The Michelson Interferometer for Passive Atmospheric Sounding global climatology of BrONO2 2002–2012: a test for stratospheric bromine chemistry
Authors: Höpfner Michael , Kirner Oliver , Wetzel Gerald , Sinnhuber Björn-Martin , Haenel Florian , Johansson Sören , Orphal Johannes , Ruhnke Roland , Stiller Gabriele , von Clarmann Thomas 
Published: December 2021

Abstract. We present the first observational dataset of vertically resolved global stratospheric BrONO2 distributions from July 2002 until April 2012 and compare them to results of the atmospheric chemical climate model
ECHAM/MESSy Atmospheric Chemistry (EMAC). The retrieved distributions are based on space-borne measurements of infrared limb-emission spectra recorded by the Michelson Interferometer for
Passive Atmospheric Sounding (MIPAS) on Envisat. The derived vertical
profiles of BrONO2 volume mixing ratios represent 10∘ latitude
bins and 3 d means, separated into sunlit observations and observations in the dark. The estimated uncertainties are around 1–4 pptv, caused by spectral noise for single profiles as well as for further parameter and
systematic errors which may not improve by averaging. Vertical resolutions
range from 3 to 8 km between 15 and 35 km altitude. All leading modes of spatial and temporal variability of stratospheric
BrONO2 in the observations are well replicated by the model
simulations: the large diurnal variability, the low values during polar winter
as well as the maximum values at mid and high latitudes during summer. Three major differences between observations and model results are observed: (1) a
model underestimation of enhanced BrONO2 in the polar winter
stratosphere above about 30 km of up to 15 pptv, (2) up to
8 pptv higher modelled values than observed globally in the lower
stratosphere up to 25 km, most obvious during night, and (3) up to 5 pptv lower modelled concentrations at tropical latitudes between 27
and 32 km during sunlit conditions. (1) is explained by the model
missing enhanced NOx produced in the mesosphere and lower
thermosphere subsiding at high latitudes in winter. This is the first time
that observational evidence for enhancement of BrONO2 caused by
mesospheric NOx production is reported. The other major
inconsistencies (2, 3) between EMAC model results and observations are studied by sensitivity runs with a 1D model. These tentatively hint at a model underestimation of heterogeneous loss of BrONO2 in the lower stratosphere, a simulated
production of BrONO2 that is too low during the day as well as strongly underestimated BrONO2 volume mixing ratios when loss via reaction with
O(3P) is considered in addition to photolysis. However, considering
the uncertainty ranges of model parameters and of measurements, an unambiguous
identification of the causes of the differences remains difficult. The observations have also been used to derive the total stratospheric bromine
content relative to years of stratospheric entry between 1997 and 2007. With
an average value of 21.2±1.4 pptv of Bry at
mid latitudes where the modelled adjustment from BrONO2 to Bry is smallest, the MIPAS data agree with estimates of
Bry derived from observations of BrO as well as from
MIPAS-Balloon measurements of BrONO2.