Dr. Sanford Sillman
Research Scientist
University of Michigan
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Summary: A 3-d model is used to investigate the performance of photochemical 'indicators' for O3-NOx-VOC sensitivity in Mexico City. The indicator ratios include O3/NOy, O3/HNO3, and H2O //NOy, described in Sillman and He, 2002 (see below). Previously, low values of these ratios were associated with predicted VOC-sensitive chemistry in models, while high values were associated with NOx-sensitive chemistry. This association suggests that measured values of the ratios can provide an indirect evaluation of the accuracy of model predictions for O3-precursor sensitivity.
Results for Mexico City show somewhat different behavior than elsewhere, in part because of the dominant role of PAN.
Summary: A modified version of CMAQ is used to simulate formation of reactive mercury in the eastern U.S., focused on Florida. The modified CMAQ includes integrated gas-phase and aqueous photochemistry, including mercury, sulfates, halogens, and gas and aqueous reactions of O3, NOx, organics and OH.
Results show that elevated reactive mercury occurs intermittantly at 3000 m., due to conversion from elemental mercury.
Photochemical conversion from elemental to reactive mercury (as opposed to direct emission of RGM) can be identified by a negative correlation between ambient Hg0 and RGM.
Elevated ozone during pollution events affects mercury and results in elevated ambient RGM. A strong correlation between ambient O3 and RGM is predicted as a test for this hypothesis.
Summary: The effect of aromatics, terpenes, isoprene nitrates and other reaction products of isoprene is investigated in a global-scale model for gas-phase photochemistry (IMPACT). We find that the isoprene nitrates have an especially large impact. When included in global models, O3 increases 20% and PAN increases 40%. Hydroxyacetone also has a large effect on global tropospheric chemistry.
Supplemental information available here
Environmental news summary from Environmental Science and Technology is available here .
Summary: Perfluorocarboxylic acids (PFCAs) have been observed widely as contaminants in natural environments, most notably in Arctic wildlife. PCFA's have also been found in Great Lakes fish and in human blood, and are currently under investigation by the USEPA for possible toxicity. Here, a photochemical reaction sequence is presented that would produce PFCAs in the atmosphere as the result of the degradation of fluorotelomer alcohols, which are emitted by industry. The proposed degradation pathway is tested in a global photochemical model (IMPACT).
The photohemical conversion from fluorotelomers to PFCAs requires several months, and PFCA's are produced mainly in remote environments with low NOx. The model predicts that PFCA's are ubiquitous in the northern hemisphere and reach maximum values in the Arctic during summer.
Sillman S., Tropospheric ozone and photochemical smog, pp. 407-432. In Environmental Geochemistry, (ed. B. S. Lollar) Vol. 9 Treatise on Geochemistry (eds. H. D. Holland and K. K. Turekian), Elsevier-Pergammon, Oxford, 2003. http://www.TreatiseOnGeochemistry.com . For availability, contact the author.
Summary: The paper shows an intercomparison between five separate calculations of 0-d atmospheric chemistry including clouds and aqueous reactions, including the effect of model iterative time intervals and intermittant exposure to clouds. It also contains a description of Dr. Sillman's numerical solution procedure for combined gas and aqueous chemistry.
Summary: Isopleths are shown for O3 vs. NOx and VOC, showing how various "indicator ratios" vary along with O3-NOx-VOC sensitivity. An analysis of chemistry shows that ozone production efficiency per NOx is higher for NOx-sensitive conditions than for VOC-sensitive conditions. Correlation plots show how patterns are different for NOx-sensitive versus VOC-sensitive locations. Smog production algorithms , often used to evaluate ozone control strategies, do not correlate well with O3-NOx-VOC sensitivity.
Summary: Measurements at the PROPHET site in rural northern Michigan showed a rapid decrease in near-surface isoprene during the nighttime, with apparent removal at a time scale of 3 hours. Unusually high OH was also observed at night, suggesting that OH chemistry might be the cause of apparent loss of isoprene. This and other possibilities are analyzed with a 1-d Lagrangian model with 25 m. vertical resolution. Results suggest that the removal of isoprene is primarily due to dynamics rather than chemistry. Isoprene decreases in part due to nighttime vertical mixing and in part due to advection of low-isoprene air from nearby Lake Michigan. Nighttime OH might be generated through the reaction of short-lived terpenes with O3, but the resulting OH would be confined to a shallow surface layer and would have relatively little impact on isoprene.
For further information: PROPHET: Program for Research on Oxidants: PHotochemistry, Emissions and Trans port.
Summary: Measurements from Ryerson et al. (1998, 2001) suggested that ozone production per NOx in plumes from large power plants is very low, even in aged downwind plumes. Here, ozone production in power plant plumes is investigated with a Lagrangian model using a 2-d grid (vertical and cross-plume) that travels with the traveling plume. Results suggest that ozone production per NOx is low in plumes from large power plants if emissions at 8 am are followed downwind for 10 hours. However, ozone production efficiency is higher for NOx emitted from power plants at nighttime and dispersed horizontally overnight. Ozone production from this dispersed power plant NOx would occur at the same rate per NOx as for emissions from urban sources.
For further information: Southern Oxidants Study
Summary: Ozone, NOy, NOz and peroxides for the Nashville urban plume and surrounding regions: model results and measurements. This includes analysis of NOx-VOC indicators, ozone production efficiency, and how true ozone production efficiency compares with the measured correlation between O3 and NOz.
For further information: Southern Oxidants Study
Summary: Summary: A 3-d global model (GRANTOUR) is used to simulate transport and photochemistry associated with ozone across the North Atlantic. Model instances of episodic transport are described. A positive correlation between O3 and CO and between O3 and NOy is predicted for sites over the North Atlantic Ocean that are affected by ozone transport. The slope of O3 versus CO decreases, and the slope of O3 versus NOy increases, as the distance from emission sources increases. A much steeper slope between O3 and CO is found at sites in the North Atlantic that are not directly affected by episodic transport. This steep slope is due to the different conditions for air masses with different latitudinal origin, rather than to the rate of ozone production per primary emissions.
Summary: Summary: Results are presented to explain the observed tendency for ozone in polluted regions to increase with temperature. Measured O3 vs. temperature are shown for urban and rural sites around the U.S. Model calculations identify the impact of various factors. The O3-temperature correlation is attributed primarily to the temperature-dependent decomposition rate of PAN. At lower temperatures PAN becomes long-lived and removes both NOx and radicals from the atmosphere, lowering the rate of ozone formation. Ozone at the global scale is not expected to change significantly in direct response to an increase in average temperatures. An increased temperature actually causes a very slight decrease in tropospheric O3, because precursors are processed more rapidly in polluted regions, where ozone production efficiency per NOx is lower.
Summary: The first presentation of the concept of O3-NOx-VOC "indicators" and the chemistry that causes different values for indicator ratios for NOx-sensitive versus VOC-sensitive conditions.
Summary: Model-measurement comparisons suggest that ozone in Atlanta is primarily sensitive to NOx rather than VOC. Regulatory models using then-existing emission inventories suggested that ozone in Atlanta was primarily sensitive to VOC. These models sharply underestimated both isoprene (a biogenic hydrocarbon critical to ozone formation) and the "indicator ratio" O3/NOy in comparison with measurements. Models with different emission rates and meteorology predicted that ozone was sensitive to NOx rather than VOC. The NOx-sensitive and VOC-sensitive model scenarios both showed good agreement with measured ozone, but only the NOx-sensitive models also showed good agreement with measured NOy and with isoprene.