Semiempirical H2SO4-H2O Cluster Standard Gibbs Reaction Energies from Nucleation Experiments: Improved Temperature Dependence.

The thermodynamics of sulfuric acid-water clusters is important for modeling new particle formation in the atmosphere. Particle number densities obtained at 296 K from a photolytic flow reactor are greatly overpredicted by the Sulfuric Acid Water Nucleation (SAWNUC) model. Empirical, temperature-dependent adjustments to the SAWNUC model allow for better agreement with the data obtained at 296 K, while maintaining reasonable agreement with the data of Hanson and Lovejoy at 242 K. Even though these adjustments result in extensive decreases in the modeled particle number densities at room temperature, the changes in the standard Gibbs reaction energies are all less than 1 kcal/mol.

Particle Formation from Photooxidation of αpinene, Limonene, and Myrcene.

We present measurements of the effect of first-generation secondary organic aerosol (SOA) material on the growth of ∼10 nanometer diameter seed particles composed of sulfuric acid and water. Experiments were performed in an atmospheric pressure, vertically aligned flow reactor where OH was produced from HONO photolysis in the presence of either SO2 or a monoterpene. For typical conditions, organic compounds at ∼300 ppbv are exposed to photooxidation for a time of ∼80 s at a [OH] of about 6 × 106 cm-3: thus, oxidation products have minimal OH exposure. The measured size changes of the sulfuric acid seed particles can then be attributed to the uptake of first-generation products. Along with descriptions of the apparatus and the procedure, the analysis to obtain SOA yields by comparing them to growth with H2SO4(g) is detailed. Results from photooxidation experiments of αpinene, limonene, and myrcene give SOA yields of 0.040, 0.084, and 0.16, respectively. These SOA yields roughly double with each addition of a double bond to the compound. The αpinene and limonene results are in accord with the results of many previous SOA experiments, while the myrcene SOA yield stands alone. Photooxidation of myrcene also led to significant nucleation, and the species responsible is comparable to H2SO4 at a 35% relative humidity in its nucleation capability.

Speciated Monitoring of Gas-Phase Organic Peroxy Radicals by Chemical Ionization Mass Spectrometry: Cross-Reactions between CH3O2, CH3(CO)O2, (CH3)3CO2, and c-C6H11O2.

Organic peroxy radicals ("RO2", with R organic) are key intermediates in most oxygen-rich systems, where organic compounds are oxidized (natural environment, flames, combustion engines, living organisms, etc). But, until recently, techniques able to monitor simultaneously and distinguish between RO2 species ("speciated" detection) have been scarce, which has limited the understanding of complex systems containing these radicals. Mass spectrometry using proton transfer ionization has been shown previously to detect individual gas-phase RO2 separately. In this work, we illustrate its ability to speciate and monitor several RO2 simultaneously by investigating reactions involving CH3O2, CH3C(O)O2, c-C6H11O2, and (CH3)3CO2. The detection sensitivity of each of these radicals was estimated by titration with NO to between 50 and 1000 Hz/ppb, with a factor from 3 to 5 of uncertainties, mostly due to the uncertainties in knowing the amounts of added NO. With this, the RO2 concentration in the reactor was estimated between 1 × 1010 and 1 × 1012 molecules cm-3. When adding a second radical species to the reactor, the kinetics of the cross-reaction could be studied directly from the decay of the first radical. The time-evolution of two and sometimes three different RO2 was followed simultaneously, as the CH3O2 produced in further reaction steps was also detected in some systems. The rate coefficients obtained are (in molecule-1 cm3 s-1): kCH3O2+CH3C(O)O2 = 1.2 × 10-11, kCH3O2+t-butylO2 = 3.0 × 10-15, kc-hexylO2+CH3O2 = 1.2 × 10-13, kt-butylO2+CH3C(O)O2 = 3.7 × 10-14, and kc-hexylO2+t-butylO2 = 1.5 × 10-15. In spite of their good comparison with the literature and good reproducibility, large uncertainties (×5/5) are recommended on these results because of those in the detection sensitivities. This work is a first illustration of the potential applications of this technique for the investigation of organic radicals in laboratory and in more complex systems.

Beyond Ecosystem Services: Valuing the Invaluable.

The ecosystem services framework (ESF) is advantageous and widely used for itemising and quantifying ways in which humans benefit from natural places. However, it suffers from two important problems: (i) incoherence of definitions and (ii) a narrow approach to valuation, inadequate to represent the full range of human motives for conservation and the diverse interests of different stakeholders. These shortcomings can lead to a range of problems including double-counting, blind spots and unintended consequences. In this opinion article, we propose an ecosystem valuing framework as a broader and more rigorous way to deliver the benefits currently sought from the ESF, without the conceptual problems.

Quantitative and time-resolved nanoparticle composition measurements during new particle formation.

The chemical composition of 20 nm diameter particles was measured with the Nano Aerosol Mass Spectrometer (NAMS) in a rural/coastal environment during days when new particle formation (NPF) occurred and days when NPF did not occur. NAMS provides a quantitative measure of nanoparticle elemental composition with high time resolution. These measurements show that nanoparticle chemical composition is dynamic on both types of days and that changes in nanoparticle chemical composition do not necessarily correlate with changes in aerosol mass or number concentration. On NPF days, NAMS can distinguish between elements associated with particle formation and early mass growth from those associated with later mass growth. In the early stage of NPF, the particle phase sulphur mole fraction (S) increases simultaneously with the increase in gas phase sulphuric acid. This composition change occurs before the mode diameter has grown into the NAMS-measured size range and is quantitatively described by sulphuric acid condensation. The nitrogen mole fraction (N) also increases during this time period. The N/S mole ratio is approximately 2, indicating that particulate sulphate is fully neutralized. As the mode diameter passes into and through the NAMS-measured size range, N increases at a faster rate than S (N/S mole ratio increases above 2), indicating that a separate, nitrogen-based growth process exists, possibly involving aminium salts, inorganic nitrate and/or organonitrates. Carbonaceous matter is the most abundant component (-50% by mass) of the growing nanoparticles, but it is the inorganic species that are preferentially enhanced during NPF relative to other times of day. Concurrent measurements of cloud condensation nucleation activity during NPF events suggest that these newly formed particles are hygroscopic. Nanoparticle composition on non-NPF days also shifts toward a more inorganic composition during the daytime, but the chemical species are different from NPF days and the particles are less hygroscopic. Incorporation of S into growing nanoparticles is adequately explained by existing models, but currently no models exist to satisfactorily explain incorporation of nitrogen-containing species or carbonaceous matter.

Acid-base chemical reaction model for nucleation rates in the polluted atmospheric boundary layer.

Climate models show that particles formed by nucleation can affect cloud cover and, therefore, the earth's radiation budget. Measurements worldwide show that nucleation rates in the atmospheric boundary layer are positively correlated with concentrations of sulfuric acid vapor. However, current nucleation theories do not correctly predict either the observed nucleation rates or their functional dependence on sulfuric acid concentrations. This paper develops an alternative approach for modeling nucleation rates, based on a sequence of acid-base reactions. The model uses empirical estimates of sulfuric acid evaporation rates obtained from new measurements of neutral molecular clusters. The model predicts that nucleation rates equal the sulfuric acid vapor collision rate times a prefactor that is less than unity and that depends on the concentrations of basic gaseous compounds and preexisting particles. Predicted nucleation rates and their dependence on sulfuric acid vapor concentrations are in reasonable agreement with measurements from Mexico City and Atlanta.

Computational fluid dynamics of a cylindrical nucleation flow reactor with detailed cluster thermodynamics.

Particle formation and growth with H(2)SO(4) molecules in an axially symmetric flow reactor was simulated with computational fluid dynamics. A warm (~310 K) gas containing H(2)SO(4) flows into a cooled section (296 K) that induces particle formation. The fluid dynamics gives flow fields, temperatures, and reactant and cluster distributions. Particle formation and growth are simulated with detailed H(2)SO(4) cluster kinetics with chemistry based on measured small cluster thermodynamics and on bulk thermodynamics for large clusters. Results show that particle number densities have power law dependencies on sulfuric acid of ~7, in accord with the thermodynamics of the cluster chemistry. The region where particle formation rates are largest has a temperature that is within 3 K of the wall. Additional simulations show that the H(2)SO(4) concentration in this region is 5 to 10 times greater than the measured H(2)SO(4): this information allows for direct comparisons of experiment and theory. Experiments where ammonia was added as a third nucleating species were simulated with a three-dimensional model. Ammonia was dispersed quickly and particle formation during this mixing was seen to be low. Downstream of the initial mixing region, however, ammonia greatly affected particle formation.

The uptake of methyl vinyl ketone, methacrolein, and 2-methyl-3-butene-2-ol onto sulfuric acid solutions.

To investigate the link between molecular structure, reactivity, and partitioning of oxygenated organic compounds in acidic aerosols, the uptake of three compounds found in the atmosphere, methyl vinyl ketone (MVK), methacrolein (MACR), and 2-methyl-3-butene-2-ol (MBO), by sulfuric acid solutions has been measured using a rotated wetted-wall reactor (RWW) coupled to a chemical ionization mass spectrometer (CIMS). MVK was found to partition reversibly into 20-75 wt % H(2)SO(4) solutions, and we report Henry's law coefficients between 20 and 7000 M atm(-1) over this range. A chemical reaction for MVK was likely responsible for the uptake observed for 80-96 wt % H(2)SO(4) solutions. We derive an upper limit to the aldol self-reaction rate coefficient for MVK in 80 wt % solution of approximately 3 M(-1) s(-1). MACR partitioned reversibly over most of the acidity range, and in contrast to that for MVK, the Henry's law coefficient was relatively independent of H(2)SO(4) content. These differences indicate that the increase of the coefficient with acidity is likely due to the ability of the carbonyl molecule to form an enol. These results indicate that aldol condensation can be facile in concentrated sulfuric acid solutions, but it should be negligibly slow in dilute acid solutions such as tropospheric aerosols. MBO uptake could be explained by a Henry's law coefficient that decreases slightly as acid content varies from 20 to 55 wt % H(2)SO(4); we also measured the value in water, 70 M atm(-1) at 298 K. A steady-state uptake of MBO was observed onto 40-80 wt % H(2)SO(4) solutions, a reaction product was observed, and the reaction was tentatively identified as Pinacol rearrangement. Similar rearrangements could be at the origin of some substituted oxygenated species found in atmospheric aerosols.