Professor Petäjä’s research has focused on aerosol–cloud–climate–biosphere interactions, combining advanced mass-spectrometric and particle-measurement techniques with long-term observations and coordinated field campaigns. As Principal Investigator of the Biogenic Aerosols–Effects on Clouds and Climate (BAECC) campaign and Scientific Director of the Pan-Eurasian Experiment (PEEX), he has contributed to international efforts to clarify the role of boreal forests in aerosol formation, cloud processes, and climate. He has also played an important role in establishing the headquarters of the ACTRIS European Research Infrastructure Consortium (ERIC) in Helsinki and in integrating Finnish atmospheric research with international infrastructures including ICOS, eLTER, and AnaEE. He has authored 390 peer-reviewed publications, with an h-index of 66, more than 18,300 citations, and 18 papers published in Nature and Science, and has supervised 29 doctoral researchers to completion. His selected works address the BAECC field campaign, aerosol–boundary-layer feedbacks and air pollution in China, experimental observations of strongly bound sulfuric-acid dimers, atmospheric sulfuric acid and hydroxyl-radical concentrations in boreal forests, and integrated approaches to understanding polar environments. Collectively, these contributions demonstrate a sustained research programme linking advanced experimental methodology and long-term observation with the broader scientific challenges of climate change, air quality, and environmental sustainability across northern Eurasia.