Reactive halogens and their measurements in the troposphere

In the earth-ocean-atmosphere system, the halogenated species take part in the cycles of several key processes involving both gas and heterogeneous interactions. The atmospheric cycles of reactive halogens are very complex specifically for those emitted from natural sources. As far as their roles in the tropospheric chemistry, the halogen compounds particularly those containing bromine (Br) and chlorine (Cl) play key roles. The reaction rate constants of many trace gases with halogen radicals are faster than those with hydroxyl radicals (OH). Near the source regions, however, halogen radicals can greatly influence the oxidizing capacity of the troposphere due to their reactive behaviors. In the lower troposphere, particularly in the marine boundary layer (MBL) and polar boundary layer, the reactive halogen compounds cause substantial destruction of ozone. The in-situ observations are available only for very limited geographical regions mainly in the midand highlatitudes of the northern hemisphere. One of the reasons for the lack of studies could be the technological constraint owing to very reactive nature of halogens hence the uncertainty in detection and quantification. Nonetheless, it is imperative to study the photochemistry of halogens in global troposphere for the better understanding of chemistry-climate interactions. Many theoretical aspects related to photochemistry of halogenated species in the troposphere need to be verified by the observations. Present study highlighted recent scientific progress about the roles of reactive halogens and their measurements in the troposphere. In spite of greater scientific opportunities in atmospheric studies of halogens, study over Indian subcontinent and surrounding marine regions are almost nil.