Inferring deposition velocities from changes in aerosol size distributions downwind of a roadway Dongzi Zhu a, * , Hampden D. Kuhns a , John A. Gillies a , Vicken Etyemezian c , Alan W. Gertler a , Scott Brown b a Desert Research Institute, 2215 Raggio Pkwy, Reno, NV 89512, USA b Nevada Tahoe Conservation District, 400 Dorla Court, Zephyr Cove, NV 89448, USA c Desert Research Institute, 755 E Flamingo Road, Las Vegas, NV 89119, USA article info Article history: Received 8 June 2010 Received in revised form 30 October 2010 Accepted 3 November 2010 Keywords: Atmospheric deposition Fugitive dust Road dust Near-source deposition Vehicle-generated dust abstract Deposition velocities for particulate matter (PM) from road traffic in the Lake Tahoe basin were estimated with a mass-balance model and measurements using four size-specific PM profilers and meteorological instruments located at 15 m upwind and 5, 30 and 100 m downwind of major highway in the Lake Tahoe basin. Coarse PM (2.5e10 mm) concentration decreased exponentially with downwind distance from the highway. With tall aspen trees barrier, at 30 m and 100 m downwind, the PM 10 mass concentrations decreased respectively to 51% and 11% from the observed concentrations at 5 m. Particles with diameters between 0.3 and 0.5 mm show almost a homogeneous distribution across the road indicating the smallest particles are unrelated to a road source. Calculated deposition velocities generally increased with particle size and ranged from <0.1 cm s 1 for particles of 1e2.5 mm diameter to 1.6 cm s 1 for particles >15 mm diameter, which is consistent with the Stokes settling velocities, implying the Stokes settling velocities may represent a reliable lower bound on the deposition rate. The wind speed and direction play important roles in determining the deposition velocities of road traffic induced particles. Higher PM concentrations were observed when strong onshore winds dominated during the daytime coupled with high traffic counts. Between late afternoon and early morning, offshore winds blew traffic induced PM towards the Lake, where they could deposit and impact the water clarity. Ó 2010 Elsevier Ltd. All rights reserved. 1. Introduction Atmospheric dry deposition of particulate matter (PM) is a significant non-point source that can impact the water clarity in water bodies like Lake Tahoe (Dolislager et al., 2006), where the water clarity has been decreasing by z0.25 m yr 1 (Goldman, 2000). Located on the CaliforniaeNevada border, Lake Tahoe is an alpine lake (elevation of 1897 m above sea level) that is surrounded by mountains on all sides (Sierra Nevada on the west side and Carson Range on the east side), creating a bowl-shaped topography. In the absence of strong synoptic weather systems (e.g., winter snow storms), strong shallow subsidence and radiation inversions develop throughout the year. Rapid radiation cooling at night generates gentle downslope (offshore) winds toward the lake center. In the daytime, winds are reversed and become (onshore) upslope as the mountains are heated by solar radiation. Fugitive road dust emissions from vehicle travel on paved and unpaved roads have been estimated to account for z45% of ambient PM 10 (PM with aerodynamic diameter  10 mm) in the Lake Tahoe basin (Kuhns et al., 2004). Using year round measure- ments from a vehicle-based road dust emissions study (TRAKER, Kuhns et al., 2001) coupled with the output from a traffic demand model, Zhu et al. (2009) calculated that z140 Mg of PM 10 road dust were emitted from z120 km road around Lake Tahoe per year. Part of this flux was attributed to dust plumes from paved roads that are often observed after traction control material is applied in the winter time. Emissions of PM 10 road dust measured at the source are not necessarily available for deposition into water bodies. Particles must travel a substantial distance over land, where some particles deposited to soil and vegetation. In order to refine the estimates of impact of road dust on lake water clarity, it is impor- tant to quantify the fractional removal of PM 10 particles over rela- tively short distances (on the order of 100s of meters) (Veranth et al., 2003) to estimate the amount of PM 10 that is available for deposition to the lake water surface. * Corresponding author. Tel.: þ1 775 674 7086; fax: þ1 775 674 7007. E-mail address: zhu@dri.edu (D. Zhu). Contents lists available at ScienceDirect Atmospheric Environment journal homepage: www.elsevier.com/locate/atmosenv 1352-2310/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.atmosenv.2010.11.004 Atmospheric Environment 45 (2011) 957e966