Accurate information about air pollutants in the troposphere, such as ozone, particulates, and their respective precursors, is essential to decision-making in domains as diverse as human health, environmental protection, climate change, and agriculture, as well as for assessments of air quality management strategies and policies. Atmospheric observations have only grown in importance as the risks posed by global climate change have come more clearly into focus, and observations will be critical in the years ahead both for tracking the impacts of mitigation strategies and predicting national needs in the realm of adaptation and preparedness. In this book the authors discuss air quality observation systems in the United States; ozone air quality standards; and the EPA’s 2013 changes to the particulate matter standard.