Tropospheric nitrogen dioxide from satellite measurements: SCIAMACHY limb/nadir matching and multi-instrument trend analysis

Abstract

Tropospheric NO2, a key air pollutant particularly in cities, has been measured from space since the mid-1990s by the GOME, SCIAMACHY, OMI, and GOME-2 instruments. These data provide a unique global long-term dataset of tropospheric pollution. The focus of this thesis is twofold: When these satellite measurements are to be used for assessing tropospheric emissions and pollution, it is necessary to separate the stratospheric from the tropospheric signal. This thesis develops a new unique technique for this separation by using the measurements performed by SCIAMACHY in limb geometry. The stratospheric NO2 measurements from SCIAMACHY are shown to be in very good agreement with NO2 fields modeled by the Oslo CTM2. However, both stratospheric datasets need to be adjusted to the level of the nadir measurements, because a time- and latitude-dependent bias to the measured nadir columns can be observed over clean regions. By combining measurements of total and stratospheric air masses taken by the same instrument, the uncertainties commonly introduced by unjustified assumptions and spatial averaging and smoothing can be significantly reduced, leading to the best dataset of tropospheric NO2 slant columns currently available. The tropospheric columns can then be used to assess the level of air pollution on a regional and even local scale. However, the observations of the four instruments differ in spatial resolution, local time of measurement, viewing geometry, and other details, and all these factors can severely impact the retrieved NO2 columns. Therefore, the analysis of temporal changes in troposheric NO2 abundances using these measurements is challenging and not straightforward. In the second part of this thesis, several methods to account for these instrumental differences are developed and applied to the analysis of trends in tropospheric NO2 columns over megacities. The first method is based on spatial averaging of the measured SCIAMACHY earthshine spectra and extraction of a spatial pattern of the resolution effect. Furthermore, two empirical corrections, which summarize all instrumental differences by including instrument-dependent offsets in a fitted trend function, are developed. These methods are applied to data from GOME and SCIAMACHY separately, to the combined time series, and to an extended dataset comprising also OMI and GOME-2 measurements. All approaches show consistent trends of tropospheric NO2 for a selection of areas on both regional and city scales, for the first time allowing consistent trend analysis of the full time series at high spatial resolution. Measured tropospheric NO2 columns have been strongly increasing over China, the Middle East, and India, with values over east-central China tripling from 1996 to 2011. All parts of the developed world, including Western Europe, the United States, and Japan, show significantly decreasing NO2 amounts in the same time period. On a megacity level, individual trends can be as large as 27.2 ± 3.9 % / yr and 20.7 ± 1.9 % / yr in Dhaka and Baghdad, respectively, while Los Angeles shows a very strong decrease of −6.00 ± 0.72 % / yr. Most megacities in China, India, and the Middle East show NO2 columns increasing by 5 to 10 % / yr, which leads to a doubling to tripling within the study period.