Abstract
The diurnal variation of pollutants such as particles and carbon monoxide (CO) in urban environments typically follow the traffic density, with two peaks coinciding with the weekday morning and evening rush-hour periods. However, observations made in central Brisbane, the third largest city in Australia, demonstrated an anomalous diurnal variation with the CO peak in the evening being significantly higher than that in the morning. This imbalance was not observed for particle concentrations. Here, we show that the imbalance is a direct result of the difference in CO emission factors from motor vehicles during warm and cold starts. Over 30,000 cars with warmed-up engines enter Brisbane city centre car parks every weekday morning. They all start their engines from cold and leave the city in the evening, producing the anomalously higher emissions of CO in the city centre. This pattern of air quality, while clearly apparent within the city car parks and within the central business district of the city, was not observed outside the city limits and in the suburbs. To the best of our knowledge, no previous study has drawn attention to this phenomenon and no explanation has been provided in the literature to-date.
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•Diurnal variation of carbon monoxide (CO) and PM2.5 concentrations in a city center .•Both concentrations exhibit two peaks coinciding with rush hour traffic periods .•However, the CO peak in the afternoon is significantly higher than in the morning .•This latter phenomenon has not been noticed or reported before .•The effect is attributed to excessive cold start emissions from motor vehicles .