Gail S. Tonnesen

Influence of increased isoprene emissions on regional ozone modeling

The role of biogenic hydrocarbons on ozone modeling has been a controversial is- sue since the 1970s. In recent years, changes in biogenic emission algorithms have resulted in large increases in estimated isoprene emissions. This paper describes a recent algorithm, the second generation of the Biogenic Emissions Inventory System (BEIS2). A sensitivity analysis is performed with the Regional Acid Deposition Model (RADM) to examine how increased isoprene emissions generated with BEIS2 can influence the modeling of elevated ozone concentrations and the response of ozone to changes to volatile organic compound (VOC) and nitrogen oxide (NOx) emissions across much of eastern North America. In- creased isoprene emissions are found to produce a predicted shift in elevated ozone concen- trations from VOC sensitivity to NOx sensitivity over many areas of eastern North America. Isoprene concentrations measured near Scotia, Pennsylvania, during the summer of 1988 are compared with RADM estimates of isoprene and provide support for the veracity of the higher isoprene emissions in BEIS2, which are about a factor of 5 higher than BEIS 1 during warm, sunny conditions.

Preliminary evaluation of the Community Multiscale Air Quality model for 2002 over the Southeastern United States.

Abstract The Visibility Improvement State and Tribal Association of the Southeast (VISTAS) is one of five Regional Planning Organizations that is charged with the management of haze, visibility, and other regional air quality issues in the United States. The VISTAS Phase I work effort modeled three episodes (January 2002, July 1999, and July 2001) to identify the optimal model configuration(s) to be used for the 2002 annual modeling in Phase II. Using model configurations recommended in the Phase I analysis, 2002 annual meteorological (Mesoscale Meterological Model [MM5]), emissions (Sparse Matrix Operator Kernal Emissions [SMOKE]), and air quality (Community Multiscale Air Quality [CMAQ]) simulations were performed on a 36‐km grid covering the continental United States and a 12‐km grid covering the Eastern United States. Model estimates were then compared against observations. This paper presents the results of the preliminary CMAQ model performance evaluation for the initial 2002 annual base case simulation. Model performance is presented for the Eastern United States using speciated fine particle concentration and wet deposition measurements from several monitoring networks. Initial results indicate fairly good performance for sulfate with fractional bias values generally within ±20%. Nitrate is overestimated in the winter by approximately +50% and underestimated in the summer by more than −100%. Organic carbon exhibits a large summer underestimation bias of approximately −100% with much improved performance seen in the winter with a bias near zero. Performance for elemental carbon is reasonable with fractional bias values within ±40%. Other fine particulate (soil) and coarse particular matter exhibit large (80–150%) overestimation in the winter but improved performance in the summer. The preliminary 2002 CMAQ runs identified several areas of enhancements to improve model performance, including revised temporal allocation factors for ammonia emissions to improve nitrate performance and addressing missing processes in the secondary organic aerosol module to improve OC performance.

Analysis of radical propagation efficiency to assess ozone sensitivity to hydrocarbons and NO x : 2. Long-lived species as indicators of ozone concentration sensitivity

We used an analysis of radical propagation efficiency and OH chain length in a simple trajectory model to propose combinations of long-lived species that distinguish conditions in which O3 concentration ([O3]) is NOx-limited and radical-limited. We further examined these indicators in a three-dimensional grid model. We proposed several new indicators including [H2O2]/([O3]+[NO2]), [O3]/[NOx], and a measure using the OH rate constant weighted concentrations of NO2 and hydrocarbons. Our analysis also supports the use of several indicators previously proposed by other researchers, including [O3]/[HNO3] and [H2O2]/[HNO3]. We found that [HCHO]/[NO2] was more useful than the previously proposed [HCHO]/[NOy]. We found that the indicators easily distinguished extremely NOx-limited or extremely radical-limited regimes but did not reliably distinguish conditions closer to the transition between these two regimes. We propose that a combined analysis using photochemical model simulations and a large set of indicators of both [O3] sensitivity and local odd oxygen production (P(Ox)) sensitivity to VOC and NOx provides the most complete and useful description of [O3] sensitivity. Time series of the indicators, at least from mid-morning to late afternoon, provide useful information about the evolution of [O3] sensitivity during the day. Values of the indicators change depending on the [O3] level due in part to the effects of miscellaneous OH and HO2 termination reactions and to the effects of the composition of the HC mixture on P(Ox). Further evaluation of these indicators using modeling studies, measurements, and test cases with NOx or VOC emissions changes are needed to determine how reliably they distinguish NOx-and radical-limited conditions.

Development of a tagged species source apportionment algorithm to characterize three‐dimensional transport and transformation of precursors and secondary pollutants

[1] We developed a new tagged species source apportionment (TSSA) algorithm for tracking the direct mass contributions of selected emissions sources to the formation of particulates such as aerosol sulfate, nitrate, ammonium, elemental carbon, and secondary organic aerosols in the U.S. EPA Community Multiscale Air Quality Model (CMAQ). The focus of this paper is on sulfate, nitrate, and elemental carbon. The objective of the TSSA algorithm is to provide useful results in modeling studies for identifying important emissions categories and identifying possible emissions reduction strategies to attain particulate matter (PM) air quality goals. TSSA differs from model sensitivity approaches because it tracks direct mass contributions from specific emissions sources to the total PM concentration at selected receptor sites, while results from sensitivity approaches are affected by nonlinear chemistry that can change the concentration of sulfate, nitrate, and organic carbon secondary particulates. We evaluated the algorithm by comparing CMAQ/TSSA results with results from zero-out CMAQ sensitivity simulations. As expected, TSSA results were almost identical to the CMAQ sensitivity results for chemical species that do not undergo nonlinear chemical reactions. For chemical species with nonlinear chemical reaction, the TSSA results are expected to differ from model sensitivity results, but for small emissions changes the results are similar in the two approaches. We also compared CMAQ/TSSA to CAMx/PSAT. Because there are significant differences in the CMAQ and CAMx model predicted concentrations, the source attribution results differed for TSSA and PSAT; however, the rank order of emissions sources were similar for the two approaches at most receptor sites.

Diagnostic evaluation of numerical air quality models with specialized ambient observations: testing the Community Multiscale Air Quality modeling system (CMAQ) at selected SOS 95 ground sites

Abstract Three probes for diagnosing photochemical dynamics are presented and applied to specialized ambient surface-level observations and to a numerical photochemical model to better understand rates of production and other process information in the atmosphere and in the model. However, care must be taken to ensure that rate and process information is not confounded by inappropriate averaging over these diurnally changing photochemical dynamics. One probe, the [O3] response surface probe [O3]/[NOX], is used here as a chemical filter to select NOX-limited hours in the observations and the simulations. Other probes used here are the fraction NOZ/NOY, a measure of chemical aging, and a measure of the production efficiency of O3 per NOX converted, [O3] to [NOZ]. The key ambient measurements for all three probes are accurate [NO2] and a reliable estimate of total NOY. Good agreement is shown between models and observations in cases where local photochemical production dominates and where model emissions inputs are thought to be mostly complete. We interpret this agreement to mean that the photochemical processing in CMAQ is substantially similar to that in the atmosphere. More importantly, we see that the three probes provide consistent information about photochemical processing, especially when used together.