An Analysis of Nitrate Reactivity with Volatile Organic Compounds in the Greater Toronto Area and Montreal

The nitrate radical (NO3) is a strong oxidant, which can react with a wide array of Volatile Organic Compounds (VOCs), particularly unsaturated compounds which are predominantly biogenic, leading to a permanent loss of odd oxygen. Alternatively, NO3 can react with NO2 to form N2O5, which may be taken up on aerosol to form particle nitrate. However, NO3 can also react with NO or photolyze to reform NO2, thus recycling nitrogen oxides. This work focuses on calculating the pseudo first order loss rate of NO3 with VOCs to determine which reaction pathway dominates in the urban atmosphere during the night. The nitrate reactivity is calculated by summing the products of the concentration of each VOC with its rate constant against NO3. The data used in this study is from the National Air Pollution Surveillance (NAPS) program which monitors VOC concentrations using 24-hour integrated observations collected every six days. The monitoring sites for this investigation are located in the Greater Toronto Area and Montreal where data was collected for long time periods between 1995 and 2020. Analysis found that the reactivity of NO3 with VOCs has declined with time due to decreasing concentrations of reactive VOCs in urban settings. It was found that two key VOCs, naphthalene and 2-methyl-2-butene, drive the reactivity and its trends through time. While the absolute reactivity of VOC towards NO3 has decreased, its importance as a sink has likely increased in relative terms.