Simulation and analysis of high ozone episodes observed in the Hong Kong airshed: An explicit model approach

The analysis and forecast of elevated boundary layer ozone is of particular concern since it is known to have adverse effects on health, vegetation and material. The main contributors to the formation of photochemical smog, and high ozone levels are the emissions of Volatile Organic Compounds (VOC) and NOx. There is a complex relationship between VOC and NOx leading to ozone generation, which varies greatly depending on the geographic and regional setting. Modelling based on observational data provides one way to determine the major components, which have the greatest impact on ozone formation. Only through a combination of measurements, emission data and modelling is it possible to perform analysis, forecast, and develop sound strategies to minimize the occurrence of these high ozone events. Hong Kong as a metropolitan city in Asia has been suffering from poor air quality for more than two decades. Therefore, the Hong Kong Environmental Protection Department (HKEPD, 2003) has developed a comprehensive monitoring network. Through this network, air pollutants including many VOC, NOx and ozone are routinely monitored. This study has analysed the available monitoring data on a series of high ozone episode days in 2003, to define the conditions for the development of an explicit airshed box model. High ozone episodes occurred from the mid-October through to November, where daily peak ozone levels exceeded 100 ppb at the most easterly monitoring station on several days. The series of high ozone day observations, supported by back trajectory analyses are used to indicate the ozone episodes are single day events. For this analysis an initial airshed model tailored to the Hong Kong environment is developed, based on the Master Chemical Mechanism (MCM) approach, to provide a comparison with the observational data. It is aimed towards gaining a detailed understanding of the sensitivities of photochemical ozone formation under the prevailing conditions in Hong Kong. Sensitivity analysis is preformed on ozone development, following targeted reductions in NOx and VOC emissions. The analysis has determined that ground level ozone production in Hong Kong is neither VOC nor NOx limited, but in a transition zone between the two.