Production and loss pathways of tropospheric ozone under different ambient conditions

Abstract

In the present study, production and loss pathways of tropospheric ozone and their rates are identified by an algorithm for the automatic determination of reaction pathways in complex chemical systems. For this purpose, reaction rates were provided by the chemistry-transport model IFS(MOZART) (Integrated Forecasting System - Model for Ozone and Related chemical Tracers). A detailed analysis is carried out for three different scenarios: clean air (Palau), intermediate emissions (Athens), and large emissions (Beijing). At each location the processes at the surface and at an altitude of 500 m are analysed. The ozone production rate is largest in Beijing, intermediate in Athens and smallest on Palau. Nevertheless, there is net ozone loss at the surface in Beijing because of a strong net conversion of ozone to NO\(_{\varvec{2}}\) by freshly emitted NO. The ozone production is dominated by methane oxidation on Palau and by the oxidation of short-lived, i.e. emitted nearby, VOCs (volatile organic compounds) in Beijing, where the strongest individual contributor at the surface is isoprene. Athens represents an intermediate situation. The pathways determined show in detail all intermediate steps of the degradation of individual VOCs, including the interaction with NO\(_{\varvec{x}}\) and HO\(_{\varvec{x}}\) species, and permit the calculation of the number of ozone molecules formed per VOC molecule consumed. For instance, at the surface in Beijing the average net production of ozone in pathways leading to the full degradation of isoprene (to CO\(_{\varvec{2}}\)) is 10.1 ozone molecules per isoprene molecule consumed. At the same location pathways producing up to 18 ozone molecules per isoprene molecules have been found. However, the rates of these extreme pathways are very small.