can cause a net cooling of climate by scattering sunlight ...and by leading to smaller but more numerous cloud droplets, which makes clouds brighter and extends their lifetimes. Atmospheric aerosols derived from human activities are thought to have compensated for a large fraction of the warming caused by greenhouse gases. However, despite its importance for climate, atmospheric nucleation is poorly understood. Recently, it has been shown that sulphuric acid and ammonia cannot explain particle formation rates observed in the lower atmosphere. It is thought that amines may enhance nucleation, but until now there has been no direct evidence for amine ternary nucleation under atmospheric conditions. Here we use the CLOUD (Cosmics Leaving OUtdoor Droplets) chamber at CERN and find that dimethylamine above three parts per trillion by volume can enhance particle formation rates more than 1,000-fold compared with ammonia, sufficient to account for the particle formation rates observed in the atmosphere. Molecular analysis of the clusters reveals that the faster nucleation is explained by a base-stabilization mechanism involving acid-amine pairs, which strongly decrease evaporation. The ion-induced contribution is generally small, reflecting the high stability of sulphuric acid-dimethylamine clusters and indicating that galactic cosmic rays exert only a small influence on their formation, except at low overall formation rates. Our experimental measurements are well reproduced by a dynamical model based on quantum chemical calculations of binding energies of molecular clusters, without any fitted parameters. These results show that, in regions of the atmosphere near amine sources, both amines and sulphur dioxide should be considered when assessing the impact of anthropogenic activities on particle formation.
We present an overview of the development of our understanding of the sources, formation mechanisms, physical and chemical transformations of atmospheric organic aerosol (OA) during the last thirty ...years. Until recently, organic particulate material was simply classified as either primary or secondary with the primary component being treated in models as nonvolatile and inert. However, this oversimplified view fails to explain the highly oxygenated nature of ambient OA, the relatively small OA concentration gradients between urban areas and their surroundings, and the concentrations of OA during periods of high photochemical activity. A unifying framework for the description of all components based on their volatility distribution (the volatility basis set) can be used for the treatment of a wide range of processes affecting organic aerosol loadings and composition in the atmosphere. These processes include direct organic particle and vapor emissions, chemical production of organic PM from volatile precursors, chemical reactions (aging) in all phases, as well as deposition of both particles and vapors and chemical losses to volatile products. The combination of this new framework with the recent results of laboratory studies can resolve some of the discrepancies between OA observations and laboratory results. The mass balance of the organic material as a function of its volatility is investigated and used to frame the corresponding constraints on the system. Finally we revisit the traditional definitions of primary and secondary organic aerosol and propose a new set of terms and definitions based on the improvements of our understanding.
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