Spatio-temporal variation of nocturnal surface ozone and regional background ozone level in Malaysia

Abstract

Air pollutant can be categorised into two categories, primary and secondary pollutant. Ozone is considered as a secondary pollutant. Understanding sources of all possible contributors to surface ozone accumulation is vital in order to come up with and effective pollution control strategies. This study attempts to assess the extent of nocturnal surface ozone pollution as well as determining the level of regional background ozone in Malaysia. Daily ozone data from 37 continuous ambient air quality monitoring stations throughout Malaysia for a period of eleven years (2005 – 2015) have been analysed in this study. These data were obtained from the Department of Environment, Malaysia. Peninsular Malaysia has been divided into four region namely northern, southern, central and east coasts while East Malaysia has been divided into Sabah and Sarawak region. Daily maximum concentrations of nocturnal surface ozone (NSO) were analysed for trend and seasonality as well as monthly diurnal variations. An overall average ratio of mean NSO to daytime surface ozone (DSO) for Malaysia was found to be 60% of DSO (0.58–0.61). The highest ratio (70% of DSO) was recorded in the east coast region and the lowest ratio (50% of DSO) was from the central region. The horizontal transboundary movement of aged ozone from Indochina was the most likely explanation for the former high ratio while titration processes by high emission of nitric oxide (NO) in the urban areas was the main reason for the low ratio in the latter. The NSO concentrations were also found to exceed the Malaysian Ambient Air Quality Standard for surface ozone at 100 ppb on several occasions in a few regions with highest exceedance observed in the central region with a maximum value of 137 ppb. Secondary/nocturnal peaks were observed at more than 50% of stations occurring around 0300 – 0500 h. Similar to DSO, the NSO is also influenced by the monsoonal season with higher NSO concentration mainly observed during boreal winter season. The long-term trend analysis of NSO revealed that most stations throughout the country were in the increasing trend with ten stations (27 %) showed statistically significant trends at p < 0.05 when tested with Mann-Kendall trend test. Daytime and nocturnal surface ozone concentration daily average were computed and analysed for the determination of regional background ozone level. Employing a regression analysis on the relationship between oxidant, OX (define as the sum of ozone and nitrogen dioxide) with the level of oxides of nitrogen, NOx (define as the sum of nitric oxide and nitrogen dioxide) at a given location, the level of OX has an NOx-independent contribution and an NOx-dependent contribution. The NOx-independent contribution, which is the intercept of the regression line, equates to the level of regional background ozone while the NOx-dependent contribution, which is the oxidant slope relative to NOx, correlates to the level of local contribution or the primary pollution. The monthly average of regional background ozone in the country was found to be in the range of 19.71 ppb to 37.79 ppb for daytime and 12.89 ppb to 23.56 ppb for nighttime. The highest level of regional background ozone was in the central region and the lowest was in the Sarawak region for both daytime and nighttime. The regional background for all regions was not markedly influence by the season except the east coast region where the regional background level showed a slight increased during the boreal winter season (Northeast monsoon). The long term yearly trend of the regional background revealed an increasing trend throughout the country at a very slow rate with the highest in the central region at 36.93 ppb/yr.