Analysis of Relationship between O3, NO, and NO2 in Riyadh, Saudi Arabia

For the first time in the city of Riyadh, the formation of O 3 and its relationship with NO and NO 2 (NO x ) was investigated. Throughout the summer O 3 , NO, and NO 2 were collected from three locations: residential, industrial, and rural areas. During the sampling period O 3 concentrations exceeded 1-hour local standards a few times yet remained consistent with the standards most of the time. The O 3 concentrations were observed highest in the rural location and lowest in the industrial area. The diurnal variation of NO followed a double peak: one in the morning and the other in the evening, representing the traffic pattern. Early morning NO peaks were observed in the rural location, which were attributed to the movement of NO from other locations. The O 3 concentrations depicted typical pattern, increasing after sunrise and reaching its maximum during midday. The highest O 3 concentrations were observed in the rural location followed by the residential and industrial. NO 2 photolysis rates were 3–4 times higher compared to other similar investigations, potentially due to intense solar radiation. A strong negative correlation was observed between NO x and O 3 values in the industrial location indicating photochemical activities around the industrial area were higher, likely due to additional NO x emissions from industries. Regression analysis of NO x and OX (O 3 +NO 2 ) indicated that in residential and industrial locations at nighttime there were large NO x independent regional contributions which is attributed to VOCs. The Weekend Effect was observed in the city potentially due to the production of the OH radical and subsequent reactions with VOCs implying that the area is VOC-sensitive.