Ground-level ozone is a serious pollutant especially in the summer season in the tropical and extra-tropical regions. It is to be noted that the ozone is not a 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 pure 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 each of the variables, all are commonly controlled by the transport phenomenon, which is complex and the intern is controlled by several things. For example, a complex surface structure such as mountainous terrain, the 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, and the transport influence on both air temperature and cloud cover.
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 lightning, agri-pesticide sprays, etc. The main mechanism of the ozone formations is that the HO radicals in the troposphere react with VOCs (CO) to form peroxy (hydro-peroxy) radicals. These peroxy radicals react with NO to form and NO2 and this NO2 dissociates in the presence of sunlight to form nascent oxygen, which can easily combine with an 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 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.