Observational assessment of the role of nocturnal residual-layer chemistry in determining daytime surface particulate nitrate concentrations

摘要

This study discusses an analysis of combined airborne and ground observations of particulate nitrate (NO3- ((p))) concentrations made during the wintertime DISCOVER-AQ (Deriving Information on Surface Conditions from COlumn and VERtically resolved observations relevant to Air Quality) study at one of the most polluted cities in the United States - Fresno, CA - in the San Joaquin Valley (SJV) and focuses on developing an understanding of the various processes that impact surface nitrate concentrations during pollution events. The results provide an explicit case-study illustration of how nighttime chemistry can influence daytime surface-level NO3- ((p)) concentrations, complementing previous studies in the SJV. The observations exemplify the critical role that nocturnal chemical production of NO3- ((p)) aloft in the residual layer (RL) can play in determining daytime surface-level NO3- ((p)) concentrations. Further, they indicate that nocturnal production of NO3- ((p)) in the RL, along with daytime photochemical production, can contribute substantially to the buildup and sustaining of severe pollution episodes. The exceptionally shallow nocturnal boundary layer (NBL) heights characteristic of wintertime pollution events in the SJV intensify the importance of nocturnal production aloft in the residual layer to daytime surface concentrations. The observations also demonstrate that dynamics within the RL can influence the earlymorning vertical distribution of NO3- ((p)), despite low wintertime wind speeds. This overnight reshaping of the vertical distribution above the city plays an important role in determining the net impact of nocturnal chemical production on local and regional surface-level NO3- ((p)) concentrations. Entrainment of clean free-tropospheric (FT) air into the boundary layer in the afternoon is identified as an important process that reduces surface-level NO3- ((p)) and limits buildup during pollution episodes. The influence of dry deposition of HNO3 ga