Jana B. Milford

The sizes of particulate trace elements in the atmosphere--a review.

Size distributions of 38 trace elements reported in the literature over the past two decades have been summarized. Those elements with aerodynamic mass median diameter less than 2 μm generally have bimodal distributions: the dominant peak occurs in the range 0.5-1 μm, with a smaller peak at about 3-5 μm. Elements with larger mass median diameter have a single peak, with relatively little mass below 1 μm. Overall, the individual distributions of many of the elements are surprisingly consistent, despite different sampling locations, time periods, and experimental methods.

Total reactive nitrogen (NO y ) as an indicator of the sensitivity of ozone to reductions in hydrocarbon and NO x emissions

For areas in the United States not meeting the federal air quality standard for ozone, an issue of continuing controversy is the emphasis to be placed on controlling nitrogen oxides (NOx) in addition to emissions of reactive organic gases (ROG). To assess conditions under which ROG or NOx controls would be most effective, we have analyzed predictions from four studies that represent different locations and meteorological conditions, distinct chemical inputs, e.g., with or without significant biogenic emissions, and different air quality models. A consistent association is found between the sensitivity of ozone to reductions in ROG versus NOx emissions and the simulated total reactive nitrogen (NOy) at the time and place of peak ozone. In the studies examined, ozone was predicted to be reduced most effectively by ROG controls at locations where NOy concentrations exceeded a threshhold value falling in the range of 10 to 25 ppb, whereas NOx controls were predicted to be more effective where NOy concentrations were below that threshhold. The NOy level explains much of the difference in ozone sensitivity at different locations and provides a basis for comparison of predicted sensitivity from different models. In contrast, the morning concentration ratio of ROG to NOx that has been used in the past is a less reliable indicator of O3 sensitivity. Measurement of NOy concentrations along with ozone would assist in empirical testing of model predictions of responses to emission reductions.

Source apportionment of exposures to volatile organic compounds. I. Evaluation of receptor models using simulated exposure data

Four receptor-oriented source apportionment models were evaluated by applying them to simulated personal exposure data for select volatile organic compounds (VOCs) that were generated by Monte Carlo sampling from known source contributions and profiles. The exposure sources modeled are environmental tobacco smoke, paint emissions, cleaning and/or pesticide products, gasoline vapors, automobile exhaust, and wastewater treatment plant emissions. The receptor models analyzed are chemical mass balance, principal component analysis/absolute principal component scores, positive matrix factorization (PMF), and graphical ratio analysis for composition estimates/source apportionment by factors with explicit restriction, incorporated in the UNMIX model. All models identified only the major contributors to total exposure concentrations. PMF extracted factor profiles that most closely represented the major sources used to generate the simulated data. None of the models were able to distinguish between sources with similar chemical profiles. Sources that contributed <5% to the average total VOC exposure were not identified.

Intercomparison of the gas-phase chemistry in several chemistry and transport models

An intercomparison of nine chemical mechanisms (e.g. ADOM, CBM-IV, EMEP, RADM2) as used by 12 contributing groups was conducted. The results for three scenarios are presented covering remote situations with a net O3 loss of around 2.7 ppb (LAND and FREE) and a moderately polluted situation with O3 formation of around 100 ppb (PLUME1) over a 5 day simulation period. The overall tendencies (i.e. the total net production/loss over 5 days) for O3 show a r.m.s. error of 38, 15 and 16%; for H2O2 the errors are 76, 23 and 30% (for LAND, FREE, PLUME1). In terms of ozone production in PLUME1, the most productive mechanisms are EMEP and IVL, the RADM-type mechanisms lie in the mid-range and the CBM-IV type mechanisms fall at the bottom of the range. The differences in H2O2 can partly be explained by an incorrect use of the HO2 + HO2 rate constant and by differences in the treatment of the peroxy radical interactions. In the PLUME1 case the r.m.s. error of the PAN tendency was found to be 29%. Differences between mechanisms for the HO radical are 10, 15 and 19% and for the NO3 radical 35, 16 and 40% (for LAND, FREE, PLUME1) in terms of the r.m.s. error of the results for a 12 h time period centred around the last noon (HO), respectively, a 8 h time period centred around the last midnight (NO3) of simulation. Especially for NO3 some differences are due to different numerical treatment of photolytic processes in the models. Large differences between mechanisms are observed for higher organic peroxides and higher aldehydes with a r.m.s. error of around 50% for the final concentration in PLUME1. The protocol of the intercomparison is given in the appendix, so that the comparison could be repeated for the purpose of mechanism development and sensitivity studies.

Urban ozone control and atmospheric reactivity of organic gases

Control strategies for urban ozone traditionally have been based on mass reductions in volatile organic compounds (VOCs). Studies show, however, that some organic gas species (such as alkanes and alcohols) form an order of magnitude less ozone than equal mass emissions of others (such as alkenes and aldehydes). Chemically detailed photochemical models are used to assess uncertainty and variability in reactivity quantification. VOC control strategies based on relative reactivity appear to be robust with respect to nationwide variations in environmental conditions and uncertainties in the atmospheric chemistry. Control of selective organic gas species on the basis of reactivity can offer cost savings over traditional strategies.

First-order sensitivity and uncertainty analysis for a regional-scale gas-phase chemical mechanism

First-order sensitivity and uncertainty analysis methods have been applied to the RADM2 mechanism, the second generation gas-phase chemical mechanism included in the Regional Acid Deposition Model. The direct decoupled method (DDM) was used to evaluate the local sensitivity of product concentrations of O3, HCHO, H2O2, PAN, and HNO3 to values of 157 rate constants and 126 stoichiometric coefficients. The sensitivity analysis results were combined with estimates of the uncertainty in each parameter in the mechanism to produce a local measure of its contribution to the uncertainty in the outputs. Several different sets of simulation conditions were used, representing summertime surface conditions for urban and nonurban areas. The analysis identified the most influential rate parameters to be those for PAN chemistry, formation of HNO3, and photolysis of HCHO, NO2, O3, and the unknown product (DCB) of aromatics oxidation. Rate parameters for the conversion of NO to NO2, such as O3 + NO, HO2 + NO and organic radical + NO, and the product yields of organic peroxy radical (XYLP) in the reaction of XYL + HO (XYL represents xylene and more reactive aromatics) and DCB in the reaction XYLP + NO are also relatively influential.

Climate Change and Western Public Lands: a Survey of U.S. Federal Land Managers on the Status of Adaptation Efforts

Climate change and its associated consequences pose an increasing risk to public lands in the western United States. High-level mandates currently require federal agencies to begin planning for adaptation, but the extent to which these mandates have resulted in policies being implemented that affect on the ground practices is unclear. To examine the status of adaptation efforts, we conducted an original survey and semistructured interviews with land managers from the four major federal land management agencies in the U.S. states of Colorado, Utah, and Wyoming. The survey was designed to examine current planning for adaptation on public lands and how it differs from prior planning, the major challenges facing land managers in this region, the major barriers preventing managers from planning for adaptation, and the major hurdles associated with implementing adaptation plans. Our results show that some adaptation planning is currently taking place, but that few adaptation projects have made it to the implementation phase. Overall, respondents considered lack of information at relevant scales, budget constraints, lack of specific agency direction, and lack of useful information to be the most common barriers to adaption planning. Budget constraints, lack of perceived importance to the public, and lack of public awareness or demand to take action were reported to be the biggest hurdles to implementation of adaptation projects. Agencies showed differing levels of adaptation activity, and reported different barriers to adaptation and hurdles to implementation. Reasons for the differences and implications for future research and policy are discussed.

Global uncertainty analysis of a regional‐scale gas‐phase chemical mechanism

A global uncertainty analysis of the Regional Acid Deposition Model version 2 chemical mechanism was conducted to estimate the effect of uncertainties in rate parameters and product yields on predicted concentrations of ozone and other secondary pollutants. Uncertainties were also estimated for changes in ozone due to 25% reductions in reactive organic gases (ROG), nitrogen oxides (NOx) or both. The analyses were performed for single-day box model simulations of summertime smog episodes, with uncertainties propagated using a Latin hypercube sampling scheme. The resulting uncertainties in peak ozone concentrations range from about 20 to 50% (±1σ relative to the mean). Uncertainties in predicted concentrations range from 15 to 30% for HNO3, 20 to 30% for HCHO, and 40 to 70% for peroxyacetyl nitrate (PAN). Except with very low concentrations, uncertainties in H2O2 range from 30 to 45%. For the cases studied the choice between ROG or NOx reductions as most effective for lowering ozone concentrations is insensitive to uncertainties in chemical parameters. Product concentrations are most strongly affected by uncertainties in rate parameters for PAN chemistry, HNO3 formation, aromatics oxidation, the reaction of HO2 + NO, and photolysis of HCHO, NO2, and O3, and uncertainties in peroxy radical yields from aromatics and olefins oxidation.

The sizes of particulate sulfate and nitrate in the atmosphere - a review

Size distributions for airborne mass of sulfate reported in the literature show a submicron peak for nearly all the locations at which data were obtained. Data from continental sites have mass median aerodynamic diameters (MMD) averaging 0.52/μm. Data from marine sites show appreciable sulfate on supermicron as well as submicron particles, with an average MMD of 2.3/μm. When used in dry deposition models, the size distribution data for continental areas yield average deposition velocities of 0.1-0.2 cm s-1 for vegetation, snow and water. The size data for marine areas yield average deposition velocities of 0.5-0.6 cm s-1 for water. In contrast to the data for sulfate, the quality of the size distribution data which have been reported for aerosol nitrate is uncertain due to potential sampling interferences.

Source apportionment of exposures to volatile organic compounds: II. Application of receptor models to TEAM study data

Abstract Four receptor-oriented source apportionment models were applied to personal exposure measurements for toxic volatile organic compounds (VOCs). The measurements are from the total exposure assessment methodology studies conducted from 1980 to 1984 in New Jersey (NJ) and California (CA) and the 1987–1990 CA Indoor Exposure study. The receptor models applied are the Chemical Mass Balance model, Principal Component Analysis/Absolute Principal Component Scores, Positive Matrix Factorization, and Graphical Ratio Analysis for Composition Estimates/Source Apportionment by Factors with Explicit Restriction. Major sources of personal exposure to toxic VOCs appear to have been aromatic sources resembling automobile exhaust, gasoline vapor, or environmental tobacco smoke, and a 1,1,1-trichloroethane-dominated source that may be associated with solvent or pesticide use. Drycleaning chemicals, deodorizers or mothballs, and building materials or carpet emissions also appear to have been significant sources of exposure. Source apportionment results from the four models agreed reasonably well for the NJ data. The performance of the models was generally poorer for the CA data, and the corresponding source apportionment results were less consistent across the models.