URBAN VENTILATION AND PHOTOCHEMICAL SMOG IN MELBOURNE FOR A FUTURE CLIMATE SCENARIO Sean Walsh †1 , Martin Cope 2 , Rob Goudey 1 , Valerio Bisignanesi 1 , Piyaratne Dewundege 1 1 EPA Victoria, Melbourne, Australia 2 CSIRO Centre for Australian Weather and Climate Research, Melbourne, Australia † Corresponding author address: Centre for Environmental Sciences, Ernest Jones Drive, MACLEOD, VIC 3085. Email: sean.walsh@epa.vic.gov.au 1. ABSTRACT Climate change may alter patterns of air pollution in urban environments, through changes to wind speeds, temperatures and other climate parameters. This project reviewed the relevant literature, then made use of CSIRO climate forecasts dynamically downscaled (Mk3  CCAM  TAPM) to a 5 km grid over Melbourne, Australia, with the aim of discerning possible climate-related trends in air quality. Summer photochemical smog is examined using a statistical method for predicting urban ozone, based on projected climate parameters and several years of ground-based ozone measurements. General air quality over the autumn-winter period is considered by calculating airshed ventilation rate and vertical stability, which provide an indication of air pollution potential. Analyses were conducted using only a single IPCC scenario (A2) to obtain preliminary estimates of likely trends. Results indicate that climate change may cause some worsening of air quality in Melbourne over the next 50 years. 2. INTRODUCTION Changes to climate are likely to affect many aspects of environmental quality, including the quality of urban air. This project attempts preliminary predictions of the effect of climate change on Melbourne’s air quality, by estimating trends in: • Ozone, • Ventilation rate (mixing height X wind speed), and • Pasquill vertical stability class. To quantify the effect of climate on photochemical smog, urban ozone measurements from 1996-2005 were matched against parameters from a climate model, dynamically down scaled to a 5 km grid over the region of interest. Regression models were developed from this data, and then used to predict ozone metrics for 2021-2030 and 2051-2060. Subsequent to this analysis, the meteorological results were further examined to look for trends in urban ventilation and Pasquill stability class in an attempt to assess the impact of climate change on the general potential for air pollution accumulation. Urban air pollution episodes in autumn and winter typically involve a low ventilation rate and stable conditions. 3. THEORY & LITERATURE Climate change may affect air quality though a range of mechanisms. Jacob (2008) provides a recent review of the effects of climate change on air quality, finding that urban ozone is likely to worsen under a warmer climate, but particle concentrations may increase or decrease depending on locality. The literature on climate change and air quality has been comprehensively reviewed, including a focus on implications for the Victorian region (Pearce, 2008; Walsh, 2008). Published studies indicate that climate change may affect background concentrations, emissions, photochemistry, dispersion and deposition, thus potentially having important effects on future air quality. A brief summary of the state of knowledge is provided below. 3.1 Tropospheric background concentrations Although higher temperatures are expected to increase urban ozone, global-averaged ozone is expected to decrease. This is due to an increase in specific humidity, causing a greater production of OH and HO 2 radicals, which in clean background air (low NO X ) act to destroy ozone (Collins, 2003). This decrease in ozone is partly offset by a projected increase the stratosphere-troposphere exchange rate (Zeng & Pyle, 2003). The actual background experienced regionally may be significantly different from the global average. Measured trends in background pollutants show regional variations; for example southern hemisphere background ozone has shown a clear upward trend between 1982 and 2003 (Galbally et al,, 2005), but northern hemisphere data do not consistently show a trend (Oltmans et al., 2006). Climate change may also alter global average fine particle concentrations, but as yet there is no clear consensus on the sign of the change (Jacob, 2008). Likely influences include changes to wet deposition (which increases with rainfall), secondary particles (sulfate formation increases with temperature, whilst nitrate and organics shift towards the vapour phase in higher temperatures), and changes to dust and fire emissions.