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

Abstract

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 Bry 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.

Mobile MAX-DOAS observations of tropospheric trace gases

“…While the sensitivity to stratospheric trace gas absorptions is almost independent on the viewing direction (e.g. Hönninger and Platt, 2002;Leser et al, 2003;Bobrowski et al, 2003;van Roozendael et al, 2003;Wittrock et al, 2004;Hönninger et al, 2004;Heckel et al, 2004;Wagner et al, 2004Wagner et al, , 2007aFrieß et al, 2006;Fietkau et al, 2007;Theys et al, 2007), the sensitivity for tropospheric trace gases and aerosols depends strongly on the elevation angle (and to a lesser degree also on the relative azimuth angle, i.e. the difference of the azimuth angles of the telescope and the sun).…”

“…In recent years Multi-AXis-Differential Optical Absorption Spectroscopy (MAX-DOAS) observations have become a widely and successfully used technique for the remote sensing of tropospheric trace gases and aerosols (Leser et al, 2003;Van Roozendael et al, 2003;Wittrock et al, 2003; Correspondence to: T. Wagner (thomas.wagner@mpch-mainz.mpg.de) Hönninger et al, 2004a, b;Sinreich et al, 2005;Heckel et al, 2005;Frieß et al, 2006;Fietkau et al, 2007;Theys et al, 2007;Wagner et al, 2004Wagner et al, , 2007aWagner et al, , b, 2009Irie et al, 2008). MAX-DOAS instruments observe scattered sun light under different (mostly slant) viewing angles, providing high sensitivity to tropospheric trace gases and aerosols.…”

Retrieval of stratospheric and tropospheric BrO profiles and columns using ground-based zenith-sky DOAS observations at Harestua, 60° N

“…In contrast, Theys et al (2007) inferred tropospheric BrO columns of about 1.1×10 13 molec/cm 2 from ground-based MAX-DOAS measurements in Reunion Island (21 • S), consistent with the abovementioned satellite and balloon estimates and with our tropospheric columns retrieved at Harestua in late spring/summer. Also from groundbased MAX-DOAS measurements in the tropics (Nairobi, 1 • S), Fietkau et al (2007) have derived slightly lower tropospheric BrO column amounts, in the range of 0.4-0.75×10 13 molec/cm 2 .…”

Retrieval of stratospheric and tropospheric BrO columns from multi-axis DOAS measurements at Reunion Island (21° S, 56° E)

“…Updated kinetic and photochemical data are taken from the JPL 2006 compilation (Sander et al, 2006). As investigated in Fietkau et al (2007), the reaction BrONO 2 +O( 3 P) can play an important role in the tropical stratosphere and has thus, been included in the model simulations. The results of the photochemical model are given at chemical time steps of 6 min.…”