Stable carbon isotope ratios of ambient secondary organic aerosols in Toronto

Abstract. A method to quantify concentrations and stable carbon isotope ratios of secondary organic aerosols has been applied to study atmospheric nitrophenols in Toronto, Canada. The sampling of five nitrophenols, all with substantial secondary formation from the photooxidation of aromatic volatile organic compounds (VOCs), was conducted in the gas phase and particulate matter (PM) together and in PM alone. Their concentrations in the atmosphere are in the low ng m⁻³ range and, consequently, a large volume of air (> 1000 m³) is needed to analyze samples for stable carbon isotope ratios, resulting in sampling periods of typically 24 h. While this extended sampling period increases the representativeness of average values, it at the same time reduces possibilities to identify meteorological conditions or atmospheric pollution levels determining nitrophenol concentrations and isotope ratios.

Average measured carbon isotope ratios of the different nitrophenols are between −34 and −33 ‰, which is well within the range predicted by mass balance. However, the observed carbon isotope ratios cover a range of nearly 9 ‰ and approximately 20 % of the isotope ratios of the products have isotope ratios lower than predicted from the kinetic isotope effect of the first step of the reaction mechanism and the isotope ratio of the precursor. This can be explained by isotope fractionation during reaction steps following the initial reaction of the precursor VOCs with the OH radical.

Limited evidence for local production of nitrophenols is observed since sampling was done in the Toronto area, an urban center with significant anthropogenic emission sources. Strong evidence for significant local formation of nitrophenols is only found for samples collected in summer. On average, the difference in carbon isotope ratios between nitrophenols in the particle phase and in the gas phase is insignificant, but for a limited number of observations in summer, a substantial difference is observed. This indicates that at high OH radical concentrations, photochemical formation or removal of nitrophenols can be faster than exchange between the two phases.

The dependence between the concentrations and isotope ratios of the nitrophenols and meteorological conditions as well as pollution levels (NO₂, O₃, SO₂ and CO) demonstrate that the influence of precursor concentrations on nitrophenol concentrations is far more important than the extent of photochemical processing. While it cannot be excluded that primary emissions contribute to the observed levels of nitrophenols, overall the available evidence demonstrates that secondary formation is the dominant source for atmospheric nitrophenols in Toronto.