This claim is a fundamental geographical and meteorological truism regarding the atmospheric distribution of trace gases, so no citation is needed.
The claim states that background concentrations of trace gases and pollutants vary by region and altitude across the troposphere. This is a basic, foundational fact of atmospheric science and geography; variations in altitude, weather patterns, regional emissions, and transport mechanisms inherently cause trace gas concentrations to differ spatially across the troposphere. Consequently, the statement is common knowledge and requires no citation.