7.4 A CLIMATOLOGY OF PHOTOCHEMICAL SMOG EPISODES IN SYDNEY AUSTRALIA Melissa Hart*, Richard de Dear and Robert Hyde Department of Physical Geography, Macquarie University, NSW, Australia In 1998, the Australian National 1. INTRODUCTION Environment Protection Council (NEPC) Concentrations of ozone often exceed produced a National Environment Protection Australian air quality goals in Sydney during Measure for Air Quality (the Air NEPM) summer. The meteorological conditions (NEPC, 1998). The Air NEPM provided for associated with photochemical smog in the the first time in Australia a set of national Sydney region and surrounds are complex. ambient air quality standards. The Air Photochemical smog episodes are generally NEPM standards for ozone are 100 ppb for confined to the months October to March, a 1-hour average concentration and 80 ppb inclusive. Sydney is located on the eastern for a 4-hour average, with an allowable coast of Australia at latitude -33.8ºS (Figure exceedance of 1 day per year. Furthermore 1), and experiences a temperate climate the New South Wales (NSW) state with warm to hot summers and cool to cold environmental regulatory body the winters (BoM, 1991). The main urban areas Department of Environment and are located within a basin bound by Conservation (DEC) has set long-term goals elevated terrain to the north, west and south for NSW based on The World Health (Figure 2). The weather in the region is Organization (WHO) ozone goals of 80 ppb affected by relatively complex topography (1-hour average) and 60 ppb (4-hour with overnight cold-air drainage flows, average). Figure 3 presents a graph particularly during cooler months and showing the number of days in which the Air frequent sea breezes during the warmer NEPM ozone goal of 100 ppb was months. exceeded over the 10-year period 1992 to 2001. Days in which the exceedance can be attributed to bushfires within Sydney and surrounding regions are presented in purple. NT QLD WA SA NSW Sydney Vic Tas Scale: 1:2 500 000 © copyright Commonwealth of Australia (Geoscience Australia), 2004 Modified from :http:// www.ga.gov.au/map/ 0 20 Figure 1 Location map of Sydney, (-33.8º S) kilometres ____________________________________ Figure 2 Topography of Sydney and surrounding * Corresponding author address: Melissa Hart, region and location of NSW DEC air monitoring sites Dept. of Physical Geography, Macquarie University, NSW, 2109, Australia. +61 (0)2 9850 9612 e-mail: [email protected] Number of Days the air NEPM for 1-hour ozone is exceeded 30 25 Exceedances of the ozone goal attributed to bushfired Days in which the ozone goal is exceeded 20 15 Days 10 5 0 Jan - Mar Oct 1992 - Oct 1993 - Oct 1994 - Oct 1995 - Oct 1996 - Oct 1997 - Oct 1998 - Oct 1999 - Oct 2000 - Oct - Dec 1992 Mar 1993 Mar 1994 Mar 1995 Mar 1996 Mar 1997 Mar 1998 Mar 1999 Mar 2000 Mar 2001 2001 Figure 3 Number of days in which the Air NEPM ozone goal of 100 ppb was exceeded Previous studies of photochemical smog This climatology has been produced using formation in Sydney are often episode- multivariate statistical techniques including based and there remain features in the principal component analysis and cluster occurrence of photochemical smog that are analysis in order to assign days into yet to be fully explained (Hyde, et al. 1995). meteorologically homogeneous synoptic A study by Leighton and Spark (1995) categories. Meteorological inputs to these produced a manual classification of mean statistical procedures include both surface sea level synoptic maps to ascertain the and upper air observations, for warm relationship between synoptic climatology months (October to March) over a ten year and pollution events in Sydney. They period (1992 – 2001). identified the main synoptic situations Days in which Sydney’s air quality may covering the eastern Australian region for have been affected by bushfires were winter and summer associated with removed from analyses. Systematic records moderate to high pollution events. The of bushfire activity in Sydney are scarce. amount of time an anticyclone remained in The most comprehensive data came from the region was also related to number of the National Parks and Wildlife Service pollution days. They found summertime (NPWS) wildfire database, which covers pollution events to be associated with a light wildfire activity from 1995 to the present. west to northwesterly gradient wind. Using this database, days in which there Meso-scale conditions have a large was significant bushfire activity in the influence on the formation of photochemical Sydney region were isolated and daily smog in the Sydney basin. High ozone maximum observations of ozone and PM10 peaks, particularly in the central and were then compared for both bushfire days western regions of Sydney, are most and non-bushfire days. Days in which there commonly associated with an afternoon sea were bushfires in the Sydney region breeze that transports precursor emissions generally showed a marked increase in across the Sydney basin. particulate concentrations compared to non- In order to gain a better understanding bushfire days. From this analysis, days in of the synoptic signatures behind which daily maximum values of ozone photochemical smog processes, this paper exceeded 80 ppb and PM10 exceed 110 presents an objective synoptic climatology µg/m3 were deemed days in which air of photochemical smog episodes in Sydney. quality in Sydney was probably being affected by bushfires. A threshold of 80 ppb as much of the residual variance as possible ozone was chosen as it corresponds to the while remaining orthogonal (i.e. completely NSW DEC long-term 1-hour ozone goal. independent) to the first in data-space. Twenty-seven days were identified between Remaining components explain less 1992-2001 using these criteria; these days variance than the proceeding components. were removed from the analyses. By retaining only a proportion of the On a number of days in which there principal components, a large amount of the were bushfires in the Sydney region ozone original variance can be explained whilst and particulate concentrations were below substantially reducing the size of the data the criteria described above. However, on matrix. some occasions synoptic, regional and Each principal component comprises a meso-scale winds may have transported series of loadings that represent the some residual bushfire air into the Sydney correlation between the principal component Basin; however the resultant impact on and the meteorological parameter. The subsequent ozone concentrations was low. larger the component loading the more Therefore, these days were not excluded important the meteorological parameter is in from the synoptic classification. the interpretation of that principal component. Once the principal components 2. SYNOPTIC CLIMATOLOGY METHODS and their respective loadings are A synoptic climatology of photochemical established it is necessary to ascertain the smog episodes in Sydney was undertaken. relationships between the original raw This analysis involved using multivariate variables for each day and the principal statistical techniques including principal components. This is achieved through the component analysis and a two-stage cluster calculation of component scores for each analysis to classify days into day, which are values for each day’s meteorologically homogenous synoptic weather observations based on the principal categories. These synoptic categories were components (PC). Thus, days with similar then related to ground level ozone meteorology conditions will exhibit concentrations. The method of synoptic proximate principal component scores classification used in this research is the (Shahgedanova, et al. 1998). circulation-to-environment approach, in To generate synoptic categories PCA which the classification is produced first and was used in conjunction with cluster is then related to the environmental variable analysis. Clustering procedures were used in question (Yarnal 1993). This method is to group days with similar component becoming more frequent in air pollution scores and to assign each day into a climatology studies, particularly in the study meteorologically homogeneous subset of of photochemical smog (e.g. Cheng, et al. the data (Kalkstein and Corrigan, 1986; 1992; Eder, et al. 1994; Davis, et al. 1998; Shahgedanova, et al. 1998; Greene, et al. Greene, et al. 1999; Schreiber, 2000). 1999). The component scores matrix The multivariate statistical techniques (number of days times the retained PCs) were used to group suites of meteorological obtained from PCA served as the input variables into synoptic categories found to matrix for cluster analysis. The reduced size be representative of the air mass and the absence of co-linearity make it ideal characteristics at that time. Principal data for clustering (Davis, et al. 1998). Component Analysis (PCA) was used as a A two-stage clustering technique was data reduction technique. Data reduction is used. Firstly, average-linkage, a achieved by finding linear combinations hierarchical, agglomerative method was (principal components) of the original used to determine the number of clusters variables, which account for as much of the and the mean conditions within each cluster. total variance in the original variables as These initial clusters were then used as possible (Statheropoulos, et al. 1998). The input in to k-means clustering, a non- components are ordered such that the first hierarchical iterative method. component explains