The ambient organic aerosol soluble in water: Measurements, chemical characterization, and an investigation of sources.

摘要

This thesis characterizes the ambient fine organic carbon (OC) aerosol and investigates its sources through the development and deployment of new analytical measurement techniques. Recognizing that OC is highly chemically complex, the approach was to develop methods capable of quantitatively measuring a large chemical fraction of the aerosol instead of specific chemical speciation. The focus is on organic compounds that are soluble in water (WSOC) since little is known about its chemical nature. The results from this thesis show that WSOC has mainly two sources: biomass burning and secondary organic aerosol (SOA). In urban areas, WSOC increases with plume age, and tracks other photochemically produced compounds. Chemical analysis of WSOC suggests that in urban Atlanta, the SOA is mainly small-chain aliphatic compounds indirectly linked to vehicle emissions.; A method was first developed for quantitative on-line measurements of WSOC by extending the application of the Particle-into-Liquid Sampler (PILS) from inorganic to organic aerosol measurements. In this approach a PILS captures ambient particles into a flow of purified water, which is then forced through a liquid filter and the carbonaceous content quantified on-line by a Total Organic Carbon (TOC) analyzer. An instrument was first developed for ground-based measurements and then modified for airborne deployment.; Ground-based measurements at the St. Louis - Midwest Supersite during the summer of 2003 showed that the fraction of OC that is water-soluble can have a highly diurnal pattern with WSOC to OC ratios reaching 0.80 during the day and lows of 0.40 during the night. During extended periods under stagnation pollution events, this pattern was well correlated with ozone concentrations. The results are consistent with formation of SOA.; Airborne PILS-TOC measurements from the NOAA WP-3D during the New England Air Quality Study/Intercontinental Transport and Chemical Transformation (NEAQS/ITCT) 2004 program investigated WSOC sources over the northeastern U.S. and Canada. Two main sources were identified: biomass burning emissions from fires in the Alaska/Yukon region and emissions emanating from urban centers. Biomass burning WSOC was correlated with carbon monoxide (CO) and acetonitrile (R 2 > 0.88). Apart from the biomass burning influence, the highest concentrations were at low altitudes in distinct plumes of enhanced particle concentrations from urban centers. WSOC and CO were highly correlated (R2 > 0.78) in these urban plumes. The ratio of the enhancement in WSOC relative to that of CO was found to be low (∼ 3 microg C/m3/ppmv) in plumes that had been in transit for a short time, and increased with plume age, but appeared to level off at ∼32 microg C/m3/ppmv after approximately one day of transport from the sources. The results suggest WSOC in fine particles is produced from compounds co-emitted with CO and that these emissions are rapidly converted to organic particulate matter within ∼1 day following emission.; To further chemically investigate the organic constituents of WSOC, a method for group speciation of the WSOC into hydrophilic and hydrophobic fractions has been developed using a XAD-8 resin column. XAD-8 resin coupled with a TOC analyzer allows for direct quantification. Based on laboratory calibrations with atmospherically relevant standards and 13C-NMR ( 13Carbon-Nuclear Magnetic Resonance) analysis, the hydrophilic fraction (compounds that penetrate the XAD-8 with near 100% efficiency at pH 2) is composed of short-chain carboxylic acids and carbonyls and saccharides. The fraction of WSOC retained by XAD-8, termed the hydrophobic fraction, includes aromatic acids, phenols, organic nitrates, cyclic acids, and carbonyls and mono-/dicarboxylic acids with greater than 3 or 4 carbons. Only aromatic compounds (or aromatic-like compounds with similar properties) can subsequently be extracted from XAD-8 with high efficiency and are referred to as th