Wind-Driven Emissions of Coarse Mode Particles in an Urban Environment
MARKUS PETTERS, Tyas Pujiastuti, Ajmal Rasheeda Satheesh, Sabin Kasparoglu, Bethany Sutherland, Nicholas Meskhidze,
North Carolina State University Abstract Number: 369
Working Group: Urban Aerosols
AbstractEmissions of coarse mode particles with a diameter greater than 0.5 µm provide giant cloud condensation nuclei and ice nuclei. Here we introduce a new method that uses lidar retrievals of the elastic backscatter and Doppler velocity to obtain surface number emissions of particles with diameters greater than 0.53 µm. Backscatter flux is calculated using the eddy-covariance technique. The attenuated backscatter is related to particle number concentration using an empirical calibration between attenuated backscatter stratified by relative humidity and particle number concentration above a certain size threshold measured by an optical particle counter at the surface. Number fluxes are then obtained from the backscatter flux through this calibration. The technique is applied to study particle number fluxes over a two month period during the TRACER campaign at an urban site near Houston, TX, USA. All of the observed fluxes were positive (upwards) indicating particle emission from the surface. The fluxes followed a diurnal pattern and peaked near noon local time. Flux intensity varied through the two months with multi-day periods of strong fluxes and multi-day periods of weak fluxes. Emission particle number fluxes peaked near ~100 cm
-2 s
-1. The daily averaged emission fluxes correlated with friction velocity and were anticorrelated to surface relative humidity. The emission flux can be parameterized as
F = 3000u*
4 where u* is the friction velocity in m s
-1 and the emission flux
F is in cm
-2 s
-1. The relationship of the flux with friction velocity is consistent with emission from wind-driven erosion. These results demonstrate that urban environments may play an important role in supplying coarse mode particles to the boundary layer.