Box Model Investigation of the Effect of Soot Particles on Ozone Downwind from an Urban Area through Heterogeneous Reactions

Soot can provide additional surface area where heterogeneous reactions can take place in the atmosphere. These reactions are dependent on the number of reactive sites on the soot surface rather than the soot surface area per se. A box model, MOCCA, is used to investigate the effects of introducing heterogeneous reactions on soot into an air parcel passing over an urban area and traveling downwind. The model was run at two soot mass concentrations of 2 μg/m3 and 20 μg/m3 with a surface density of n-hexane and decane. Signifcant change in gas-phase concentration was only observed for the higher soot concentration. Due to the noncatalytic nature of the heterogeneous reactions, soot sites are rapidly consumed, and soot site concentrations are greatly reduced shortly after emissions are turned off. Notable changes in gaseous concentrations due to the introduction of heterogeneous reactions are not observed in the urban setting. The impact of heterogeneous reactions is more evident after emissions are turned off (i.e. downwind from the urban center). These changes are minimal for the condition that used n-hexane surface density. For conditions that used decane soot, NO x concentrations showed a slight increase, with NO being higher in the day time and NO2 at night. The maximum O3 reduction observed when using the higher soot concentration is 7 ppb, downwind of the urban center. Change in O3 concentration was less than 1 ppb when using the lower soot loading. The observed effects of heterogeneous reactions on soot decrease with time.