Regime‐Dependence of Nocturnal Nitrate Formation via N2O5 Hydrolysis and Its Implication for Mitigating Nitrate Pollution

The heterogeneous hydrolysis of dinitrogen pentoxide (N2O5) is an important pathway in nitrate formation; however, its formation rate and relative contribution to total particulate nitrate (pNO3‐ ${{\text{pNO}}_{3}}^{\mbox{-}}$) are highly variable. Here we report that nocturnal pNO3‐ ${{\text{pNO}}_{3}}^{\mbox{-}}$ formation via N2O5 hydrolysis is dependent on the regime defined by the ratio of NO2 to O3. Nocturnal pNO3‐ ${{\text{pNO}}_{3}}^{\mbox{-}}$ formation via N2O5 hydrolysis is suppressed in an O3‐limited regime but enhanced in a NO2‐limited regime. The results have crucial implications for effective control of nitrate pollution in the future. An exclusive decrease in NO2 will decrease nocturnal pNO3‐ ${{\text{pNO}}_{3}}^{\mbox{-}}$ formation in a NO2‐limited regime but may be less effective or even increase nocturnal pNO3‐ ${{\text{pNO}}_{3}}^{\mbox{-}}$ formation in an O3‐limited regime. Plain Language Summary Our observations show that nocturnal pNO3‐ ${{\text{pNO}}_{3}}^{\mbox{-}}$ formation via dinitrogen pentoxide (N2O5) hydrolysis in the residual layer over megacity Beijing is more efficient than at ground level. Further investigations suggest nocturnal pNO3‐ ${{\text{pNO}}_{3}}^{\mbox{-}}$ formation via N2O5 hydrolysis is dependent on the regime defined by the ratio of NO2 to O3. Nocturnal pNO3‐ ${{\text{pNO}}_{3}}^{\mbox{-}}$ formation via N2O5 hydrolysis is suppressed in an O3‐limited regime but enhanced in a NO2‐limited regime. As a result, an exclusive decrease in NO2 will decrease nocturnal pNO3‐ ${{\text{pNO}}_{3}}^{\mbox{-}}$ formation in a NO2‐limited regime but may be less effective or even increase nocturnal pNO3‐ ${{\text{pNO}}_{3}}^{\mbox{-}}$ formation in an O3‐limited regime. The above result is also substantiated by observations during the COVID‐19. Key Points Nocturnal pNO3‐ ${{\text{pNO}}_{3}}^{\mbox{-}}$ formation via N2O5 hydrolysis is dependent on the regime defined by the ratio of NO2 to O3 Nocturnal pNO3‐ ${{\text{pNO}}_{3}}^{\mbox{-}}$ formation via N2O5 hydrolysis in the residual layer over megacity Beijing is more efficient than at ground level Nocturnal pNO3‐ ${{\text{pNO}}_{3}}^{\mbox{-}}$ formation via N2O5 hydrolysis is suppressed in an O3‐limited regime but enhanced in a NO2‐limited regime