The claim that Earth's atmospheric composition and particle count have changed significantly over billions of years is a matter of common geological knowledge, well-documented by the changing records of oxygen, carbon dioxide, and aerosol proxies across Earth's history, so no citation is needed.
The claim states that Earth's atmospheric composition and particle count have changed significantly over geological timescales. This is a fundamental, empirically established truism in Earth sciences (e.g., the Great Oxidation Event, transition from high to low CO2 eras, evolution of proxies as shown in papers [1], [2], [6], [8], [11]). According to the system instructions, a claim that is trivially true by definition or direct everyday/geological observation is common knowledge, requiring no citation.