Barrierless HNO3 formation from the hydrolysis reaction of NO2 with Cl atom in the atmosphere

Abstract

Nitric acid (HNO3) is one of the main pollutants in the atmosphere and has an important effect on the air pollution. However, the formation pathways of HNO3 are still poorly understood in the marine boundary layer (MBL). Here, we show evidence of the formation of HNO3 from chlorine (Cl) radical and nitrogen dioxide (NO2), with 1–3 water molecules, as well as on the air-water interface, based on the direct Born-Oppenheimer molecular dynamics (BOMD) simulation. The relative free energy change along the reaction coordinates for the (Cl)(NO2)(H2O)1 (monohydrate) system was calculated using thermodynamic integration, revealing that the monohydrate system exhibited no free energy barrier and was 26.5 kcal mol−1 exergonic. This suggests that the formation of HNO3 from the monohydrate system is thermodynamically favorable. For all the systems studied, the formation of HNO3 was directly observed using BOMD simulations, and only one water molecule participates in the reaction. However, loop structures, which were expected to be ubiquitous on the air-water interface, were not observed. This study identifies the importance of the reactions of Cl atom, NO2, and water molecules in the formation of HNO3 in the MBL region.