Fluctuations in nighttime ground-level ozone concentrations during haze events in Malaysia

Abstract

This study focused on O3 variations and the titration effects of NOx during nighttime at urban, industrial, sub-urban and background sites. Nighttime O3 concentration variations and the presence of high particles with an aerodynamic diameter of less than 10 μm (PM10) were examined because haze disturbs the photochemical reactions of O3. Hourly data on O3, NO2, NO and PM10 concentrations provided by the Air Quality Division of the Department of Environment were divided into two groups of daytime and nighttime and analysed. The maximum O3 concentrations during daytime were generally observed during noon. At nighttime, the concentration of O3 decreased, indicating that destruction activities occurred mainly via titration. The retention of O3 during daytime caused the nighttime O3 during haze events to be higher than that during normal days. Apparent fluctuations in nighttime O3 concentrations were observed in the urban site (20 ± 13 ppb) during haze events. The NO2/NO ratio in the urban site during haze was higher than that on normal days; amongst the sites, the urban one had the highest value (6.6). Results indicated that during haze, the reactions between NO and O3 were enhanced at nighttime, leading to low nighttime NO concentrations. The low nighttime NO concentrations led to low nighttime NO titration rates, which enabled O3 to persist in ambient air. Nighttime O3 was not completely absent due to anthropogenic sources. This condition accelerated NO titration to NO2, thus promoting O3 production even during haze.