Bruce Ainslie

Spatiotemporal Trends in Episodic Ozone Pollution in the Lower Fraser Valley, British Columbia, in Relation to Mesoscale Atmospheric Circulation Patterns and Emissions

Abstract A cluster analysis of wind measurements from two meteorological stations in the Lower Fraser Valley, British Columbia, Canada, has been performed to identify mesoscale circulation regimes that are common to days on which ozone mixing ratios at one or more measuring stations in the region’s fixed monitoring network exceed the National Ambient Air Quality Objective of 82 ppb. The analysis, using 20 yr of data (1984–2003), identifies the following four regimes: two with morning winds at the coastal Vancouver International Airport (YVR) meteorological station from the northwest direction and two with southerly YVR morning winds. Not all exceedance days are associated with sea-breeze circulations, but days with southerly morning winds have a higher proportion of well-developed sea-breeze circulations. Composite synoptic patterns associated with each regime all show high pressure over the eastern Pacific Ocean with a thermal trough over Washington State and southwestern British Columbia. Composite ozon...

Models of Exposure for Use in Epidemiological Studies of Air Pollution Health Impacts

Observational epidemiological studies have had an important role in understanding the public health impacts of air pollution. In such studies, accurate assessment of exposure remains a major challenges, especially in studies involving large populations. Here we review state-of-the-art approaches to assessment of population exposure in epidemiological studies with a focus on approaches applied in the Border Air Quality Study (www.cher.ubc.ca⋏qs.htm). The strengths and limitations of these methods are discussed and future research needs identified.

Data Verification Tools for Minimizing Management Costs of Dense Air-Quality Monitoring Networks

Aiming at minimizing the costs, both of capital expenditure and maintenance, of an extensive air-quality measurement network, we present simple statistical methods that do not require extensive training data sets for automated real-time verification of the reliability of data delivered by a spatially dense hybrid network of both low-cost and reference ozone measurement instruments. Ozone is a pollutant that has a relatively smooth spatial spread over a large scale although there can be significant small-scale variations. We take advantage of these characteristics and demonstrate detection of instrument calibration drift within a few days using a rolling 72 h comparison of hourly averaged data from the test instrument with that from suitably defined proxies. We define the required characteristics of the proxy measurements by working from a definition of the network purpose and specification, in this case reliable determination of the proportion of hourly averaged ozone measurements that are above a threshold in any given day, and detection of calibration drift of greater than ±30% in slope or ±5 parts-per-billion in offset. By analyzing results of a study of an extensive deployment of low-cost instruments in the Lower Fraser Valley, we demonstrate that proxies can be established using land-use criteria and that simple statistical comparisons can identify low-cost instruments that are not stable and therefore need replacing. We propose that a minimal set of compliant reference instruments can be used to verify the reliability of data from a much more extensive network of low-cost devices.

Modelled Recirculation of Pollutants During Ozone Episodes in the Lower Fraser Valley, B. C.

Recirculation of pollutants is often invoked as a cause of degraded air quality episodes. In this modelling study, we investigated recirculation in order to explore its behaviour in the Lower Fraser Valley (LFV), British Columbia. HYSPLIT was used to produce trajectories from WRF output for seven severe episodes, covering the four main circulation regimes conducive to ozone episodes. Both internal and external recirculations within our domain of interest were observable, but they do not have the same frequency for all regimes.

Pattern-Based Evaluation of Coupled Meteorological and Air Quality Models

Abstract A novel pattern-based model evaluation technique is proposed and demonstrated for air quality models (AQMs) driven by meteorological model (MM) output. The evaluation technique is applied directly to the MM output; however, it is ultimately used to gauge the performance of the driven AQM. This evaluation of AQM performance based on MM performance is a major advance over traditional evaluation methods. First, meteorological cluster analysis is used to assign the days of a historical measurement period among a small number of weather patterns having distinct air quality characteristics. The clustering algorithm groups days sharing similar empirical orthogonal function (EOF) representations of their measurements. In this study, EOF analysis is used to extract space–time patterns in the surface wind field reflecting both synoptic and mesoscale influences. Second, simulated wind fields are classified among the determined weather patterns using the measurement-derived EOFs. For a given period, the leve...

A Simple Model for Pollution Dispersion in a Convective Boundary Layer

Abstract A simplified model for dispersion in a convective boundary layer is presented and is used to diagnose crosswind-integrated concentrations, ground-level concentrations, and vertical plume spread over flat terrain for various release heights. The model parameterizes the long-wavelength oscillation of the time-averaged plume centerline versus downwind distance under unstable conditions, using a simple sine wave. This wave is phase shifted to account for the influence of source height and is damped toward the mid–mixed layer to account for the well-mixed end state of convective dispersion. This model represents an improvement over a previous model in two ways. First, vertical dispersion about the oscillating time-averaged centerline is parameterized using a lognormal distribution instead of a Gaussian distribution so as to give better ground-level concentration. Second, to account for the addition of surface-layer shear-generated turbulence to a convective boundary layer, the wavelength of the time-a...

Scaling Analysis of Ozone Precursor Relationships

The importance of ozone production from mixtures of NOx and volatile organic compounds (VOCs) in the presence of sunlight was first recognized in the Los Angeles basin in the 1940s (Haagen-Smit, 1952). Since that time, a great deal of effort has been directed at understanding tropospheric photochemistry. Complicating this effort is the multitude of VOC species found in a polluted environment, and the extremely large number of ways these species can react — it is estimated that an explicit treatment of ozone formation would contain more than 20,000 reactions involving several thousand reactants and products (Dodge, 2000). To limit the number of species and reactions, different means of simplifying the chemistry have been employed which typically yield mechanisms with 30 to 50 species and between 80 and 200 reactions (Dodge, 2000). While this represents a significant reduction, the resulting mechanisms remain complex; solving the resulting chemical transformation require the largest fraction of computing time in a photochemical simulation.

A Source-Receptor Analysis of NOx Emissions in the Lower Fraser Valley, B. C.

Though the Lower Fraser Valley (LFV) has generally good air quality (AQ), ozone episodes may occur under a narrow set of synoptic and mesoscale conditions. These conditions give rise to complex flow systems, which further complicate the chemical sensitivity of the airshed. In this study, we use the MLDP0 dispersion model to investigate source-receptor relationships between NOx emissions and receptor locations (AQ stations) subject to high ozone concentrations in the NOx-limited portion of the valley.

A Retrospective Analysis of Ozone Formation in the Lower Fraser Valley, Canada

We conducted a study of ozone formation in the Lower Fraser Valley (LFV), using WRF-SMOKE-CMAQ models, observations and emission inventories in order to understand relationships between the reduction in both amount and location of precursor emissions and spatio-temporal changes in episodic ambient ozone concentrations over the last 20 years. A dynamical model evaluation shows that the modeling framework is able to capture the changes in both magnitude and spatio-temporal structure of ozone concentrations over the 20-year period. We model ozone formation for four episodes, which both capture the observed changes in ozone reduction and the different meteorological regimes that occur during LFV ozone episodes. The SMOKE emission inventories are adjusted to account for temporal changes in amount, and location of emissions, based on population shifts. Model runs allow us to isolate the effects of emission changes from meteorological changes. Results show that the western LFV has been, and remains VOC-sensitive; the central LFV has changed from VOC-limited to NOx-limited; and the eastern LFV has been, and remains NOx-limited. Analysis shows that the ozone production efficiency as a function of NO has increased noticeably in the eastern LFV. This has likely offset some of the benefits resulting from local NOx emission reductions.

The Use of Meso-Scale Atmospheric Circulation Types as a Strategy for Modelling Long-Term Trends in Air Pollution

It has been shown that ozone air pollution in the Lower Fraser Valley (LFV), B.C. has improved markedly over the past two decades. Ainslie and Steyn (2007) show that this improvement is associated with a statistically significant West-East shift the position of the ozone plume arising from emissions in the city of Vancouver. They hypothesize that this improvement and shift is due to emissions changes (rather than shifts in meteorological regimes). In order to test this hypothesis, we have performed a sequence of runs with the integrated mesoscale meteorology and air quality numerical model MC2-AQ. Ainslie and Steyn (2007) also show (by a statistical cluster analysis) that ozone episodes in this region occur under only four distinct mesoscale meteorological flow regimes: two with morning winds at the coastal Vancouver International Airport (YVR) meteorological station from the northwest direction, and two with southerly YVR morning winds. Because of the small number of distinct regimes, our modelling may be restricted only to a few representative days a year, rather than full, multi-year model runs. Emissions changes are based on published emissions inventories, and meteorological clusters are represented by archetypes drawn from the Pacific 2001 field study.