Diurnal variations in nighttime ground-level ozone concentration and its chemical removal in Peninsular Malaysia

Nighttime ground-level ozone is a crucial period of ozone depletion due to the absence of sunlight and the lack of ozone produced. This study presents an in-depth analysis of the major cause behind ozone deflection from 7 PM to 12 AM in consideration of ozone precursors, meteorological factors and other indirect chemical reactions. Malaysia is a tropical area with two distinctive monsoons and an average temperature ranging between 21 °C and 32 °C. A total of 10 continuous air quality monitoring stations, 3 years (2013–2015), nighttime hours (7 PM to 12 AM) and 6 parameters (O3, NO, NO2, T, RH and PM10) were considered in the analysis. Box and whisker plots (descriptive statistics), diurnal plots and principal component analysis (PCA) were used to highlight the ozone depletion in Malaysia from 7PM to 12 AM. The descriptive statistics of O3 concentration differed across all locations due to different atmospheric surroundings (land-use types), and the highest ozone distribution during nighttime was observed in the suburban area of Malaysia. Meanwhile, the diurnal analysis reveals a decreasing trend in O3 concentration and an increasing trend in NO and NO2 concentrations. The suburban area (LS04-S) of Malaysia reported the highest O3 concentration, whereas the urban (LS02-U; LS03-U) and industrial areas (LS07-I; LS10-I) recorded the highest NO and NO2 concentrations due to their highly congested roads and heavy industries. The PCA results show variations ranging from 52.195% to 79.532% and that PC1 can be separated into two main factors, namely, meteorological factors and traffic-originated emissions.