Measurements of submicron organonitrate particles: Implications for the impacts of NO_x pollution in a subtropical forest

Organonitrates (ONs) are an important component of atmospheric aerosols that signify the perturbation of nitrogen oxides (NOx) pollution in the remote atmosphere. This study investigated the mass concentration and molecular composition of ONs in submicron particulate matters (PM1) at a subtropical forest site (Xitou), which located downwind the Taichung metropolis in the Central Taiwan. Here we showed that transport of urban air pollution with local circulations caused elevation in the ambient concentration of ONs in the forested area. In addition, the levels of ONs were positively correlated with ozone mixing ratio and were negatively correlated with the relative humidity, suggesting that the ONs were regulated by photochemistry and hydrolysis. The results also showed that the mass concentration of the ONs was significantly correlated with the number density of nanoparticles (d < 100 nm), implying the role of ONs in the formation and/or growth processes of atmospheric aerosols. Mass spectra analysis revealed five major ONs, which were characterized by the molecular skeletons of either isoprene or monoterpenes. The molecular profiles suggested that the reactions between nitrate radicals and biogenic volatile organic compounds (BVOCs) significantly contributed to the formation of ONs, which thereby represented a major sink of the functional BVOCs in the forested area.