The atmosphere is composed of a plethora of different gases in different amounts, and of particles, suspended in these gases (aerosols). Atmospheric composition may vary considerably depending on the area of the globe and the proximity to sources (natural and anthropogenic) and can impact on climate, air quality and weather. In this context, APCG Atmospheric Composition focus area studies the variations in aerosol and trace gases levels within the Athens Metropolitan Area. This research, combined with observations, data assimilation, and modeling, improves society’s ability to predict how future changes in atmospheric composition will affect climate, weather, and air quality.