Hygroscopic Properties of Oxidation Products of Terpenes
To understand the hygroscopic growth factor (HGF) of secondary organic aerosol (SOA) formed by the oxidation of terpenes, a series of seeded and nucleation experiments were conducted at the York University smog chamber facility. Oxidation of terpenes by OH was carried out in a dry chamber (RH∼5%). In nucleation experiments particles formed were pure organic and their hygroscopic growth factor was measured as function of relative humidity by using a tandem differential mobility analyzer (HTDMA). Humidograms of these particles don't show any deliquescence or efflorescence. Humidograms of pure organic particles formed by the oxidation products of β-pinene show slight but smooth take up of water while particles formed by α-pinene and δ3-carene exhibit very little or no water uptake. Experimental results were fitted with an empirical equation and the hygroscopicity parameter for the particles formed by β-pinene was found to be 0.019±0.009. To examine the interaction of organic and inorganic phases, monodisperse ammonium sulfate seed particles injected into the smog chamber were allowed to undergo condensational growth due to partitioning of terpenes oxidation products from the gas phase. Humidograms of seeded particles show both smooth hygroscopic growth and deliquescence. These experimental results were fitted with a numerical model that accounts for water uptake by both phases and for the gradual dissolution of ammonium sulfate. The results show that volume additivity is a reasonable approximation for this system and that HTDMA results can be inverted to obtain the organic hygroscopicity parameter and the relative amounts of organic and inorganic material
Parameterization of Aerosol Particle Hygroscopic Growth Factors
A model was developed to fit hygroscopic growth as a function of relative humidity in order to retrieve information on aerosol particle hygroscopic properties and chemical composition. The parameters obtained are the "hygroscopicity parameter" and volume fraction for the organic components; the remainder of the aerosol is assumed to be ammonium sulphate. The model was used to analyze Hygroscopic Tandem Differential Mobility Analyzer (HTDMA) data from two field studies in southern Ontario. Comparisons between the organic fractions from the model fits and ones measured with an Aerosol Mass Spectrometer (AMS) were performed for 100 nm and 150 nm diameter particles. The two data sets generally agreed within their error limits. After validating the model at the larger sizes, it was then used to analyze HTDMA data for particle sizes of 30 and 50 nm, at which the AMS is unable to provide useful data. As with the larger particles, organic fraction often made up more than 50 percent of the total particle volume. The organic hygroscopicity parameter was typically in the range of 0.01 to 0.37; this is in agreement with values obtained in previous studies.
Long range atmospheric transport of Aerosols: First ARCTIC measurements using Quadrupole Aerosol Mass Spectrometer
Atmospheric aerosols can directly affect climate by scattering and absorbing solar radiation, thereby modifying the radiative balance of the atmosphere. Aerosols can also act as cloud condensation nuclei, which alter cloud properties and precipitation rates, thereby indirectly influencing the climate. Aerosol surfaces are a medium for heterogeneous reactions and carry many different compounds, which naturally affect their properties. Since aerosol lifetimes in the troposphere are on the order of days to a week, they are transported throughout the atmosphere. To study this transport, we installed a Q - AMS in the Polar Environment Atmospheric Research Laboratory in August 2006. The laboratory is located on Ellesmere Island, (80°N 86°W at 610 m above sea level). It provides a unique location for observing transport to the sensitive Arctic ecosystem because it is far from anthropogenic sources of contamination. In this presentation, we will report the analysis of aerosol mass concentrations, size, and chemical compositions covering the time period from August, 2006 to May 2009. Our measurements show that sulphate dominates the aerosol composition most of the time, with a maximum concentration of 0.655 µg/m3 and minimum concentration of 0.030 µg/m3. The second most abundant species was organic aerosols, with concentrations in the range from 0.440 µg/m3 to 0.050 µg/m3. Although the sulphate dominates in general, plots of concentration time series show a seasonal change in the relative concentrations of sulphate and organic species. Relatively lower concentrations of nitrate and ammonium species were detected during the period of our observations. Occasional episodes of concentrations up to 0.050 µg/m3 nitrate and 0.080 µg/m3 ammonium were detected; otherwise these were below our detection limit (0.009 µg/m3). In addition to the above results, we will briefly report the ionic components and discuss possible aerosol transportation routes determined with the semi-Lagrangian trajectory model, FLEXPART.
Water Mediated Equilibrium Between Pyruvic Acid and its Geminal Diol Counterpart
The purpose of our study is a spectroscopic infrared (IR) investigation of water mediated equilibrium of an oxidized atmospheric chromophore, pyruvic acid, and its geminal diol counterpart. Their fundamental vibrational spectra were assigned and analyzed for the effects of intra- and intermolecular hydrogen bonding. The IR spectra are useful in probing the differences in their structures and hydrogen bonding tendencies. The equilibrium between the two molecules was determined for conditions where only very small hydrates are present by isolating these in a carbon tretrachloride (CCl4) matrix, using the frequencies, intensities and linewidths determined from the IR spectra. The implications of this water mediated chemistry to the rate growth of secondary organic aerosols will be discussed.
Heterogeneous and Photochemical Reactions Involving Surface Adsorbed Organics: Common Lignin Pyrolysis Products With Nitrogen Dioxide.
Solid-air interfaces, such as airborne particulate matter and ground level surfaces, provide unique supports for tropospheric heterogeneous chemistry. These interfaces commonly contain surface adsorbed organics, such as lignin pyrolysis products, that can significantly alter their physical and chemical properties. Attenuated total reflectance infrared spectroscopy (ATR-FTIR) provides an ideal tool for monitoring chemical changes in thin organic films during heterogeneous and photochemical reactions. Phenolic compounds, with and without co- adsorbed photosensitizers, were exposed to NO2 concentrations in the parts-per-billion range at 300 K and 20% relative humidity. Catechol, when mixed with benzophenone or dicyclohexylketone, formed 4- nitrocatechol as the dominant product under dark conditions. Deuterating the catechol alcohol groups caused the initial rate of reaction to decrease by a factor of 3.3±0.5, consistent with formation of the ortho- semiquinone radical as the rate determining step. The rate of 4-nitrocatechol formation did not increase under illuminated conditions, even with the presence of benzophenone a well known photosensitizer. UV-A/visible radiation did, however, initiate a photochemical reaction between benzophenone and 4-nitrocatechol, likely forming high molecular weight polymerization products. In contrast, 2-ethoxyphenol displayed no reactivity with NO2, even under illuminated conditions with a photosensitizer. Implications for the fate of lignin pyrolysis products, which are prevalent in biomass combustion smoke, will be discussed.