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Characteristics, Sources, and Formation of Organic Aerosol in the Central Rocky Mountains

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November 7, 2013
Misha Schurman
Hosted by Jeff Collett (advisor), Sonia Kreidenweis, Emily Fischer, Chuck Henry (Chemistry)


Particulate matter in the atmosphere has wide-ranging health, environmental, and climate effects, many of which are attributed to fine-mode secondary organic aerosols. High-altitude ecosystems in the Rocky Mountains are sensitive to increased nutrient, and particularly nitrogen, deposition, with documented changes in dominant diatom, lichen, and vegetative communities; visibility is also affected by fine particle pollution. Submicron particle size, composition, and source apportionment were explored at the Rocky Mountain and Grand Teton National Parks using a High-Resolution Time-of-Flight Aerosol Mass Spectrometer and Positive Matrix Factorization. A July 2-August 31, 2010 campaign at Rocky Mountain National Park found low-concentration submicron particulate matter (max = 93.1 µg/m3, avg. = 5.13 ± 2.72 µg/m3) of which 75.2 ± 11.1% is organic, with significant contributions from low-volatility (LV-OOA, 39.3% of PM1 on average) and semi-volatile oxidized organic aerosol (SV-OOA, 27.6%) punctuated by short, high-concentration biomass burning organic aerosol episodes (BBOA, 8.4%) associated with increased organic nitrogen. Nitrate (4.3%), sulfate (16.6%), and ammonium (3.9%), for which local sources are sparse, are enhanced with upslope (SW) surface winds from the densely populated Front Range; similar transport of oxidized organic aerosols is indicated by advanced oxidation and relative monodispersity (both indicative of ageing), correlation with inorganic anthropogenic tracers (LV-OOA with ammonium sulfate and SV-OOA with ammonium nitrate), and concentration correlation with upslope winds. A local BBOA source is suggested by cellulose combustion markers (m/zs 60 and 73) limited to brief, high-concentration, polydisperse events (suggesting fresh combustion emission), a diurnal maximum at 22:00 LST, when campfires were set at adjacent summer camps, and surface wind associations consistent with local campfire locations. Submicron particulate mass is lower at the Grand Teton site, averaging 2.08 μg/m3 of which 75.0% is organic; LV-OOA averages 48.9% of PM1, with sporadic but higher-concentration BBOA events contributing another 26.1%. Sulfate (12.5%), ammonium (8.7%), and nitrate (3.9%) are low in mass. Oft-anthropogenic ammonium and sulfate have correlated time-series and association with upslope winds from the Snake River valley. A regionally disperse and/or in situ photochemical LV-OOA source is suggested by 1) afternoon concentration enhancement not correlated with upslope winds, anthropogenic NOx, or ammonium sulfate, 2) smaller size and higher polydispersity during the day and in comparison to a biomass burning plume inferred to have travelled ~480 km, and 3) lower degree of oxidation than is usually observed in transported urban plumes. Organic nitrogen in the form of nitriles and/or pyridines is indicated during the day by CHN fragment spectra, with the addition of amines at night. Fires near Boise, ID may be the source of a high-concentration biomass-burning event on August 15-16, 2011 associated with W-SW winds (upslope from the Snake River Valley) and increased sulfate, ammonium, nitrate, and CHN fragments (nominally, amines). Comparison of these campaigns to GEOS-Chem model simulations echoes the literature in which models under-predict organic aerosols; aqueous SOA formation is a possible, albeit poorly understood, explanation for this discrepancy. This motivates a comparison of aqueous SOA (aqSOA) production in (1) ambient cloud water, (2) cloud water with pinonic acid added to simulate uptake of biogenic VOCs, and (3) single-precursor ethylglyoxal and pinonic acid solutions; solutions were photooxidized using UVC light and added hydrogen peroxide (producing hydroxyl radical at cloud-relevant [OH]= 4.5×10-14 M) in a temperature-stabilized vessel and then continuously atomized, dried, and analyzed via AMS with 1-minute time resolution. During photo-oxidation of cloud samples, organics increase in mass and level of oxidation to 110-140% of the initial value and a m/z 44 (CO2+) to total organic aerosol mass fraction f44 ≈ 0.23 ± 0.05, respectively, before decreasing in organic mass to 60-80% of the initial value. Decreases in organics are explained by chemical decomposition causing functional group loss in some experiments, and volatile product formation in others. Pinonic acid addition appears to reduce organic mass losses and favor carbonyl formation. In contrast, the single-precursor solutions, which are similar to those used to understand aqSOA in the literature, gained 150-300% of initial organic concentrations, with a larger increase in O:C than observed in the (albeit already fairly oxidized) ambient cloud samples. Combined, these experiments illustrate an aqSOA production sequence in which organic mass is first gained through formation of lower-volatility carbonyls and acids and then either a) lost through molecular decomposition or volatile product formation, or b) maintained or increased further by addition of fresh precursors from the gas phase. These experiments also suggest that aqSOA formation in complex ambient cloud water is not well represented by the simple solutions often used for aqSOA parameterization and, further, that the rate of aqSOA production may decrease as oxygenation of organics increases. The high O:C and f44 ratios observed in aqueous SOA formed during the laboratory experiments is broadly consistent with the high degree of oxygenation in oxidized organic aerosol (OOA) at Rocky Mountain and Grand Teton National Parks, though other mechanisms may also contribute to the observed aerosol characteristics.