title: Impact of atmospheric boundary layer depth variability and wind reversal on the diurnal variability of aerosol concentration at a valley site.
abstract: The development of the atmospheric boundary layer (ABL) plays a key role in affecting the variability of atmospheric constituents such as aerosols, greenhouse gases, water vapor, and ozone. In general, the concentration of any tracers within the ABL varies due to the changes in the mixing volume (i.e. ABL depth). In this study, we investigate the impact on the near-surface aerosol concentration in a valley site of 1) the boundary layer dilution due to vertical mixing and 2) changes in the wind patterns. We use a data set obtained during a 10-day field campaign in which a number of remote sensing and in-situ instruments were deployed, including a ground-based aerosol lidar system for monitoring of the ABL top height (zi), a particle counter to determine the number concentration of aerosol particles at eight different size ranges, and tower-based standard meteorological instruments. Results show a clearly visible decreasing trend of the mean daytime zi from 2900 m AGL (above ground level) to 2200 m AGL during a three-day period which resulted in increased near-surface pollutant concentrations. An inverse relationship exists between the zi and the fine fraction (0.3-0.7 μm) accumulation mode particles (AMP) on some days due to the dilution effect in a well-mixed ABL. These days are characterized by the absence of daytime upvalley winds and the presence of northwesterly synoptic-driven winds. In contrast, on the days with an onset of an upvalley wind circulation after the morning transition, the wind-driven local transport mechanism outweighs the ABL-dilution effect in determining the variability of AMP concentration. The interplay between the ABL depth evolution and the onset of the upvalley wind during the morning transition period significantly governs the air quality in a valley and could be an important component in the studies of mountain meteorology and air quality.
authors.affiliation: Department of Environmental Sciences, University of Virginia, Charlottesville, VA, USA. Electronic address: sp5hd@Virginia.EDU.
authors.forename: S
authors.initials: S
authors.lastname: Pal
authors.affiliation: Department of Environmental Sciences, University of Virginia, Charlottesville, VA, USA.
authors.forename: T R
authors.initials: TR
authors.lastname: Lee
authors.affiliation: Department of Environmental Sciences, University of Virginia, Charlottesville, VA, USA.
authors.forename: S
authors.initials: S
authors.lastname: Phelps
authors.affiliation: Department of Environmental Sciences, University of Virginia, Charlottesville, VA, USA.
authors.forename: S F J
authors.initials: SFJ
authors.lastname: De Wekker
mesh_terms: D000336:Aerosols / Q000032:analysis*; D000393:Air Pollutants / Q000032:analysis*; D000397:Air Pollution / Q000706:statistics & numerical data*; D001272:Atmosphere / Q000737:chemistry*; D004784:Environmental Monitoring; D055904:Meteorology; D014919:Wind
chemical_list: D000336:Aerosols; D000393:Air Pollutants
keywords: Accumulation mode particles; Air quality; Atmospheric boundary layer; Dilution effect; Lidar; Valley-wind