Session D36: Focus Session: Environment I: Aerosols and Aqueous Solutions

Uncovering the formation mechanism of atmospheric nanoparticles

Atmospheric aerosol affect human health, visibility and radiation budget of the Earth. The current estimate is that 20-80\% of aerosols particles are formed in the atmosphere by condensable gases. Experimental and theoretical data indicates that the formation of new particles in the atmosphere in most cases very likely involves sulphuric acid assisted with some base molecules. The role of ions in atmospheric particle formation is has been widely discussed during recent years. The diameter of the forming clusters is 1-2nm, falling between the smallest size where brute force quantum mechanical treatment is possible, and macroscopic size where bulk thermodynamics in applicable. Recent experiments at the CLOUD chamber in CERN have provided molecular-level information on the charged fraction of the nucleating clusters, but the theoretical framework needed to convert this into information on neutral clusters is still lacking. We have used a cost-effective multi-step computational chemistry method involving automated configurational sampling, density functional theory geometry optimizations and coupled-cluster energy calculations, to study the stability of charged and neutral sulfuric acid clusters containing ammonia and dimethylamine. Combined with a cluster dynamics model ACDC, we are able to replicate the formation rates observed in the CLOUD chamber, as well as match observed formation rates in Hyyti\"al\"a Smear II station in Finland.   

Molecular dynamics of binary and ternary nanodroplets with a miscibility gap

The structure of nanodroplets plays an important role in many natural processes including particle nucleation and aerosol formation in the atmosphere. Among other factors, chemical miscibility and surface tension strongly affect the structure of multicomponent nanodroplets at low temperature. Here, we investigate the structure of water/nonane and water/butanol/nonane nanodroplets using molecular dynamics (MD). Our MD results confirm our theoretical predictions of nonspherical nanodroplet (Russian-Doll) structures at low temperatures using density functional and lattice Monte Carlo techniques. We systematically study the variation of the droplet structure with temperature and with butanol concentration.   

Organization at the Air-Aqueous Interface by Heterodyne-detected Phase-Sensitive Sum Frequency Spectroscopy

Water and ions organize at the air -- aqueous interface and the ion distributions within this region give rise to interfacial electric double layers. Geochemical solid-aqueous and atmospheric aerosol relevant air-aqueous interfaces were studied using vibrational sum frequency generation (VSFG), and in some cases, heterodyne-detected VSFG spectroscopy. Solid-aqueous and air-liquid interfaces were also investigated using total internal reflection Raman spectroscopy, infrared reflection absorption spectroscopy (IRRAS), and Brewster angle microscopy (BAM), respectively. Here, we show results from aqueous solutions containing salts such as sulfates and chlorides where surface adsorption and electric field direction reversal was observed. Of the salts studied at the air-aqueous interface with heterodyne-detected VSFG, the magnitude of the electric field in the surface extending to the subsurface regions decreases in the order: (NH$_{4})_{2}$SO$_{4} \quad >$ Na$_{2}$SO$_{4} \quad >$ Na$_{2}$CO$_{3 }\ge $ CaCl$_{2} \quad >$ NaCl; the electric field is opposite in direction for the sulfates and carbonate relative to the chloride salts.   

First-principles study of the infrared spectrum of the ice Ih (0001) surface

Ice particles catalyze a number of processes relevant to atmospheric and environmental chemistry, and the elucidation of these reactions require knowledge of the ice surface structure. Although it is well known that the structure of bulk ice-Ih is proton disordered, the understanding of the microscopic structure of the ice surface is still limited. Recent theoretical studies $^{1,2}$ suggest that the basal (0001) surface of ice Ih is significantly more proton ordered than the bulk. In this work we compute infrared (IR) spectra of several ordered and disordered models of the (0001) surface of ice, and investigate the sensitivity of these spectra to the microscopic details of the surface structure. In particular we discuss possible signatures of disorder in the computed spectra. [1] D. Pan et al., Phys. Rev. Lett. 101, 155703 (2008); [2] V. Buch et al., Proc. Natl. Acad. Sci. U.S.A. 105, 5969 (2008)   

1st principle simulations of ions in water solutions: Bond structure and chemistry in the hydration shells of highly charged ions

Methods of direct simulation (Monte Carlo and molecular dynamics) have provided new insights into the structure and dynamics of electrolyte solutions. However, these methods are limited by the difficulty of developing reliable ion-solvent and solvent-solvent potential interactions in the highly perturbed hydration region. To model the interactions in this region methods of simulation that are based on the direct on the fly solution to the electronic Schr\"{o}dinger equation (ab-initio molecular dynamics, AIMD) are being developed. However, 1st principle methods have their own problems because the solution to the electronic structure problem is intractable unless rather uncontrolled approximations are made (e.g. density functional theory, DFT) and there is high computational cost to the solution to the Schr\"{o}dinger equation. To test the accuracy of AIMD methods we have directly simulated the XAFS spectra for a series of transition metal ions Ca$^{2+}$, Cr$^{3+}$, Mn$^{2+}$, Fe$^{3+}$, Co$^{2+}$, Ni$^{2+}$, Cu$^{2+}$, and Zn$^{2+}$. Despite DFT's well know deficiencies, the agreement between the calculated XAFS spectra and the data is almost quantitative for these test ions. This agreement supports the extension of the interpretation well beyond that of the usual XAFS analysis to include higher-order multiple scattering signals in the XAFS spectra, which provide a rigorous probe of the first shell distances and disorders. Less well resolved features of the spectra can still be analyzed and are related to 2nd shell structure. The combination of XAFS measurements and the parameter free AIMD method leads to new insights into the hydration structure of these ions. While strictly local DFT +gga provides excellent agreement with data, the addition of exact exchange seems to provide slightly better structural agreement. The computational complexity of these calculations requires the development of simulation tools that scale to high processor number on massively parallel supercomputers. Our present algorithm scales to nearly 100,000 processors. However, even with high scaling the time to solution is very long. We are also developing and testing new methods to improve the performance of simulation and new sampling methods that more efficiently explore phase space and can reach longer time frames. Results of calculation of the hydration structure and dynamics of highly charges ions and free energy calculations of ion association will be presented.   

Reactions of Solvated Electrons Initiated by Sodium Atom Ionization at the Vacuum-Liquid Interface

Solvated electrons are powerful reagents in the liquid phase that break chemical bonds and create new reactive species, including hydrogen atoms. Electrons and hydrogen atoms born near the surface, however, behave differently than those created within the liquid. We explore this behavior by exposing liquid glycerol to a beam of sodium atoms. The Na atoms ionize in the surface region, generating electrons that react with deuterated glycerol, C$_{3}$D$_{5}$(OD)$_{3}$, to produce D atoms, D$_{2}$, D$_{2}$O, and glycerol fragments. Surprisingly, 40{\%} of the D atoms desorb into vacuum before attacking C-D bonds to produce D$_{2}$. These D atoms must traverse the interfacial region before desorbing, demonstrating that Na ionization prepares reactive species that reside momentarily at the surface and often escape before reacting with the solvent.   

Heterogeneous Photochemistry and Optical Properties of Mineral Dust Aerosol

It is now widely recognized that heterogeneous reactions of mineral dust aerosol with trace atmospheric gases impact the chemical balance of the atmosphere and the physicochemical properties of these particles. Field studies using single particle analysis, have now shown that the chemistry is mineralogy specific and follows the trends expected from laboratory studies. These laboratory studies, which were initiated over a decade ago, have focused on the nighttime chemistry of mineral dust aerosol which is really only ``half'' the story. This talk will focus on two aspects of solar light interaction with mineral dust aerosol. First, the heterogeneous photochemistry of adsorbed chromophores (e.g. nitrate ion) and light absorbing components of mineral dust (iron oxides and titanium dioxide) is discussed. These heterogeneous photochemical reactions are poorly understood and laboratory studies to better quantify these reactions in order to determine the impact on the chemical balance of the atmosphere are needed, as will be discussed. Second, the optical properties of mineral dust aerosol measured by extinction infrared spectroscopy and visible light scattering show that shape effects are extremely important for mineral dust aerosol.