LV PERF

Sensitivity Analysis of Weekday/Weekend Differences in Photochemical Air Pollution: 

Sensitivity Analysis of Weekday/Weekend Differences in Photochemical Air Pollution Laurent Vuilleumier, Lawrence Berkeley National Laboratory ﴀ

Collaborators: 

Collaborators Robert A. Harley University of California, Berkeley Nancy J. Brown Lawrence Berkeley National Laboratory

Presentation Outline: 

Presentation Outline Motivations The weekend effect: What is it? Why choosing it? What can sensitivity analysis bring? Model description Emissions. Results Is it possible to model the weekend effect? Using sensitivity analysis to understand the weekend effect. Accuracy of sensitivity analysis prediction.

Motivations: 

Motivations Despite lower emissions and concentration of NOx, VOC (volatile organic compound) and CO on weekend, average ozone concentrations are higher on weekend than on weekdays in many California urban areas. Provide the detailed understanding of weekend effect needed for designing strategies to attain National Ambient Air Quality Standard for ozone. Demonstrate the usefulness of sensitivity analysis for obtaining detailed understanding of model mechanism. Develop and evaluate sensitivity analysis techniques to be used in model assessment and uncertainty analysis.

The Model: 

The Model Lagrangian box model Equations for both the pollutant concentrations and their sensitivities to the model input parameters are solved in parallel. Model governing equation:

Sensitivities: 

Sensitivities Analytical derivation of sensitivities: The model governing equation is derived with respect to input parameters under the assumption that they are constant within a simulation.

Solution with a Green’s function method: 

The solution of the sensitivity equation is given by: The Green’s function is computed as follows: Solution with a Green’s function method

Emissions: 

Emissions Vehicle and stationary source emissions are treated separately. The pollutant concentration changes due to emissions are computed with: Emphasis was put on vehicle emissions when analyzing weekday/weekend differences. Emission fluxes (g m-2 day-1) Emission time profile Mixing height

Emission Fluxes: 

Emission Fluxes Alameda County, CA, emission fluxes are used. Vehicle emissions fluxes for weekdays, Saturday and Sunday are different. For stationary sources, the same value is used for weekdays, Saturday and Sunday. A published emission inventory for California is used.

Vehicle Emission Time Profile: 

Vehicle Emission Time Profile Time profiles are derived from traffic intensity measurements at 3 sites. The sum of two gaussians is fitted to the data. The average site-to-site variability is used as a measure of the error.

Site to site variability: 

Site to site variability Data from the 3 sites were averaged. The residuals between the individual site measurements and the averages were used to estimate the error.

Chemical mechanism: 

Chemical mechanism Used two published mechanisms: The Carbon Bond IV mechanism (CB4) and the condensed version of the surrogate species mechanism of Lurmann, Carter and Coyner (LCC). The chemical mechanism is the only part that allows influence of one species on another: Parameter vector Concentration vector

Tropospheric ozone production: 

Tropospheric ozone production

Dilution and Mixing Height: 

Dilution and Mixing Height Due to surface air heating, the height of the well-mixed layer above ground varies during the day. Resulting in dilution, entrainment of pollutants aloft, and changing the volume of air in which the pollutants are emitted. A mixing height profile was produced with data from acoustic sounding in Claremont, CA.

Results: 

Results Simulations conducted with weekend conditions resulted in ozone concentrations about 20% higher than simulations with weekday conditions. A

Ozone concentration (ppm) vs. time (hour) for weekdays, Saturday and Sunday: 

Ozone concentration (ppm) vs. time (hour) for weekdays, Saturday and Sunday Environmental Energy Technologies Division ﴀ 16

Results: 

Results Ozone sensitivities to emission fluxes and initial conditions are typical of a VOC-limited system. B

Ozone sensitivities (ppm) to emissions and initial concentrations vs. time (hour): 

Ozone sensitivities (ppm) to emissions and initial concentrations vs. time (hour) Environmental Energy Technologies Division ﴀ 18

Results: 

Results Sensitivity analysis estimation of the differences in peak ozone concentrations between weekdays and weekend are of correct sign and correct order-of-magnitude. Sensitivity analysis shows that the decrease in NOx emissions on weekends is the main cause for the ozone concentration increase. C

Sensitivity analysis predictions for weekday/Saturday differences in [O3]: 

Sensitivity analysis predictions for weekday/Saturday differences in [O3] Environmental Energy Technologies Division ﴀ 20

Sensitivity analysis predictions for weekday/Sunday differences in [O3]: 

Sensitivity analysis predictions for weekday/Sunday differences in [O3] Environmental Energy Technologies Division ﴀ 21

Validity of sensitivity analysis approximations: 

Validity of sensitivity analysis approximations Why is the sensitivity analysis approximation for the Sunday ozone concentration change  60%higher than the real prediction? Is it possible to resolve the discrepancy by computing higher orders?

First- and second-order sensitivity analysis approximations: 

First- and second-order sensitivity analysis approximations First-order: Second-order:

Test of sensitivity approximations for [O3] when ESNO value is changed: 

Test of sensitivity approximations for [O3] when ESNO value is changed Environmental Energy Technologies Division ﴀ 24

To be or not to be VOC-limited?: 

To be or not to be VOC-limited? A change in model mechanism is often the reason for sensitivity analysis inaccuracies. The VOC to NOx ratio suggest that the model is less likely to be in VOC-limited mode on weekend days than on weekdays. Green’s function analysis can reveal the mechanistic information.

Green’s function relating [O3](t) to [NO] (t’) for weekdays and Sunday: 

Green’s function relating [O3](t) to [NO] (t’) for weekdays and Sunday Environmental Energy Technologies Division ﴀ 26

Slide27: 

Conclusions It is possible to reproduce the weekend effect with a simulation. Sensitivity analysis showed the reason for these differences. Green’s function analysis is a powerful tool to obtain a detailed picture of the model mechanism.