18 research outputs found
Evaluation of the Community Multiscale Air Quality Model for Simulating Winter Ozone Formation in the Uinta Basin
The Weather Research and Forecasting (WRF) and Community Multiscale Air Quality (CMAQ) models were used to simulate a 10 day high-ozone episode observed during the 2013 Uinta Basin Winter Ozone Study (UBWOS). The baseline model had a large negative bias when compared to ozone (O3) and volatile organic compound (VOC) measurements across the basin. Contrary to other wintertime Uinta Basin studies, predicted nitrogen oxides (NOx) were typically low compared to measurements. Increases to oil and gas VOC emissions resulted in O3 predictions closer to observations, and nighttime O3 improved when reducing the deposition velocity for all chemical species. Vertical structures of these pollutants were similar to observations on multiple days. However, the predicted surface layer VOC mixing ratios were generally found to be underestimated during the day and overestimated at night. While temperature profiles compared well to observations, WRF was found to have a warm temperature bias and too low nighttime mixing heights. Analyses of more realistic snow heat capacity in WRF to account for the warm bias and vertical mixing resulted in improved temperature profiles, although the improved temperature profiles seldom resulted in improved O3 profiles. While additional work is needed to investigate meteorological impacts, results suggest that the uncertainty in the oil and gas emissions contributes more to the underestimation of O3. Further, model adjustments based on a single site may not be suitable across all sites within the basin
Modeling NH4NO3 over the San Joaquin Valley During the 2013 DISCOVER-AQ Campaign
The San Joaquin Valley (SJV) of California experiences high concentrations of PM2.5 (particulate matter with aerodynamic diameter 2.5 m) during episodes of meteorological stagnation in winter. Modeling PM2.5 NH4NO3 during these episodes is challenging because it involves simulating meteorology in complex terrain under low wind speed and vertically stratified conditions, representing complex pollutant emissions distributions, and simulating daytime and nighttime chemistry that can be influenced by the mixing of urban and rural air masses. A rich dataset of observations related to NH4NO3 formation was acquired during multiple periods of elevated NH4NO3 during the DISCOVER-AQ (Deriving Information on Surface Conditions from Column and Vertically Resolved Observations Relevant to Air Quality) field campaign in SJV in January and February 2013. Here, NH4NO3 is simulated during the SJV DISCOVER-AQ study period with the Community Multiscale Air Quality (CMAQ) model version 5.1, predictions are evaluated with the DISCOVER-AQ dataset, and process analysis modeling is used to quantify HNO3 production rates. Simulated NO3- generally agrees well with routine monitoring of 24-h average NO3-, but comparisons with hourly average NO3- measurements in Fresno revealed differences at higher time resolution. Predictions of gas-particle partitioning of total nitrate (HNO3 + NO3-) and NHx (NH3 + NH4+) generally agreed well with measurements in Fresno, although partitioning of total nitrate to HNO3 was sometimes overestimated at low relative humidity in afternoon. Gas-particle partitioning results indicate that NH4NO3 formation is limited by HNO3 availability in both the model and ambient. NH3 mixing ratios are underestimated, particularly in areas with large agricultural activity, and the spatial allocation of NH3 emissions could benefit from additional work, especially near Hanford. HNO3 production via daytime and nighttime pathways is reasonably consistent with the conceptual model of NH4NO3 formation in SJV, and production peaked aloft between about 160 and 240 m in the model. During a period of elevated NH4NO3, the model predicted that the OH + NO2 pathway contributed 46% to total HNO3 production in SJV and the N2O5 heterogeneous hydrolysis pathway contributed 54%. The relative importance of the OH + NO2 pathway for HNO3 production is predicted to increase as NOx emissions decrease
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Assessment Of Nitrogen Deposition: Modeling And Habitat Assessment
This study reviewed four widely used air quality models and concluded that simple Gaussian dispersion models, ISCST3 and AERMOD, are not suitable for modeling N deposition because they fail to represent chemical and phase transformations of nitrogen oxides (NOX) and ammonia (NH3) emissions. It reviewed two models that do represent chemical speciation and formation of aerosols: CALPUFF and the Community Multiscale Air Quality (CMAQ) model. CALPUFF is a trajectory model that adopts several simplifications that raise important concerns regarding its accuracy. CMAQ is a photochemical grid model and includes state-of-the-art science algorithms. The study performed simulations using each model to assess nitrogen (N) deposition from a power plant. CALPUFF predicted slightly lower deposition rates than did CMAQ, and the spatial features of CALPUFF were poorly resolved. For CMAQ, the study simulated calendar year 2002 to develop baseline N deposition estimates throughout California, and then performed a sensitivity simulation with the new power plant to calculate the change in N deposition. The CMAQ predicted higher deposition rates than the CALPUFF model and provided finer spatial resolution. However, the CMAQ model exhibited numerical noise. The authors recommend exploring other photochemical grid models that might have less numerical noise than CMAQ. Perhaps the most significant outcome of this work is the CMAQ model-simulated baseline annual N deposition for a 4-kilometer resolution grid on a domain that includes much of California. These data have also been converted into an ASCII format that can be readily imported into ArcGIS or other GIS software and used in future ecosystem studies of the effects of N deposition and soil nitrification
Investigation Of Voc Reactivity Effects
The CAMx grid model was used to assess ozone reactivity effects for Carbon Bond (CB4) VOC species and ethane using the CRC-NARSTO database for the July 12-15, 1995 NARSTO-NE episode in the Eastern United States. The ozone sensitivities to emissions changes in NO x , total VOCs, total anthropogenic VOCs, CO, ethane and the 8 CB4 species used to represent major anthropogenic VOC emissions were calculated using DDM sensitivity analysis. A number of different ozone reactivity scales were derived using various methods to quantify the ozone impacts of the VOC species on the regional scale. These were based on effects of VOCs on daily maximum 1-hour averages in four different episode days, on effects on daily maximum 8-hour averages in three different episode days, and on using six different methods or metrics to derive regional reactivity scales from the varying impacts throughout the modeling domain. The results were compared to relative reactivities calculated with the same chemical mechanism in an EKMA box model used previously to derive the Carter reactivity scales