Thomas F. Mentel

A large source of low-volatility secondary organic aerosol

Forests emit large quantities of volatile organic compounds (VOCs) to the atmosphere. Their condensable oxidation products can form secondary organic aerosol, a significant and ubiquitous component of atmospheric aerosol, which is known to affect the Earth’s radiation balance by scattering solar radiation and by acting as cloud condensation nuclei. The quantitative assessment of such climate effects remains hampered by a number of factors, including an incomplete understanding of how biogenic VOCs contribute to the formation of atmospheric secondary organic aerosol. The growth of newly formed particles from sizes of less than three nanometres up to the sizes of cloud condensation nuclei (about one hundred nanometres) in many continental ecosystems requires abundant, essentially non-volatile organic vapours, but the sources and compositions of such vapours remain unknown. Here we investigate the oxidation of VOCs, in particular the terpene α-pinene, under atmospherically relevant conditions in chamber experiments. We find that a direct pathway leads from several biogenic VOCs, such as monoterpenes, to the formation of large amounts of extremely low-volatility vapours. These vapours form at significant mass yield in the gas phase and condense irreversibly onto aerosol surfaces to produce secondary organic aerosol, helping to explain the discrepancy between the observed atmospheric burden of secondary organic aerosol and that reported by many model studies. We further demonstrate how these low-volatility vapours can enhance, or even dominate, the formation and growth of aerosol particles over forested regions, providing a missing link between biogenic VOCs and their conversion to aerosol particles. Our findings could help to improve assessments of biosphere–aerosol–climate feedback mechanisms, and the air quality and climate effects of biogenic emissions generally.

Direct Observations of Atmospheric Aerosol Nucleation

Aerosol Formation Most atmospheric aerosol particles result from a growth process that begins with atmospheric molecules and clusters, progressing to larger and larger sizes as they acquire other molecules, clusters, and particles. The initial steps of this process involve very small entities—with diameters of less than 2 nanometers—which have been difficult to observe. Kulmala et al. (p. 943; see the Perspective by Andreae) developed a sensitive observational protocol that allows these tiny seeds to be detected and counted, and they mapped out the process of aerosol formation in detail. Detailed aerosol measurements provide a consistent framework for the formation of particles from atmospheric gases. [Also see Perspective by Andreae] Atmospheric nucleation is the dominant source of aerosol particles in the global atmosphere and an important player in aerosol climatic effects. The key steps of this process occur in the sub–2-nanometer (nm) size range, in which direct size-segregated observations have not been possible until very recently. Here, we present detailed observations of atmospheric nanoparticles and clusters down to 1-nm mobility diameter. We identified three separate size regimes below 2-nm diameter that build up a physically, chemically, and dynamically consistent framework on atmospheric nucleation—more specifically, aerosol formation via neutral pathways. Our findings emphasize the important role of organic compounds in atmospheric aerosol formation, subsequent aerosol growth, radiative forcing and associated feedbacks between biogenic emissions, clouds, and climate.

Aging of biogenic secondary organic aerosol via gas-phase OH radical reactions

The Multiple Chamber Aerosol Chemical Aging Study (MUCHACHAS) tested the hypothesis that hydroxyl radical (OH) aging significantly increases the concentration of first-generation biogenic secondary organic aerosol (SOA). OH is the dominant atmospheric oxidant, and MUCHACHAS employed environmental chambers of very different designs, using multiple OH sources to explore a range of chemical conditions and potential sources of systematic error. We isolated the effect of OH aging, confirming our hypothesis while observing corresponding changes in SOA properties. The mass increases are consistent with an existing gap between global SOA sources and those predicted in models, and can be described by a mechanism suitable for implementation in those models.

The Formation of Highly Oxidized Multifunctional Products in the Ozonolysis of Cyclohexene

The prompt formation of highly oxidized organic compounds in the ozonolysis of cyclohexene (C6H10) was investigated by means of laboratory experiments together with quantum chemical calculations. The experiments were performed in borosilicate glass flow tube reactors coupled to a chemical ionization atmospheric pressure interface time-of-flight mass spectrometer with a nitrate ion (NO3(-))-based ionization scheme. Quantum chemical calculations were performed at the CCSD(T)-F12a/VDZ-F12//ωB97XD/aug-cc-pVTZ level, with kinetic modeling using multiconformer transition state theory, including Eckart tunneling corrections. The complementary investigation methods gave a consistent picture of a formation mechanism advancing by peroxy radical (RO2) isomerization through intramolecular hydrogen shift reactions, followed by sequential O2 addition steps, that is, RO2 autoxidation, on a time scale of seconds. Dimerization of the peroxy radicals by recombination and cross-combination reactions is in competition with the formation of highly oxidized monomer species and is observed to lead to peroxides, potentially diacyl peroxides. The molar yield of these highly oxidized products (having O/C > 1 in monomers and O/C > 0.55 in dimers) from cyclohexene ozonolysis was determined as (4.5 ± 3.8)%. Fully deuterated cyclohexene and cis-6-nonenal ozonolysis, as well as the influence of water addition to the system (either H2O or D2O), were also investigated in order to strengthen the arguments on the proposed mechanism. Deuterated cyclohexene ozonolysis resulted in a less oxidized product distribution with a lower yield of highly oxygenated products and cis-6-nonenal ozonolysis generated the same monomer product distribution, consistent with the proposed mechanism and in agreement with quantum chemical modeling.

Nitrate effect in the heterogeneous hydrolysis of dinitrogen pentoxide on aqueous aerosols

The heterogeneous hydrolysis of N2O5 was investigated on NaHSO4, Na2SO4, and NaNO3 aerosols. The experiments were performed in the large Aerosol Chamber at FZ Julich at room temperature and ambient pressure for several relative humidities. These salts are components of aerosols in the marine and coastal boundary layer. For the sodium sulfate aerosols at relative humidities of 50–70% the reaction probabilities γN2O5 were in the range of 0.02–0.04. For NaNO3 aerosol at similar relative humidities we observed γN2O5 of 0.0018–0.0032. With increasing relative humidity, i.e. with increasing dilution of the nitrate concentration in the aerosol droplets, γN2O5 increases to 0.023 at 90% relative humidity. Our observation of decreasing γN2O5 with increasing nitrate concentration can be explained within the framework of an ionic mechanism for the hydrolysis of N2O5, if the recombination reaction of NO2+ with NO3- to N2O5 is considered. By a steady state analysis we derived analytical expressions of γN2O5 as a function of the nitrate concentration for a reaction either throughout the aerosol volume or in a thin surface shell. Accordingly, increasing nitrate concentration should enhance the lifetime of physically dissolved N2O5(aq) and as a consequence the heterogeneous hydrolysis of N2O5 should change from a near-surface to a volume reaction. The observation of such a specific nitrate effect can be regarded as further experimental evidence for the ionic reaction mechanism in the uptake of N2O5 on aqueous aerosols. A nitrate effect may gain (local) importance in the atmosphere if increasing NOX emissions translate in an increasing nitrate fraction in the secondary aerosol of anthropogenic origin.

Missing gas-phase source of hono inferred from zeppelin measurements in the troposphere

On a Zeppelin Nitrous acid (HONO) is an important atmospheric trace gas that acts as a precursor of tropospheric hydroxyl-radicals (OH), which is responsible for the self-cleansing capacity of the atmosphere and which also controls the concentrations of greenhouse gases, such as methane and ozone. How HONO is made is a mystery. Flying onboard a Zeppelin over the Po Valley in Northern Italy, Li et al. (p. 292) discovered HONO in the undisturbed morning troposphere, indicating that HONO must be produced there, rather than mixed from the surface. The high HONO concentrations are likely to have been formed by a light-dependent gas-phase source that probably consumed OH or HO2 radicals, which hints that the impact of HONO on the abundance of OH in the entire troposphere may be substantially overestimated. The tropospheric production of HONO from a light-dependent gas-phase source raises questions about its impact on OH. Gaseous nitrous acid (HONO) is an important precursor of tropospheric hydroxyl radicals (OH). OH is responsible for atmospheric self-cleansing and controls the concentrations of greenhouse gases like methane and ozone. Due to lack of measurements, vertical distributions of HONO and its sources in the troposphere remain unclear. Here, we present a set of observations of HONO and its budget made onboard a Zeppelin airship. In a sunlit layer separated from Earth’s surface processes by temperature inversion, we found high HONO concentrations providing evidence for a strong gas-phase source of HONO consuming nitrogen oxides and potentially hydrogen oxide radicals. The observed properties of this production process suggest that the generally assumed impact of HONO on the abundance of OH in the troposphere is substantially overestimated.

Soluble Mass, Hygroscopic Growth, and Droplet Activation of Coated Soot Particles during LACIS Experiment in November (LExNo)

The LACIS Experiment in November (LExNo) campaign was conducted in November 2005 at the Atmospheric Composition Change the European Network of Excellence (ACCENT) site Leipzig Aerosol Cloud Interaction Simulator (LACIS). The goal of LExNo was to provide deeper insight into the activation properties of coated soot particles imitating aged combustion aerosol particles. The aerosols were prepared by starting with spark-generated soot particles. In some experiments the soot particles were compacted by exposure to propanol vapor; in others this step was bypassed. The soot was thermally coated with ammonium sulfate, levoglucosan, or a mixture of both ammonium sulfate and levoglucosan. The synthesized particles were investigated using aerosol mass spectrometry, a Hygroscopicity Tandem differential mobility analyzer, two Wyoming static diffusion cloud condensation nuclei (CCN) instruments, a Droplet Measurement Technologies continuous flow CCN instrument, and LACIS. A close correlation between the hygroscopic growth factor at 98% relative humidity and the critical supersaturation of CCN activation was observed. Closure between hygroscopic growth, CCN activation, and chemical composition of the investigated particles was achieved with two different single-parameter Kohler model approaches and with a third approach, a standard Kohler model using as input parameter the soluble mass as determined by aerosol mass spectrometry. (Less)

Gas-phase reaction of N2O5 with water vapor: Importance of heterogeneous hydrolysis of N2O5 and surface desorption of HNO3 in a large Teflon chamber

Laboratory measurements of the gas-phase hydrolysis of N2O5 are always preturbed by wall reactions. Their contribution could be quantified in a large double-walled teflon bag chamber by introducing HNO3 into the chamber and analysing its temporal evolution in the gas-phase at low and high relative humidities, and by varying the surface-to-volume ratio of the chamber. By fitting a model to the experimental results, a consistent set of rate coefficients for adsorption and desorption of HNO3 from the chamber walls could be obtained. The desorption is so slow that the gas-phase hydrolysis of N2O5 via the reactions N2O5 + H2O → 2HNO3 (k1 = 2.5 · 10−22cm³molec.−1s−1) and N2O5 + 2H2O → 2HNO3 + H2O (k2 = 1.8 · 10−39cm6molec.−2s−1) is influenced by wall effects to less than 5%. The importance of the gas-phase N2O5 hydrolysis in the troposphere is discussed.

Heterogeneous reaction of N2O5 on sodium nitrate aerosol

The heterogeneous reactive uptake coefficients of N2O5 on metastable and deliquescent aerosols were determined in a large aerosol chamber at different humidities to 0.0018±0.0004, 0.0032±0.0006, and 0.023±0.005 at 48, 62, and 88% relative humidity and 293±2 K. The uptake coefficients at the lower two relative humidities are ∼1 order of magnitude smaller than the uptake coefficients of N2O5 on other aqueous surfaces. This effect is explained by the ionic hydrolysis mechanism of N2O5 and the influence of the significantly enhanced recombination reaction NO2+ + NO3− ⇒ N2O5 due to the large molality of NO3− in the metastable aerosols. About 70% of the heterogeneously hydrolyzed N2O5 are found to be released as HNO3 into the gas phase. The results present experimental support for the ionic hydrolysis mechanism of N2O5. The low reaction probabilities lead to lower heterogeneous NOx turnover in specific areas of the atmosphere as assumed so far.

Aerosol Mass Spectrometric Features of Biogenic SOA: Observations from a Plant Chamber and in Rural Atmospheric Environments

Secondary organic aerosol (SOA) is known to form from a variety of anthropogenic and biogenic precursors. Current estimates of global SOA production vary over 2 orders of magnitude. Since no direct measurement technique for SOA exists, quantifying SOA remains a challenge for atmospheric studies. The identification of biogenic SOA (BSOA) based on mass spectral signatures offers the possibility to derive source information of organic aerosol (OA) with high time resolution. Here we present data from simulation experiments. The BSOA from tree emissions was characterized with an Aerodyne quadrupole aerosol mass spectrometer (Q-AMS). Collection efficiencies were close to 1, and effective densities of the BSOA were found to be 1.3 +/- 0.1 g/cm(3). The mass spectra of SOA from different trees were found to be highly similar. The average BSOA mass spectrum from tree emissions is compared to a BSOA component spectrum extracted from field data. It is shown that overall the spectra agree well and that the mass spectral features of BSOA are distinctively different from those of OA components related to fresh fossil fuel and biomass combustions. The simulation chamber mass spectrum may potentially be useful for the identification and interpretation of biogenic SOA components in ambient data sets.