The MIPAS climatology of BrONO2: a test for stratospheric bromine chemistry

Besides chlorine, bromine is the major halogen species affecting stratospheric ozone with both anthropogenic and natural sources. Despite the significantly lower concentrations of bromine in the atmosphere, its potential for ozone depletion is similar to that of chlorine. An important prerequisite for the effectiveness of bromine ozone destruction cycles versus those of chlorine is the larger instability of bromine reservoir gases, especially the faster photolysis of bromine nitrate (BrONO₂) compared to chlorine nitrate (ClONO₂). With BrONO₂ abundances in the stratosphere available from observations, (1) homogeneous, heterogeneous as well as photochemical processes involving bromine as implemented in atmospheric models can be assessed, and (2) independent information on the total stratospheric bromine content can be gained which is important, e.g. to analyse the amount of short-lived bromocarbons entering the stratosphere.

The first detection of BrONO₂ in the atmosphere had been achieved by analysis of infrared limb-emission spectra from the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) on the Envisat satellite (doi: 10.5194/acp-9-1735-2009). On availability of improved infrared cross-sections, this was followed by the analysis of the behaviour of BrONO₂ during sunrise and sunset through MIPAS balloon observations (doi: 10.5194/acp-17-14631-2017). Here we present a novel dataset of global stratospheric BrONO₂ distributions based on the recently available MIPAS version 8 dataset of calibrated level-1b spectra. The altitude profiles of BrONO₂ volume mixing ratios are zonally averaged in 10° latitude and 3-day bins, separated between day- and night-time observations, with a vertical resolution of 3-8 km between 15 and 35 km altitude for the whole MIPAS period from July 2002 until April 2012. The typical characteristics of this new dataset will be discussed. Furthermore, we will compare it to a multi-annual simulation of the chemistry climate model EMAC. Specific differences between observation and model simulation of BrONO₂ will be highlighted and discussed by means of sensitivity 1-d model runs. Finally, a time series of the derived stratospheric Br_y content normalized to the time of the entry into the stratosphere on basis of MIPAS age-of-air information will be discussed with regard to estimated uncertainties as well as independent observations.