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This section covers observed trends in stratospheric water vapour; stratospheric and tropospheric ozone (O3); the O3 precursor gases, nitrogen dioxide (NO₂) and carbon monoxide (CO); and column and surface aerosol. Since trend estimates from the cited literature are used here, issues such as data records of different length, potential lack of comparability among measurement methods and different trend calculation methods, add to the uncertainty in assessing trends.