Ground-based measurements of tropospheric and stratospheric bromine monoxide above Nairobi (1° S, 36° E)

Abstract. Ground based observations of stratospheric and tropospheric bromine monoxide, BrO, from a multi axial differential optical absorption spectrometer, MAXDOAS, located at the UNEP/UNON site in Nairobi (1° S, 36° E) are presented for the year 2003. Differences in BrO slant column densities at 90° and 80° solar zenith angle retrieved from the zenith-sky measurements are used to study stratospheric BrO. They show only small variations with season, as expected for the small seasonality in stratospheric Br_y and NO₂ in this region. A pronounced diurnal variation can be observed, the average value for the morning being 1.3×10¹⁴ molecules/cm² and for the evening 1.5×10¹⁴ molecules/cm². The measurements are compared with simulations from a one-dimensional photochemical stacked box model which is coupled with a radiative transfer model to allow direct comparisons between the observations and the model calculations. In general the model reproduces the measurements very well. The differences in the absolute values are 15% for the evening and 20% for the morning which is within the limits of the combined uncertainties. Both seasonality and diurnal variation are well reproduced by the model. A sensitivity study shows that inclusion of the reaction BrONO₂ + O(³P) significantly improves the agreement between model calculations and measurements, indicating an important role of this reaction in the stratosphere near to the equator. Tropospheric BrO columns and profile information is derived from the combined results obtained in the different viewing directions for the average over several clear days. The resulting tropospheric BrO columns are in the range of 4–7.5×10¹² molecules/cm² which is significant but lower than in previous studies at mid and high latitudes. The vertical distribution of the tropospheric BrO peaks at about 3 km indicating the absence of local sources at this high altitude site.