Transport Pathways of Nitrate Formed from Nocturnal N 2 O 5 Hydrolysis Aloft to the Ground Level in Winter North China Plain.

Particulate nitrate (NO ₃ ^- ) has currently become the major component of fine particles in the North China Plain (NCP) during winter haze episodes. However, the contributions of formation pathways to ground NO ₃ ^- in the NCP are not fully understood. Herein, the NO ₃ ^- formation pathways were comprehensively investigated based on model simulations combined with two-month field measurements at a rural site in the winter NCP. The results indicated that the nocturnal chemistry of N ₂ O ₅ hydrolysis aloft could contribute evidently to ground NO ₃ ^- at the rural site during the pollution episodes with high aerosol water contents, achieving the contribution percentages of 25.2-30.4% of the total. In addition to the commonly proposed vertical mixing of breaking nocturnal boundary layer in the early morning, two additional transport pathways (frontal downdrafts and downslope mountain breezes) in the nighttime were found to make higher contributions to ground NO ₃ ^- . Considering the dominant role (69.6-74.8%) of diurnal chemistry in NO ₃ ^- formation, reduction of NO _x emissions in the daytime may be an effective control measure for reducing regional NO ₃ ^- in the NCP.