General Audience Oral Presentations Abstracts

TOPIC #5

back to guest contributor abstracts list

1. Size distributions and formation mechanisms of water-soluble organic aerosols in a coastal environment

Jian Zhen Yu
Department of Chemistry, Hong Kong University of Science & Technology, Clear Water Bay, Kowloon, Hong Kong
Phone: 852-2358-7389, fax: 852-2358-1594, email: chjianyu@ust.hk

Aerosol mass size distributions of water-soluble organic carbon (WSOC) were studied at a coastal site in Hong Kong. Eleven sets of 72-hr samples were collected using an 8-stage impactor sampler over three seasons in 2000-2001. Regardless of air mass origins being mainly marine or continental, WSOC exhibited bimodal size distributions with a dominant fine mode and a minor coarse mode in the size range of > 0.43 mm. The two modes had a mass mean aerodynamic diameter (MMAD) of 0.7 ± 0.1 mm and 4.0 ± 0.3 mm, respectively. The fine WSOC accounted for the major proportion of the total WSOC, ranging from two-thirds to four-fifths. The WSOC mixtures were resolved into 3 to 5 peaks under controlled progressive heating and combustion conditions. A positive matrix factorization analysis deconvoluted the WSOC mixtures into three component groups of different thermal behaviors. The three groups of WSOC species were postulated to be low, medium, and high molecular weight (MW) polar compounds on the basis of their thermal evolution features. On average, they accounted for 21%, 39%, and 40% of fine WSOC (<2.1 mm) mass, respectively. Coarse WSOC (>2.1 mm) was largely made of the low MW polar compound group. The low MW group had a bimodal distribution with a dominant coarse mode, likely associated with sea salt aerosols. The medium and the high MW groups had a single mode distribution in the droplet mode, both with an average MMAD of 0.7 ± 0.1 mm. Their droplet mode presence indicated that they were associated with cloud-processed particles. There is evidence to suggest that the high MW group was likely secondary in origin, formed during cloud processing.

Key Words: Water-soluble organic compounds, Formation mechanisms, Cloud processing, Positive matrix factorization, Secondary organic aerosols.

2. Characterization of low molecular weight carboxylic acids in atmospheric aerosols using capillary electrophoresis

Xiao-Ying Yu*a, Susanne V. Hering c, Jeffrey L. Collett, Jr.a, and Charles Henryb

a Colorado State University, Department of Atmospheric Science, Fort Collins, CO 80523-1371
b Colorado State University, Department of Chemistry, Fort Collins, CO 80523-1872
c Aerosol Dynamics Inc., 2329 Fourth Street, Berkeley, CA 94710

Low molecular weight carboxylic and dicarboxylic acids are important airborne carbonaceous pollutants. They are found in a variety of marine and continental environments. Some of these compounds contribute to ambient and precipitation acidity and some are good indicators of secondary organic aerosol production. Therefore, the capability to measure atmospheric concentrations of these species with high time resolution would greatly aid our understanding of organic acid sources and effects.

By applying high voltage across the length of an electrolyte-filled capillary, different ions can be separated depending on their size and charge. Capillary electrophoresis (CE) is considered to be a powerful tool in environmental applications due to its high efficiency, speed, small analyte mass requirements, and economy of analysis. However, the use of CE so far in atmospheric chemistry has been quite limited. In order to take advantage of the small analyte mass requirement and efficient separation capabilities of CE for semi-continuous characterization, collaboration between Colorado State University (CSU) and Aerosol Dynamics Inc. (ADI) was initiated.

Using an analytical method based on indirect uv-vis detection [Dabek-Zlotorzynska et al, 2001] more than a dozen low molecular weight carboxylic acids can be separated in less than I 0 minutes with a detection limit better than 1 µm. For aerosol measurement, this analytical approach was used for samples collected by impaction, with direct extraction from the collection surface. To attain high collection rates with small particle cutoffs, particles are enlarged by water condensation in a laminar, thermally diffusive flow (ADI, patent pending). In work reported here, CE analysis of laboratory and ambient aerosol samples was achieved with aqueous aerosol extracts of I µm, or less, from aerosol particles deposited on different substrate materials.

Preliminary results of low molecular weight carboxylic and dicarboxylic acid measurements from this prototype system are presented. The observed ability to distinctly separate organic anions in ambient aerosol samples utilizing I µL, injection indicate that the approach is capable of monitoring atmospheric relevant carboxylic and dicarboxylic acids. By changing separation and detection schemes, the system can be readily adapted to measure other organic and inorganic compounds as well. The combined system promises to provide a powerful new technique for continuous aerosol composition measurement in the future.

References
Dabek-Zlotorzynska, E., M. Piechowski, M. McGrath, and E.P.C Lai, Determination of low-molecular-mass carboxylic acids in atmospheric aerosol and vehicle emission samples by capillary electrophoresis, J. Chromatogr. A, 910, 331-45, 2001.

back to top