This chapter introduces the importance of atmospheric chemistry, focusing on air pollution and its health impacts. It highlights that air pollution, often referred to as an "invisible pandemic," affects millions of people annually. The chapter discusses the history of air pollution and its global impact, emphasizing the role of chemical composition in defining the severity of emissions. Part 1 explores why air pollution matters, citing the World Health Organization's statement that breathing polluted air kills millions and harms billions. It also delves into the chemical composition of the atmosphere, noting that while most of it is composed of nitrogen, oxygen, and氩, trace amounts of volatile organic compounds (VOCs) play a crucial role. Part 3 examines chemical reactions in the troposphere, comparing the atmosphere to a combustion engine. It explains how VOCs, similar to fuel, react with other atmospheric components like NOₓ, sunlight, and ozone to form a complex autocatalytic cycle. The chapter details various oxidants and their reactions, including hydroxyl radicals, nitrate radicals, atomic chlorine, and peroxyl radicals, highlighting their roles in cleaning and degrading the atmosphere. Part 4 discusses chemical reactions in the stratosphere, focusing on the ozone layer and the Chapman Cycle. It explains how ozone production and destruction are balanced by number density, photon flux, and absorption cross sections of O₂ and O₃. The chapter also covers the catalytic destruction of ozone by chlorine and bromine compounds, leading to the formation of the ozone hole. Overall, the chapter underscores the complexity and importance of atmospheric chemistry in understanding and mitigating air pollution and its health consequences.This chapter introduces the importance of atmospheric chemistry, focusing on air pollution and its health impacts. It highlights that air pollution, often referred to as an "invisible pandemic," affects millions of people annually. The chapter discusses the history of air pollution and its global impact, emphasizing the role of chemical composition in defining the severity of emissions. Part 1 explores why air pollution matters, citing the World Health Organization's statement that breathing polluted air kills millions and harms billions. It also delves into the chemical composition of the atmosphere, noting that while most of it is composed of nitrogen, oxygen, and氩, trace amounts of volatile organic compounds (VOCs) play a crucial role. Part 3 examines chemical reactions in the troposphere, comparing the atmosphere to a combustion engine. It explains how VOCs, similar to fuel, react with other atmospheric components like NOₓ, sunlight, and ozone to form a complex autocatalytic cycle. The chapter details various oxidants and their reactions, including hydroxyl radicals, nitrate radicals, atomic chlorine, and peroxyl radicals, highlighting their roles in cleaning and degrading the atmosphere. Part 4 discusses chemical reactions in the stratosphere, focusing on the ozone layer and the Chapman Cycle. It explains how ozone production and destruction are balanced by number density, photon flux, and absorption cross sections of O₂ and O₃. The chapter also covers the catalytic destruction of ozone by chlorine and bromine compounds, leading to the formation of the ozone hole. Overall, the chapter underscores the complexity and importance of atmospheric chemistry in understanding and mitigating air pollution and its health consequences.