Ground-level ozone is a serous pollutant especially in the summer season in the tropical and extra-tropical regions. It is to be noted that the ozone is not primary pollutant, meaning that the ozone is not directly emitted from the activities at the surface. However, it is a secondary pollutant formed in the atmosphere by primary pollutants such as nitrogen oxides (NOx) and volatile organic compounds (VOCs) under the sunlight, the chemical reaction speeds are augmented by atmospheric temperature. Therefore, we can now understand that the ozone levels are controlled by the emissions of NOx, VOCs, cloud-cover, air temperature, season, and the time of day. In this, the season and the specific time of the day are invariant, but the NOx, VOCs, cloud-cover and air temperature are variables. While studying the ozone modeling studies, these should be critically taken care of.
Let’s now dissect these variables in terms of meteorology and chemistry. The former two are purely chemicals and the latter two are meteorological variables. What many air pollution modelers miss is the interaction between the meteorology and chemistry. Let’s see what it is. Although there are specifics of the each of the variables, all are commonly controlled by transport phenomenon, which is complex and in tern is controlled by several things. For example, complex surface structure such as mountainous terrain, roughness of the surface by forests, urban building, etc., etc. significantly impact on the transport of heat, moisture, momentum and chemical tracers. Although the heat and moisture fluxes are controlled by the land surface characteristics, for example, plants and water bodies, the transport of them significantly influence on both air temperature and cloud-clover.
The NOx and VOCs are emitted by both human-activities and natural sources. Vehicular emissions, industrial emissions, biogenic emissions are the sources of NOx and VOCs, despite having other sources such as lightening, agri-pesticide sprays, etc. The main mechanism of the ozone formations is that the HO radicals in the troposphere reacts with VOCs (CO) to form peroxy (hydro-peroxy) radicals. These peroxy radicals reacts with NO to form and NO2 and this NO2 dissociates in the presence of sunlight to form nascent oxygen, which can easily combine with atmospheric oxygen molecule (O2) to form ozone. It looks simple but the chemistry is very complex and complexity increases with the meteorology as explained. In addition to this, the complexity is further increased by the sea-sprays at the Ocean surfaces as the these contain reactive halogenated compounds that tightly interact with the ozone itself and destroys. The whole chemistry is taught at my YouTube channel at https://www.youtube.com/channel/UC2RUKupzrKrgUceSr-8nw1g . Stay tuned at the channel for the periodic lecture videos.
In a nutshell, while modeling the ozone, one has to keep in mind that the emission levels of the precursors, season, surface terrain, surface characteristics such as vegetation and roughness, water bodies and the most importantly the chemical mechanisms. Please stay tuned and subscribe my YouTube channel at https://www.youtube.com/channel/UC2RUKupzrKrgUceSr-8nw1g to walk you through all of these essentials.