This paper presents a study of the evolution of particles and gases downwind of a highway, with a focus on the diurnal variation of pollutant gradients and its controlling variables. A mobile laboratory was used to measure the concentration gradients of ultra-fine particles (UFP), black carbon (BC), CO2, NO, and NO2 at varying distances up to 850 m from a major highway. The horizontal distributions of pollutants show a strong diurnal pattern. Results suggest that the horizontal gradients are predominantly influenced by traffic levels, friction velocity, and atmospheric stability. The results were compared to a dispersion model, which showed good agreement with the measurements and was able to qualitatively capture the observed diurnal cycles. Emission rates [g km(-1)] calculated from the model fits are within 10% of the Mobile 6.2C inventory for CO2 and demonstrate good agreement for NOx, but are higher than the inventory by a factor between 2.0 and 5.9 for black carbon. Hourly NOx emission rates correlate with the fraction of heavy-duty vehicles in the total fleet and agree with inventory values based on maximum vehicle emission rates. Crown Copyright (C) 2012 Published by Elsevier Ltd. All rights reserved.
In August and September of 2010, measurements of turbulent fluxes and turbulent kinetic energy were made on highways in the Toronto area (Ontario, Canada). In situ turbulence measurements were made with a mobile laboratory while driving on the highway with traffic. Results demonstrate that the turbulent kinetic energy (TKE) spectrum is significantly enhanced on and near the highway by traffic for frequencies above 0.015 Hz. The decay of TKE with distance behind vehicles is well approximated by power-law curves. The strongest increase in TKE is seen while following heavy-duty trucks, primarily for frequencies above 0.7 Hz. From these results, a parameterization of on-road TKE enhancement is developed that is based on vehicle type and traffic-flow rate. TKE with distance downwind of the highway also decays following a power law. The enhancement of roadside TKE is shown to be strongly dependent on traffic flow. The effect of vehicle-induced turbulence on vertical mixing was studied by comparing parameterized TKE enhancement with the typical TKE predictions from the Global Environmental Multiscale weather forecast to predict the potential increase in vertical diffusion that results from highway traffic. It is demonstrated that this increase in TKE by traffic may be locally significant, especially in the early morning.
Reliable characterization of particles freshly emitted from the ocean surface requires a sampling method that is able to isolate those particles and prevent them from interacting with ambient gases and particles. Here we report measurements of particles directly emitted from the ocean using a newly developed in situ particle generator (Sea Sweep). The Sea Sweep was deployed alongside R/V Atlantis off the coast of California during May of 2010. Bubbles were generated 0.75 m below the ocean surface with stainless steel frits and swept into a hood/vacuum hose to feed a suite of aerosol instrumentation on board the ship. The number size distribution of the directly emitted, nascent particles had a dominant mode at 55-60 nm (dry diameter) and secondary modes at 30-40 nm and 200-300 nm. The nascent aerosol was not volatile at 230 degrees C and was not enriched in SO4=, Ca++, K+, or Mg++ above that found in surface seawater. The organic component of the nascent aerosol (7% of the dry submicrometer mass) volatilized at a temperature between 230 and 600 degrees C. The submicrometer organic aerosol characterized by mass spectrometry was dominated by non-oxygenated hydrocarbons. The nascent aerosol at 50, 100, and 145 nm dry diameter behaved hygroscopically like an internal mixture of sea salt with a small organic component. The CCN/CN activation ratio for 60 nm Sea Sweep particles was near 1 for all supersaturations of 0.3 and higher indicating that all of the particles took up water and grew to cloud drop size. The nascent organic aerosol mass fraction did not increase in regions of higher surface seawater chlorophyll but did show a positive correlation with seawater dimethylsulfide (DMS).