Flux Measurements for the Ronald H. Brown
The near-surface concentrations of pollutant gases and aerosols results from a complicated balance of sources, sinks, chemical reactions, aerosol dynamics, advection, and turbulent transport. Boundary layer (BL) turbulence is the dominant form of vertical mixing near the surface. The sources of BL turbulence are friction and convection (surface warmer than air) at the surface plus velocity shear and cloud top cooling (upside down convection) in the upper BL. Over the ocean, there tend to be few sources of pollutants of current interest, although the ocean may be a source of gases that react to produce or remove specific pollutants. Thus, when pollutants happen to be brought ashore by a local sea breeze, they typically have their source region somewhere over land (elsewhere) and have been subject to a variety of physical and chemical processes both in and above the BL. The balance of BL versus above the BL processes is highly dependent on the depth of the BL and the strength of turbulent mixing in the BL - BL characteristics that are highly sensitive the original meteorological properties of the BL air overland and the history of its thermal interactions with the ocean surface.
For the BL itself and for gaseous/aerosol properties, key BL variables to consider are surface fluxes, depth of the BL, entrainment velocity/fluxes (BL exchange with the free troposphere), and the BL turbulence intensity (strength of mixing). The BL wind profile may also be important, particularly when there is directional shear. For example, an afternoon convective BL overland (with depth 2 km) might advect over the ocean and quickly form a stable BL with depth 0.1 km. Turbulence in the original BL will decay in about an hour and contact with the surface will be lost for pollutants between 0.1 and 2.0 km. In this layer changes in concentration that depend on surface and entrainment processes will essentially cease until the BL is re-energized by passing over warm land or water. Opposite effects can occur when overland, night time stable BL's are advected over the ocean; highly stratified concentrations from, say elevated smokestacks, could be quickly mixed to the surface over the ocean.
The ETL effort for NEAQS will focus primarily on defining the BL properties and the physical processes that led to the observed properties. This latter aspect is important for improving numerical model realizations of BL properties on the coastal region and for interpreting the observed temporal/spatial evolution of gaseous/aerosol concentrations. The properties of interest will be separated into meteorological and chemical.
The meteorological measurements will feature a combination of episodic (rawinsondes) and continuous (various remote sensors) definition of the mean meteorological profiles on roughly 1-hr time scales. The remote sensors will primarily feature Doppler wind and turbulence profiling (radar, lidar, and sodar). Direct measurements of turbulent fluxes of momentum, heat, and moisture (and associated bulk meteorological sea-air near surface properties) from the ship's foremast will yield key scaling parameters for BL evolution and characterization of surface removal of pollutants. We also propose to do direct turbulent flux measurements for CO2 and O3 to check the NOAA/COARE gas transfer model. A high-resolution ozone lidar will be used to provide continuous monitoring of ozone vertical structure. The combination of O3 surface fluxes and O3 profiles will allow a good 1-D look at the local processes affecting O3 concentration. Solar and IR radiative flux sensors are part of the ETL flux package; they will allow us to close the surface energy budget to provide an important check on model BL results. A ceilometer is included as a simple way to monitor the low cloud statistics; this system also yields some crude information on aerosol backscatter profiles.
High vertical resolution and the ability to resolve the near surface part of the profiles will be important because of the expected occurrences of stable BL cases. Several steps will be taken to improve on the results from NEAQS-2002.
We also suggest a feasibility study of the scanning C-band Doppler radar's ability to resolve spatial information on the BL. This radar is expected to have sufficient sensitivity to acquire usable signal from the marine BL out to ranges on the order of 30 km. Three-dimensional volume scans can be processed to map the inversion within 30-km of the ship. We also anticipate a sharp change in properties at the land-sea breeze front.
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