To understand air chemistry, we need to quantify the reactive compounds in the atmosphere: their sources, concentrations and impacts on atmospheric chemistry. One of our goals is to increase our process-based understanding of chemical mechanisms in the lower part of the troposphere. This includes the emissions of biogenic volatile organic compounds from different ecosystems with a special focus on boreal region, their vertical and horizontal distribution and their chemical reaction paths. Another primary research objective is a fundamental physical and chemical understanding of the atmospheric transformation of volatile organic emissions, especially related to their potential form condensable vapors that contribute to organic particulate matter. The cornerstone of this study is mass spectrometry, used to investigate compounds both in the gas and particle phase.