Session V26: Focus Session: Advances in Atmospheric Aerosol Science III

Measurements of the Chemical Composition of Atmospheric Nanoparticles

The Thermal Desorption Chemical Ionization Mass Spectrometer (TDCIMS) is an instrument that is capable of measuring the chemical composition of particles as small as 4 nm. It accomplishes this with a sensitivity that makes it possible to measure the molecular composition of nanoparticles at ambient concentrations in the atmosphere. For the past five years, the TDCIMS has been performing measurements of the smallest particles in the atmosphere in order to determine the chemical species and mechanisms responsible for the growth of aerosols formed by nucleation. In this talk I will summarize what we've learned from these measurements, which took place in urban areas (Atlanta and Mexico City), a remote location (the boreal forests of Finland), and regions that are combinations of both (Boulder). With the exception of one study in urban Atlanta, in which sulfur species were seen to dominate, most measurements indicate a crucial role played by organic species in the growth of atmospheric nanoparticles. Positive ion TDCIMS measurements in a variety of locations show the presence of methyl and dimethyl amines in particles as small as 8 nm. Other oxidized organics detected in positive ion TDCIMS measurements are presumed to be alcohols, aldehydes, or ketones. Negative ion TDCIMS measurements show the presence of multifunctional organics with carboxylic acid moieties. Laboratory studies using pure and multi-component aerosols are assisting us in identifying the many ions that were observed during our campaigns. Our measurements suggest that reactions of organic acids and organic bases on particle surfaces or within particles may form organic ions and/or salts in particles. Based on these measurements, we hypothesize that the organic salt formation mechanism may be the dominant mechanism by which nanoparticles grow in the atmosphere.   

Laboratory-Measured Nucleation Rates of Sulfuric Acid and Water from the SO$_{2}$ + OH Reaction

We present results of the laboratory study of sulfuric acid-water binary nucleation system. H$_{2}$SO$_{4}$ was produced through the reaction of SO$_{2}$ + OH $\to$ HSO$_{3}$ in the presence of SO$_{2}$, OH, O$_{2}$, and H$_{2}$O in a fast flow reactor at 288 K and atmospheric pressure. OH was produced from the photolysis of water vapor. The power dependence of nucleation rate ($J)$ on sulfuric acid concentration ([H$_{2}$SO$_{4}$]) was 2 - 10 in the [H$_{2}$SO$_{4}$] range from 3$\times $10$^{6}$ - 1$\times$ 10$^{9}$ cm$^{-3}$. This power dependence increased with decreasing RH and increasing nucleation time. The power dependence of $J$ on RH was 10 - 15 for the RH values from 10 - 50{\%}. The measured aerosol sizes ranged from 4 - 20 nm. These aerosol sizes were larger for higher [H$_{2}$SO$_{4}$], higher RH, and higher nucleation times. The effects of RH on aerosol growth were also more pronounced at higher [H$_{2}$SO$_{4}$] and with higher nucleation times.   

The enhancement of aqueous aerosol formation by ions and radicals

The formation of aqueous aerosols in the atmosphere is of significant importance due the role of these particles in heterogeneous chemistry. One important mechanism for the formation of these aerosols is the multi-component nucleation of water with other compounds present in the atmosphere, such as ions and radicals. We have applied the AVUS-HR approach developed in our group for to examine the nucleation of water in the presence of both single ions and ion pairs, and to the binary nucleation of water with hydroxyl and peroxyl radicals. This method allows us to efficiently calculate the free energy profile for these nucleation processes as a function of the cluster size and composition. This information can give us a clear picture of the role that these ions and radicals may play in forming aqueous aerosols.   

Scaled Nucleation in a Lennard-Jones System

Scaling of the vapor-to-liquid nucleation rate, $J$, is examined in a model Lennard-Jones system using Monte Carlo derived rate constant ratios for growth and decay of small clusters. \ The model assumes a dilute vapor system of non-interacting clusters and the steady-state nucleation rate formalism expressed as a summation over products of rate constant ratios. The nucleation rates so obtained are examined in a scaling plot of $\log J$ \ \textit{vs.} $\ \ln S/[T_{c}/T-1]^{3/2}$ [Hale, B. N., \textit {J. Chem. Phys}. 122, 204509 (2005)], the general form of which has been recently used to test the consistency of nucleation rate data [Gharibeh, M., Kim, Y., Dieregsweiler, U., Wyslouzil, B., Ghosh, D. and Strey, R., \textit{J. Chem. Phys}. 122, 094523 (2005); Brus, D., Zdimal, V., and Stratmann, F., \textit{% J. Chem. Phys}. 124, 164306 (2006)].   

The Nucleation Rate and the Gibbs Free Formation Energy of a Cluster

In this work, we present an atomistic/molecular model along with the classical approximation for the Gibbs free formation of nuclei. The free formation energy of the critical cluster plays an essential role in the calculation of nucleation rates. Thus, we have constructed a nucleation rate relation which is easy to calculate and its result is relatively simple to compare with experimental data. The energy formation of a cluster has a few more terms than the traditional classical model. Furthermore, the extra terms in this approach have their roots in the molecular treatment of a cluster formation and they are temperature dependent. We have compared the result of this approach with the original classical theory along with some experimental data. Our initial results seem promising and the temperature correction has a correct trend.   

Dynamical investigation of water clusters in atmospheric conditions

Addressing environmental challenges via first principle calculations is one of the most promising subjects of numerical simulations. Here, we investigate the dynamical evolution of water clusters, namely the dimer and the hexamer, which are abundant in our atmosphere. We use these two clusters as prototypes to clarify long-standing dilemma of greenhouse effects and ozone depletion. To begin with, I will show the behavior of HCl on water hexamers [1]. Our calculations show that at zero temperature the most energetically favorable structure is obtained when the HCl is completely dissociated. At temperatures T $\sim$ 200 K, the vibrational entropic effects stabilize the non-dissociated clusters. This behavior is traced back to the large dynamic effects associated with the flexibility of the planar cluster. Water vapor absorption in the far-infrared region accounts a large portion of the total radiative absorption responsible for the greenhouse effect. We found that at T close to 200K, the dimer dynamics is fully anharmonic and the calculated adsorption strength throughout the far-infrared spectra is smaller than the measured vapor absorption continuum [2]. \newline [1] U.F.T. Ndomgouo et al. JPCA accepted \newline [2] M-S. Lee, et al. submitted   

Modeling the Growth of H$_{2}$O-D$_{2}$O Nanodroplets

Using experimental data for water condensation in supersonic nozzles, including SAXS measurements of position-resolved nanodroplet size distributions [Wyslouzil, et al., \textit{Phys. Chem. Chem. Phys. }\textbf{9}, 5353 (2007)], we test five different droplet growth models. Three nonisothermal growth models estimate temperature differences between the droplets and the carrier gas; the two isothermal models do not. In general, we found that none of the growth laws agrees well with the experimental data. Although the droplets should be hotter than the carrier gas for our experimental conditions, our results suggest that the nonisothermal models over predict the average droplet temperatures. This leads us to hypothesize that the average temperature is not a good estimate of the most likely temperature of the growing droplets. To accurately predict growth in the nanodroplet regime, droplet growth models will need to account better for the full distribution of temperatures of growing droplets because the main contribution to growth is likely to come from droplets cooler than the average.