Reactive bromine chemistry in the Rann of Kachchh salt desert

The Rann of Kachchh is a salt desert in the southern border area of India and Pakistan. Recently, high amounts of bromine monoxide (BrO) were observed there in satellite measurements of the Ozone Monitoring Instrument (OMI). Release mechanisms of reactive bromine, dominating chemical processes, the influence of the ambient atmosphere and transport processes, etc. are not well understood in general. Furthermore, due to their short time scales these processes are difficult to assess with satellite instruments, which only offer a single measurement per day with limited spatial resolution.

Here, we present BrO, HCHO and nitrogen dioxide (NO₂) measurements from ground-based MAX DOAS performed at two different locations in the Rann of Kachchh salt desert in Gujarat, India during three weeks in March and April 2019. We observe large amounts of BrO building up during daytime reaching maxima of several tens of ppt in the late afternoon. Additional mobile measurements performed directly over the salt gave similar results to the measurements at 5-15 km distance from the salt surface, suggesting that the BrO formation time scale and effective life times during daytime are at least of the order of several minutes to a few hours. Additional in-situ ozone measurements indicate ozone depletion events linked to the episodes of high BrO abundance. This indicates that BrO is formed by bromine atoms reacting with ozone and then being recycled via BrO self-reaction and heterogeneous processes involving aerosol surfaces, as proposed for other environments (Polar Regions, volcanic plumes).

While we found high but steady HCHO levels, the observed NO₂ levels showed a distinct anti-correlation to BrO, indicating coupling of bromine- and NO_x-chemistry and thereby the influence of the pollution level of the ambient atmosphere. Formation of bromine nitrate probably delays the formation of large BrO amounts, but might also support the recycling of bromine atoms through heterogeneous chemistry.