Homogeneous and heterogeneous photolysis of nitrate in the atmosphere:state of the science,current research needs,and future prospects

Nitrate is an important component of atmospheric particulate matter and affects air quality,climate,human health,and the ecosystem.Nitrate was previously considered a permanent sink for nitrogen oxides(NO_(x)).However,this viewpoint has been challenged in recent years because growing research evidence has shown the transformation of nitrate into NO_(x)(i.e.,renoxification).The photolysis of nitrate/HNO3,especially in the particulate phase or adsorbed on particles,can be a significant renoxification process in the atmosphere.The formation and photolysis of nitrate in aerosol not only change the diurnal variation of NO_(x),but also provide long-distance transport of NO_(x)in the form of nitrate,which affects local and regional atmospheric chemistry and air quality.This review summarizes recent advances in the fundamental understanding of the photolysis of nitrate/HNO3 under various atmospheric conditions,with a focus on mechanisms and key factors affecting the process.The atmospheric implications are discussed and future research is recommended.