A photochemical model based on a scaling analysis of ozone photochemistry
Creators: name: Ainslie, Bruce
Publisher: name: University of British Columbia
Year: 2009

description: A scaling-level model of a photochemical mechanism has been developed and integrated into an air quality model used to study ozone formation in an urban environment. A scaling analysis was used to capture the internal workings of a photochemical mechanism using the OZIPR trajectory model to simulate a smog chamber for a wide range of precursor concentrations and a variety of environmental conditions. The Buckingham P i method of dimensional analysis was used to express the relevant variables in terms of dimensionless groups. These grouping show maximum ozone, initial NOx and initial VOC concentrations to be made non-dimensional by the average NO₂ photolysis rate (j[sub av]) and the rate constant for the NO-O3 titration reaction ([sup k]NO); temperature by the NO-O₃ activation energy (E/R) and time by the cumulative NO₂ photolysis rate (J). The analysis shows dimensionless maximum ozone concentration can be described by a product of powers of dimensionless initial NOx concentration, dimensionless temperature (θ(T)) and a similarity curve (f) directly dependent on the ratio of initial VOC to NOx concentration (R) and internally dependent on the cumulative NO₂ photolysis rate: [Chemical Equation Diagram] When Weibull transformed, the dimensionless model output cluster onto two line segments. This is interpreted as-a break in ..the scaling and can be understood,in-terms of a change in governing feedback mechanisms separating low- and high-NOx chemistry regimes. The similarity relationship can be modeled by two Weibull distributions using four parameters: two describing the slopes of the line segments (01,02) and two giving the location of their intersection (β, λ): [Chemical Equation Diagram] A fifth parameter (γ) is used to normalize the model output. The most important parameter, β, the VOC to NOx ratio at the scaling break, defines a characteristic process scale for ozone photochemistry. The scaling analysis, similarity curve and parameterization appear to be independent of the details of the chemical mechanism, hold for a variety of VOC species and mixtures and are applicable over a wide range of VOC and NOx concentrations. The similarity relationship is used to generalize ozone-precursor relationships in terms of four rules governing ozone production (P(O₃)), to quantify NOx-inhibition and define isopleth slope. The scaling framework is used to study VOC reactivity, explore the scaling properties of a simple reaction mechanism and collapse a wide range of smog chamber measurements onto a single similarity curve. To complement the scaling analysis, a meteorological model and an emissions inventory were developed. These were incorporated into an air quality model used to explore the sensitivity of a regional ozone plume to environmental conditions, and precursor .concentrations. The air quality model consisted of a series of box models being advected by the mean wind, for a single day, where photochemistry of the precursors emissions was modeled using the similarity relationships developed from the scaling analysis. The chosen domain was the Lower Fraser Valley B.C., a complex coastal region that experiences moderate ozone episodes during summertime fair-weather conditions. Emission fields were developed using published emission totals, four land-use categories and generic temporal emissions curves and were found to be comparable with fields based on more detailed inventories. Wind observations (speed and direction), from 53 stations, on a typical episode day, were interpolated to produce hourly wind fields. Mixing depths were determined using a simple slab model incorporating the interpolated wind fields and measured heat fluxes. The most problematic aspect of the model was determining the effects of pollutant build up in the boundary layer, prior to the modeling day. This was handled by emitting precursors into the boundary layer and advecting them, without chemical reactions, until steady state concentrations were reached. These were dependent on the choice of background concentrations used to initialize the pre-conditioning scheme and were set so resulting boundary layer NOx and VOC concentrations were in agreement with the limited available data and peak ozone concentrations were typical of recent episodes. In departure from previous modeling studies, model validation was not through point by point analysis of model output and observations but through high level comparison of model sensitivity with a range of modeling techniques and observations. The model appears to capture ozone sensitivity to meteorological conditions and precursor concentrations; justifying its use as a screening tool. The model suggests: the region to be VOC limited; projected emissions reductions may not improve present episodic ozone concentrations; larger than anticipated reductions in NOx emissions, without equivalent additional VOC reductions, could increase episodic concentrations and future emissions reductions, stemming from TIER 2 LDV standards, which target NOx emissions to a greater extent than VOC emissions, may not result in appreciable changes in episodic ozone concentrations. These conclusions are intended to guide comprehensive modeling studies.