Shipping nitrogen oxides recycling via isoprene nitrate amplifies forest ozone

Forests are essential for climate stability, yet elevated ozone levels-exacerbated by poorly understood impacts of shipping-related nitrogen oxides, stress these ecosystems. Here we combine field sampling at a coastal site southeast of Hong Kong and numerical air quality modelling to analyze nitrogen oxides dynamics in marine and forested environments. Results show that marine air masses influenced by shipping emissions have a higher formation potential for isoprene nitrate (a nitrogen oxides reservoir) than high- nitrogen oxides urban areas like Beijing and Hong Kong. Shipping-derived nitrogen oxides is transported and recycled deep into forests, triggering a cascade that enhances ozone formation. Shipping-linked isoprene nitrate accounts for ~20% of total recycled nitrogen oxides and increases ozone by up to 1 part per billion. The study reveals that nitrogen oxides distribution is more spatially uniform than previously recognized due to these processes, with implications for regional-scale ozone management. Marine air influenced by shipping emissions display greater formation rate of isoprene nitrate than urban areas, which can be transported to forests and enhance ozone formation, according to air sampling in East Asia combined with air quality modelling.