Impact of atmospheric boundary layer depth variability and wind reversal on the diurnal variability of aerosol concentration at a valley site

• Published in: The Science of the total environment 2014-10, Vol.496, p.424-434
• Main Authors: Pal, S., Lee, T.R., Phelps, S., De Wekker, S.F.J.
• Format: Article
• Language: English
• Subjects: Accumulation mode particles; Aerosols - analysis; Air Pollutants - analysis; Air Pollution - statistics & numerical data; Air quality; Atmosphere - chemistry; Atmospheric boundary layer; Dilution effect; Environmental Monitoring; Lidar; Meteorology; Valley-wind; Wind
• ISSN: 0048-9697; 1879-1026
• DOI: 10.1016/j.scitotenv.2014.07.067

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 2900m AGL (above ground level) to 2200m 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 (0.3–0.7μm) 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. •Role of atmospheric boundary layer depth on particle concentration variability•Only accumulation mode particles are affected by boundary layer dilution effect.•Effect of wind reversal in a valley on the diurnal cycle of particle concentrations.•Decreasing trend in the boundary layer depths resulted in an increasing trend in aerosol concentration.•Effect of boundary layer depth growth rate on aerosol concentration.