Air pollution research is rapidly expanding from compliance metrics to source-resolved, health-relevant evidence that supports action. This book presents international case studies and perspectives on key air pollutants, with emphasis on PM2.5, ultrafine particles, and atmospheric ammonia. Chapters cover chemical characterization and source apportionment, toxicity potential and guideline benchmarking, and the analytical methods needed for accurate measurement of reactive species that drive secondary aerosol formation. The book discusses mitigation options, ranging from source-focused strategies to nature-based approaches such as tolerant plant species, and bridges atmospheric science with practical monitoring and control applications.