Dr. Alexei Khalizov
Assistant Research Scientist
Ph.D., 1997, Physical Chemistry, Ufa Research Center of the Russian Academy of Sciences
B.S., 1994, Chemistry (honors), Bashkir State University (Russia)
My research is focused on the formation, growth, and effects of atmospheric aerosols. Submicron aerosol particles originate from two major sources: direct emissions and gas-to-particle conversion of semi-volatile species from atmospheric gas-phase oxidation. Increased aerosol loading of the atmosphere has negative impacts on air quality and human health and contributes to local and global climate change. Studies of aerosol formation and evolution will help to deduce appropriate policies to alleviate these impacts.
Aerosols affect climate by scattering and absorbing solar and thermal radiation and by acting as cloud condensation nuclei (CCN). Carbon soot is the major absorbing component of atmospheric aerosols. Freshly emitted soot aggregates undergo complex transformations during atmospheric transport, changing their optical and hygroscopic properties. In laboratory experiments, I study how initially hydrophobic carbon soot particles within a time period of a few days acquire sufficient fraction of sulfate or organic coating to become efficient CCN. The changes in soot morphology and hygroscopic properties are investigated using a combination of Tandem Differential Mobility Analyzer (TDMA) and Aerosol Particle Mass analyzer (APM). The internal mixing with condensable atmospheric species has a profound effect on the optical properties of soot. Transparent coating increases both the scattering and the absorption efficiency of the light-absorbing core. This effect is investigated by measuring the extinction and scattering of light by fresh and aged soot particles using a Cavity Ring-Down Spectrometer (CRDS) and a Nephelometer in combination with twin DMA.
Despite the intensive research over the last decade, the mechanism of new particle formation in the atmosphere is still unclear. Field measurements show atmospheric nucleation events for sulfuric acid concentrations about 107 molecules cm-3. In contrast, much higher concentrations of sulfuric acid (1010 molecules cm-3) are necessary to produce particles in the laboratory. There is emerging evidence that chemical species other than sulfuric acid can contribute to new particle formation and growth in the atmosphere. For instance, a recent study from our laboratory has shown that minute amounts of organic acids greatly enhance nucleation in the sulfuric acid - water system. I study the contribution of organic species, such as carboxylc acids and carbonyls, to the nucleation and growth of nanoparticles. Nanoparticles produced in a laminar flow reactor are exposed to organic vapor and then analyzed for changes in size and chemcial composition using nano-TDMA and Thermal Desorption - Ion Drift Chemical Ionization Mass Spectrometer. Quantum chemical ab initio and density functional calculations are used to corraborate the experimental measurements.
Chemical analysis of aerosol nano-particles is an extremely challenging task because of their incredibly small size. Each particle contains only few hundred to few thousand molecules. This difficulty can be overcome by collecting particles on an electrically charged platinum wire in a thermal precipitator (TD) unit. When enough material is collected, the wire is extended into the front part of the ion drift tube (ID) of a chemical ionization mass spectrometer (ID-CIMS) where the sample is flash-evaporated by heating the wire with electrical current. The sample vapor is carried into the drift tube where the molecules are ionized through reactions with the reagent ions produced by the chemical ionization source. Finally, the sample ions are detected with a quadrupole mass spectrometer. This technique, called TD-ID-CIMS, is extremely sensitive, allowing to detect aerosol mass of the order of a few picograms, which can be collected within less then ten minutes.
Heterogeneous interaction of nitrogen dioxide with soot aerosol can lead to the formation of nitrous acid (HONO). Photolysis of HONO formed during the night can increase the concentration of OH radical in the morning upon sunrise. To estimate the contribution of the NO2-soot dark reaction to the OH formation, one needs to know the heterogeneous reaction probability and HONO yield, which depend on the type of soot and can change when soot particles become internally mixed with other chemical species present in the urban troposphere. I study the heterogeneous HONO formation by exposing soot films to ambient level concentrations of nitrogen dioxide in a fast flow reactor. The kinetics of the NO2 uptake and the yield of HONO are monitored using a chemical ionization mass spectrometer. Experiments are conducted with fresh soot produced from different fuels under rich and lean flame conditions and also with soot films exposed to condensable atmospheric species such as sulfuric acid and organic acids.
Mercury is a persistent, bioaccumulative pollutant present in the atmosphere mainly in its elemental form (Hg0) with a lifetime of the order of 1-2 years, which explains the observation of nearly uniform mixing ratios of Hg0 within the Earth’s atmosphere. It has been long recognized that mercury needs to be converted into oxidized form before it can be efficiently removed from the atmosphere. Chemical oxidation of atmospheric mercury can be driven by halogen chemistry, but the mechanism of this transformation is largely unknown and there is a significant discrepancy in the available kinetic data. I study the kinetics and mechanism of mercury – halogen reactions using a fast-flow turbulent reactor coupled to ion drift-chemical ionization mass spectrometer (ID-CIMS). This approach allows direct kinetic investigation with minimal wall effects and real-time identification of reaction intermediates and products.
Honors and Awards
Selected Publications (of total 51 with an h-index of 15)