The chemical landscape of leaf surfaces and its interaction with the atmosphere

Atmospheric chemists have historically treated plant leaves as inert surfaces that merely emit volatile hydrocarbons through their stomata. However, a growing body of evidence suggests that leaves are ubiquitous substrates for multiphase reactions of atmospheric relevance – implying the presence of chemicals on their surfaces. This Review provides an evidence-based overview of the chemistry and reactivity of the leaf surface’s “chemical landscape”, the dynamic ensemble of molecules and particles that cover plant leaves. We classified chemicals as endogenous (originating from the leaf and its biome) or exogenous (delivered from the environment), highlighting the biological, geographical, and meteorological factors driving their relative contributions. Based on available data, we predicted ≫ 2 µg cm-2 of organics on a typical leaf, leading to a global estimate of ≫ 3 Tg available for multiphase reactions. Our work also highlighted three major knowledge gaps: (i) the key (but still overlooked) role of ambient water in enabling the leaching of endogenous substances and mediating aqueous chemistry; (ii) the role of phyllosphere biofilms in shaping leaf surface chemistry and reactivity; (iii) the paucity of studies on the multiphase reactivity of main atmospheric oxidants with leaf-adsorbed chemicals. Although biased towards available data, we hope this Review will spark a renewed interest on the leaf surface’s chemical landscape and motivate the establishment of multidisciplinary collaborations to move the field forward.