Direct observational evidence of amplified nitrate formation by the coupled chemical–boundary layer interactions in a Northern China valley

The interactions of aerosols and planetary boundary layer and its critical impacts on environment, weather and climate is a hot issue in the atmospheric environment field at present. The coupled chemical–boundary interactions that drive severe haze events are less understood due to the limited comprehensive vertical measurements，especially for valley cities, in which the formation and development of haze pollution are more complicated and unique. High ozone concentrations promote the formation of nitrate in the nocturnal residual layer, but this phenomenon has not been confirmed and quantified by direct observation. Here, gaseous pollutants, water-soluble ions in PM_2.5 and meteorological factors were simultaneously observed in the stable boundary layer (SBL), residual layer (RL) and mixing layer (ML) in a typical valley city in Northern China. Strong photochemical formation was found in the upper ML during the daytime. At night, nitrate formation was weak due to poor reaction conditions with low ozone and relative humidity (RH) in the SBL near ground level. However, enhanced reaction conditions were observed in the RL, which was associated with the increased precursors transported by the upward valley breeze during the daytime and the increased humidity caused by the temperature reduction; the increased relative humidity and precursors in turn promoted nitrate formation. The mixing down of nitrate from the RL contributes 70% of the surface-level nitrate in the next morning. This vertical feedback, together with the pollutant-trapped terrain, constitutes a key mechanism that links boundary evolution, mountain-valley breeze and nitrate formation in the valley city. This mechanism is self-amplifying, leading to faster chemical production, accumulation, and more severe haze pollution，which was significant to air quality in the valley or basin cities in the world.