Single-source impact analysis using three-dimensional air quality models

Isolating the effects of an individual emissions source on secondary air pollutants such as ozone and some components of particulate matter must incorporate complex nonlinear processes, be sensitive to small emissions perturbations, and account for impacts that may occur hundreds of kilometers away. The ability to evaluate these impacts is becoming increasingly important for efficient air quality management. Here, as part of a recent compliance enforcement action for a violation of the Clean Air Act and as an evaluation of ozone response to single-source emissions plumes, two three-dimensional regional photochemical air quality models are used to assess the impact on ozone from approximately 2000 to 3000 excess t/month of nitrogen oxides emitted from a single power plant in Ohio. Periods in May, July, and August are evaluated. Two sensitivity methods are applied: the "brute-force" (B-F) method and the decoupled direct method (DDM). Using DDM, maximum 1-hr averaged ozone concentrations are found to increase by up to 1.8, 1.3, and 2.2 ppbv during May, July, and August episodes, respectively, and concentration increases greater than 0.5 ppbv occur in Ohio, Pennsylvania, Maryland, New York, West Virginia, Virginia, and North and South Carolina. B-F results for the August episode show a maximum 1-hr averaged ozone concentration increase of 2.3 ppbv. Significant localized decreases are also simulated, with a maximum of 3.6 ppbv in Ohio during the August episode and decreases of 0.50 ppbv and greater in Ohio, Pennsylvania, Maryland, West Virginia, and Virginia. Maximum increases are compared with maximum decreases for the August period using second-order DDM and are found, in aggregate, to be greater in magnitude by 42%. When evaluated during hours when ozone concentrations exceed 0.060 ppm, the maximum increases in ozone are higher than decreases by 82%. The spatial extent of ozone increase in both cases is about triple that of reduction.     

Determination of the Absolute Rate of Solar Photolysis of NO2

Improvements made on the direct NO₂ photolysis actinometer developed by J. O. Jackson have produced more precise data. A plot of measured values of the photolysis rate J₁ vs Eppley uv photometer readings produces a curved rather than the straight line correlation previously reported. This curvature arises from the Eppley and NO₂ absorption spectrum overlap, backround surface albedoes, the Ep-pley's cosine response and inherent errors in the chemical equation used. New J₁ measurement vs Eppley data is shown, and a procedure for calculating instantaneous J-i values from an Eppley output is suggested.     

An observation-based model for analyzing ozone precursor relationships in the urban atmosphere

An Observation-Based Model (OBM) is described, which uses in-situ atmospheric observations to determine the sensitivity of ozone concentrations in an urban atmosphere to changes in the emissions of ozone precursors (i.e., volatile organic compounds and nitrogen oxides). The model is formulated following the concept of Relative Incremental Reactivity (RIR) developed by Carter and Atkinson. In the OBM, however, observed concentrations rather than emission inventories are used to drive the photochemical simulations and thereby ensure that the calculations are carried out for the proper mix of nitrogen oxides and volatile organic compounds. From these calculations, a series of sensitivity factors, or RIRs, are inferred that can be used to (1) determine whether reducing emissions of nitrogen oxide or emissions of hydrocarbons would be most effective in abating ozone in a given urban area, and (2) identify the most critical subset of hydrocarbons present in an urban atmosphere causing ozone exceedances. Because the OBM is relatively easy and inexpensive to operate and makes use of data that are increasingly available, it can be used to analyze a wide array of ozone episodes and, thus, could prove to be a relatively cost-effective tool for the analysis of ozone precursor relationships in an urban atmosphere. On the other hand, because the OBM is diagnostic rather than prognostic, it cannot be used in a predictive mode to estimate exactly how much emission reduction is needed to reduce ozone concentrations. For this reason, the OBM should be viewed as a complement to, rather than a substitute for, more sophisticated gridded, emission-based models. To illustrate the characteristics of the OBM and to demonstrate its applicability, we first compare the results of the OBM to those obtained from a series of simulations of the Atlanta metropolitan area using the Urban Airshed Model (UAM), a three-dimensional Eulerian grid model. The OBM is then used to analyze a dataset obtained from the 1990 Atlanta Ozone Study, an EPA field sampling program conducted during the summer of 1990. Because of limitations and potential flaws in the 1990 Atlanta dataset, the results of this OBM analysis are largely illustrative rather than definitive. Nevertheless, a few important issues are elucidated by the analysis. These include (1) the importance of accounting for biogenic hydrocarbons produced from urban vegetation; (2) the potential flaw in using early-morning VOC-to-NOx ratios to infer whether ozone production is limited by VOC or NOx; (3) the critical need for high-sensitivity nitrogen oxide measurements to quantify the sub-ppbv concentrations of NO during the afternoon hours; and (4) the need to consider a number of individual ozone episodes in studying an urban atmosphere because of the possibility that the degree of VOC- and NOx-limitation may vary from one episode to another.     

黏性颗粒在滤料表面沉积的动态模拟及规律

为研究颗粒黏性对过滤除尘性能的影响,分析了颗粒沉积过程的受力情况,应用颗粒流计算软件PFC2D对颗粒在滤料表面的沉积行为进行了数值模拟。结果表明:颗粒形成粉尘层的过程中主要受到风流力、惯性力等压缩力和范德华力、滚动摩擦力偶矩等压缩阻力的作用,颗粒黏性的增加,会使颗粒间滚阻系数和摩擦系数显著增加;PFC可实现粉尘颗粒在滤料表面沉积过程的可视化,得到不同黏性颗粒在滤料表面的沉积形态,滚阻系数或摩擦系数越大,粉尘层孔隙率越大,过滤阻力越小,其中,摩擦系数的影响作用弱于滚阻系数。     

Short-term temporal variation in PM2.5 mass and chemical composition during the Atlanta Supersite Experiment, 1999

Measurements in urban Atlanta of transient aerosol events in which PM2.5 mass concentrations rapidly rise and fall over a period of 3-6 hr are reported. The data are based on new measurement techniques demonstrated at the U.S. Environmental Protection Agency (EPA) Atlanta Supersite Experiment in August 1999. These independent instruments for aerosol chemical speciation of NO3-, SO4(2-), NH4+, and organic and elemental carbon (OC and EC), reconstructed the observed hourly dry PM2.5 mass to within 20% or better. Data from the experiment indicated that transient PM2.5 events were ubiquitous in Atlanta and were typically characterized by a sudden increase of EC (soot) and OC in the early morning or SO4(2-) in the late afternoon. The frequent temporal decoupling of these events provides insights into their origins, suggesting mobile sources in metro Atlanta as the main contributor to early morning PM2.5 and more regionally located point SO2 sources for afternoon PM2.5 events. The transient events may also have health implications. New data suggest that short-term PM2.5 exposures may lead to adverse health effects. Standard integrated filter-based techniques used in PM2.5 compliance monitoring networks and in most past PM2.5 epidemiologic studies collect samples over 24-hr periods and thus are unable to capture these transient events. Moreover, health-effects studies that focus on daily PM2.5 mass alone cannot evaluate the health implications of the unique and variable chemical properties of these episodes.     

大气细粒子的快速捕集及化学成分在线分析方法研究

本文介绍了一种新型的大气细粒子快速捕集及其化学成分在线分析方法。该方法中大气细粒子与过饱和水蒸气绝热混和而增长成大粒子 ,并在气流的带动下惯性撞击到一垂直玻璃板上 ,一含有内标物的去离子水流自上而下将撞击到玻璃板上的粒子收集 ,收集液被直接输送到离子色谱、总有机碳分析仪 ,在线分析其中的化学成分。该方法每 12min可完成单个样品分析 ,每小时可连续采集分析 5个样品。离子色谱最低检测限约为 0 .0 5 μg/m3(NH+4 )到 0 .2 0 μg/m3(SO2 -4 ) ,不确定度约为 3 % ,TOC最低检测限为 5× 10 -5μg/m3,不确定度约为 3 %。真实的大气细粒子观测结果表明 ,该方法快速、简捷、灵敏度高 ,是实时监测大气中细粒子化学成分的有效工具