This chapter discusses the chemistry of troposphere, focusing on the chemical loss processes and physical loss processes. The altitude of the tropopause varies from about 10 km to 18 km, depending on latitude and season; it is highest in the tropics and lowest in the polar regions. This chapter discusses photolysis and then introduces the most important gas-phase species responsible for chemical attack in the troposphere. Atmospheric (gas-phase) compounds undergo chemical transformation by three possible routes: photolysis; chemical attack by free radical and other species; and dissolution in droplets followed by aqueous phase chemical reaction. Along with atmospheric physical removal processes, agents of chemical attack in the troposphere, and nitrogen oxides chemistry, the chapter highlights methane oxidation cycle, chemistry of urban and regional atmosphere, atmospheric reactions of selected nitrogen, and tropospheric aerosols and gas-to-particle conversion.