Particles formed in the atmosphere via nucleation provide about half the number of atmospheric cloud condensation nuclei, but in many locations, this process is limited by the growth of the newly ...formed particles. That growth is often via condensation of organic vapors. Identification of these vapors and their sources is thus fundamental for simulating changes to aerosol-cloud interactions, which are one of the most uncertain aspects of anthropogenic climate forcing. Here we present direct molecular-level observations of a distribution of organic vapors in a forested environment that can explain simultaneously observed atmospheric nanoparticle growth from 3 to 50 nm. Furthermore, the volatility distribution of these vapors is sufficient to explain nanoparticle growth without invoking particle-phase processes. The agreement between observed mass growth, and the growth predicted from the observed mass of condensing vapors in a forested environment thus represents an important step forward in the characterization of atmospheric particle growth.
Secondary organic aerosols (SOA) forms a major fraction of organic aerosols in the atmosphere. Knowledge of SOA properties that affect their dynamics in the atmosphere is needed for improving climate ...models. By combining experimental and modeling techniques, we investigated the factors controlling SOA evaporation under different humidity conditions. Our experiments support the conclusion of particle phase diffusivity limiting the evaporation under dry conditions. Viscosity of particles at dry conditions was estimated to increase several orders of magnitude during evaporation, up to 109 Pa s. However, at atmospherically relevant relative humidity and time scales, our results show that diffusion limitations may have a minor effect on evaporation of the studied α‐pinene SOA particles. Based on previous studies and our model simulations, we suggest that, in warm environments dominated by biogenic emissions, the major uncertainty in models describing the SOA particle evaporation is related to the volatility of SOA constituents.
Key Points
Evaporation of secondary organic aerosol from α‐pinene ozonolysis was studied under different relative humidity
At dry conditions the evaporation was likely slowed down by diffusion limitations resulting from viscosity of the particles
At atmospherically relevant relative humidity, diffusion limitations had a minor effect on evaporation of the particles
Aerosol particles cool the climate by scattering solar radiation and by acting as cloud condensation nuclei. Higher temperatures resulting from increased greenhouse gas levels have been suggested to ...lead to increased biogenic secondary organic aerosol and cloud condensation nuclei concentrations creating a negative climate feedback mechanism. Here, we present direct observations on this feedback mechanism utilizing collocated long term aerosol chemical composition measurements and remote sensing observations on aerosol and cloud properties. Summer time organic aerosol loadings showed a clear increase with temperature, with simultaneous increase in cloud condensation nuclei concentration in a boreal forest environment. Remote sensing observations revealed a change in cloud properties with an increase in cloud reflectivity in concert with increasing organic aerosol loadings in the area. The results provide direct observational evidence on the significance of this negative climate feedback mechanism.
We present ambient observations of dimeric monoterpene oxidation products (C16–20HyO6–9) in gas and particle phases in the boreal forest in Finland in spring 2013 and 2014, detected with a chemical ...ionization mass spectrometer with a filter inlet for gases and aerosols employing acetate and iodide as reagent ions. These are among the first online dual‐phase observations of such dimers in the atmosphere. Estimated saturation concentrations of 10−15 to 10−6 µg m−3 (based on observed thermal desorptions and group‐contribution methods) and measured gas‐phase concentrations of 10−3 to 10−2 µg m−3 (~106–107 molecules cm−3) corroborate a gas‐phase formation mechanism. Regular new particle formation (NPF) events allowed insights into the potential role dimers may play for atmospheric NPF and growth. The observationally constrained Model for Acid‐Base chemistry in NAnoparticle Growth indicates a contribution of ~5% to early stage particle growth from the ~60 gaseous dimer compounds.
Key Points
First online observations of dimeric monoterpene oxidation products in both gas and particle phases in the boreal forest
Measured concentrations and inferred saturation vapor pressure of gaseous dimers indicate that these compounds are formed in the gas phase
An observationally constrained model indicates a contribution of up to ~5% to early stage particle growth from the gaseous dimer compounds
Plain Language Summary
Atmospheric aerosol particles influence climate and air quality. We present new insights into how emissions of volatile organic compounds from trees are transformed in the atmosphere to contribute to the formation and growth of aerosol particles. We detected for the first time over a forest, a group of organic molecules, known to grow particles, in the gas phase at levels far higher than expected. Previous measurements had only measured them in the particles. This finding provides guidance on how models of aerosol formation and growth should describe their appearance and fate in the atmosphere.
Cloud droplets form in the atmosphere on aerosol particles, many of which result from nucleation of vapors. Here the authors comment on current knowledge and open questions regarding the ...condensational growth of nucleated particles to sizes where they influence cloud formation.
Natural aerosol feedbacks are expected to become more important in the future, as anthropogenic aerosol emissions decrease due to air quality policy. One such feedback is initiated by the increase in ...biogenic volatile organic compound (BVOC) emissions with higher temperatures, leading to higher secondary organic aerosol (SOA) production and a cooling of the surface via impacts on cloud radiative properties. Motivated by the considerable spread in feedback strength in Earth System Models (ESMs), we here use two long-term observational datasets from boreal and tropical forests, together with satellite data, for a process-based evaluation of the BVOC-aerosol-cloud feedback in four ESMs. The model evaluation shows that the weakest modelled feedback estimates can likely be excluded, but highlights compensating errors making it difficult to draw conclusions of the strongest estimates. Overall, the method of evaluating along process chains shows promise in pin-pointing sources of uncertainty and constraining modelled aerosol feedbacks.
One barrier to predicting biogenic secondary organic aerosol (SOA) formation in a changing climate can be attributed to the complex nature of plant volatile emissions. Plant volatile emissions are ...dynamic over space and time, and change in response to environmental stressors. This study investigated SOA production from emissions of healthy and aphid-stressed Scots pine saplings via dark ozonolysis and photooxidation chemistry. Laboratory experiments using a batch reaction chamber were used to investigate SOA production from different plant volatile mixtures. The volatile mixture from healthy plants included monoterpenes, aromatics, and a small amount of sesquiterpenes. The biggest change in the volatile mixture for aphid-stressed plants was a large increase (from 1.4 to 7.9 ppb) in sesquiterpenesparticularly acyclic sesquiterpenes, such as the farnesene isomers. Acyclic sesquiterpenes had different effects on SOA production depending on the chemical mechanism. Farnesenes suppressed SOA formation from ozonolysis with a 9.7–14.6% SOA mass yield from healthy plant emissions and a 6.9–10.4% SOA mass yield from aphid-stressed plant emissions. Ozonolysis of volatile mixtures containing more farnesenes promoted fragmentation reactions, which produced higher volatility oxidation products. In contrast, plant volatile mixtures containing more farnesenes did not appreciably change SOA production from photooxidation. SOA mass yields ranged from 10.8 to 23.2% from healthy plant emissions and 17.8–26.8% for aphid-stressed plant emissions. This study highlights the potential importance of acyclic terpene chemistry in a future climate regime with an increased presence of plant stress volatiles.
Accurate information on gas-to-particle partitioning is needed to model secondary organic aerosol formation. However, determining reliable
saturation vapor pressures of atmospherically relevant ...multifunctional organic compounds is extremely difficult. We estimated saturation vapor
pressures of α-pinene-ozonolysis-derived secondary organic aerosol constituents using Filter Inlet for Gases and AEROsols (FIGAERO)–chemical ionization mass spectrometer (CIMS) experiments and conductor-like screening model for real solvents (COSMO-RS). We found
a good agreement between experimental and computational saturation vapor pressures for molecules with molar masses around 190 g mol−1
and higher, most within a factor of 3 comparing the average of the experimental vapor pressures and the COSMO-RS estimate of the isomer closest to
the experiments. Smaller molecules likely have saturation vapor pressures that are too high to be measured using our experimental setup. The
molecules with molar masses below 190 g mol−1 that have differences of several orders of magnitude between the computational and
experimental saturation vapor pressures observed in our experiments are likely products of thermal decomposition occurring during thermal
desorption. For example, dehydration and decarboxylation reactions are able to explain some of the discrepancies between experimental and
computational saturation vapor pressures. Based on our estimates, FIGAERO–CIMS can best be used to determine saturation vapor pressures of compounds
with low and extremely low volatilities at least down to 10−10 Pa in saturation vapor pressure.
•For exhaust gas cleaning, H2SO4 and HNO3 can be converted to aerosol particles by NH3.•A model set-up to simulate the gas-to-particle conversion of H2SO4 and HNO3 is built.•The model framework can ...be applied to optimize the pollutant conversion.•Key parameters for optimization include NH3 level, temperature and relative humidity.
Sulfur and nitrogen oxides (SOx and NOx) are harmful pollutants emitted into the atmosphere by industry and transport sectors. In addition to being hazardous gases, SOx and NOx form sulfuric and nitric acids which contribute to the formation of airborne particulate matter through nucleation and condensation, hence magnifying the environmental impact of these species. In this work, we build a modeling framework for utilizing this phenomenon for low-temperature exhaust gas cleaning. It has been reported that ammonia gas can be used to facilitate particle formation from the aforementioned acids, and thus remove these gaseous pollutants by converting them into ammonium sulfate and nitrate particles. Here we provide comprehensive modeling tools for applying this idea to exhaust gas cleaning by combining detailed models for nucleation, gas–particle mass exchange and particle population dynamics. We demonstrate how these models can be used to find advantageous operating conditions for a cleaning unit. In particular, the full model is computationally cheap and enables optimization of the particle formation efficiency and particle growth, hence ensuring sufficient conversion of gaseous pollutants into collectable particulate matter. This constitutes a ground for future engineering tools for designing next-generation sustainable exhaust gas cleaners.
Accurate information on gas-to-particle partitioning is needed to model secondary organic aerosol formation. However, determining reliable saturation vapor pressures of atmospherically relevant ...multifunctional organic compounds is extremely difficult. We estimated saturation vapor pressures of α-pinene-ozonolysis-derived secondary organic aerosol constituents using Filter Inlet for Gases and AEROsols (FIGAERO)-chemical ionization mass spectrometer (CIMS) experiments and conductor-like screening model for real solvents (COSMO-RS). We found a good agreement between experimental and computational saturation vapor pressures for molecules with molar masses around 190 g mol.sup.-1 and higher, most within a factor of 3 comparing the average of the experimental vapor pressures and the COSMO-RS estimate of the isomer closest to the experiments. Smaller molecules likely have saturation vapor pressures that are too high to be measured using our experimental setup. The molecules with molar masses below 190 g mol.sup.-1 that have differences of several orders of magnitude between the computational and experimental saturation vapor pressures observed in our experiments are likely products of thermal decomposition occurring during thermal desorption. For example, dehydration and decarboxylation reactions are able to explain some of the discrepancies between experimental and computational saturation vapor pressures. Based on our estimates, FIGAERO-CIMS can best be used to determine saturation vapor pressures of compounds with low and extremely low volatilities at least down to 10.sup.-10 Pa in saturation vapor pressure.
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