Retrieval and analysis of tropospheric bromine monoxide enhancements in polar regions using satellite measurements

Abstract

Bromine compounds have received growing attention due to their impact on the atmospheric chemistry in particular ozone chemistry and the resulting oxidation capacity. Reactive bromine is involved in chain reactions that deplete ozone in the stratosphere as well as catalytically destroy ozone in the troposphere leading to nearly complete removal near the surface called as an ozone depletion event (ODE). Bromine monoxide (BrO) is the most commonly observed bromine species and large amounts of BrO are observed in the polar regions during spring time through bromine explosion events. Over the last decades, BrO vertical column density has been retrieved with satellite-based UV–visible instruments such as GOME, SCIANACHY, OMI and GOME-2 for the monitoring of bromine amounts in various source regions. In particular, TROPOMI which was launched in October 2017 provides daily global coverage as the predecessor instruments, but with its unprecedented high spatial resolution (3.5x5.5 km2), this instrument enables to detect spatial variations and small-scale emission sources in more detail. The aim of my doctoral project is to develop an advanced algorithm to derive accurate total and tropospheric BrO columns by using the improved spatial resolution of TROPOMI data. In the first part of this thesis, a description of total/tropospheric BrO retrieval algorithm is presented. Optimized DOAS settings for the BrO retrieval are determined based on sensitivity tests under various measurement conditions. Approaches for the stratospheric correction to separate the tropospheric partial columns from total BrO columns are tested using several different methods. Also, air mass factor calculations for TROPOMI are performed by considering various observation conditions. As a consistency test, TROPOMI BrO columns are compared with OMI and GOME-2 retrieval results on both global and regional scales. Satellite BrO retrievals for a large spatial coverage over a long-term period can be used to explain the causes of BrO enhancements in the polar regions during spring and the role and characteristics of meteorological factors contributing to them. The relationship between satellite total BrO vertical columns and meteorological fields including sea level pressure, surface level wind speed and direction, surface air temperature, and tropopause height are investigated in this study. Statistical analysis using decadal GOME-2A/B observations reveals spatial and temporal behaviors of total BrO column enhancements in the Arctic and Antarctic. Also, differences in meteorological conditions and their regional characteristics between enhanced BrO situations and the mean field are identified, which shows effects of meteorological parameters on BrO enhancement processes. Finally, the satellite BrO retrieval algorithm developed in this thesis is verified by comparing satellite BrO columns with ground-based MAX-DOAS BrO measurements in Ny-Ålesund, Svalbard, operated by IUP-Bremen DOAS group. The possibilities and needs for improving the BrO retrieval algorithm are discussed based on the satellite retrieval algorithm validation results. Also, using both satellite and ground-based MAX-DOAS tropospheric BrO retrievals, bromine explosion event cases in Ny-Ålesund are investigated in detail, and moreover, capabilities of TROPOMI to investigate small-scale halogen chemistry are discussed.