Techniques for analysing Ground-based UV-Visible Long-term BrO and NO2 observations for satellite validation and trend analysis

NIWA operates a network of zenith-sky viewing DOAS (Differential Optical Absorption Spec- troscopy) instruments to measure NO2 and BrO. The longest existing time series (1981 present) of NO2 has been measured at Lauder (45oS), New Zealand and the trend of this long- term data set has been studied extensively. Here we present a summary of stratospheric NO2 trends observed at several Northern and Southern Hemisphere stations (including Lauder) and an update of our understanding of the observed hemispheric asymmetry. These trends provide an important anchor for the interpretation of NO2 trends measured by satellites.BrO observations are currently made by NIWA at two Southern Hemisphere sites, Lauder and Arrival Heights (78oS) with each data set spanning more than 15 years. The zenith sky BrO observations are complemented with direct sun observations at Lauder since 2001and with MAX-DOAS (Multi-axis Differential Optical Absorption Spectroscopy) observations at Arrival Heights (78oS) since 1998. A retrieval technique to separate the tropospheric and stratospheric partial columns of BrO was developed for the combination of zenith sky and direct sun measurements with the zenith sky observations providing predominantly the information on the stratospheric partial column and the direct sun observations providing the tropospheric contribution. This retrieval has now been applied to Lauder BrO UV-visible measurements for the whole time period (2001 - present) and the updated results including an upper limit of BrO in the troposphere and the stratospheric bromine loading will be presented.The retrieval method has now also been extended so that it can be applied to zenith sky data only. Furthermore, an independent retrieval algorithm has been developed including a forward model capable of dealing with multiple scattering (Monte Carlo radiative transfer model) to enable us to retrieve altitude information in the boundary layer and lower troposphere. This retrieval method has been applied to MAX-DOAS measurements made at Arrival Heights for the last 12 years with the aim to investigate bromine explosion events observed in McMurdo Sound during Antarctic springtime and the results of this investigation will be presented.