Observations of Plume Transport and Continental Outflow in the Lower Troposphere during the ICARTT 2004 Campaign

An analysis of pollution plumes emitted from coastal sources in the Northeastern U.S. is presented based on observations from the ICARTT 2004 field campaign. Particular attention is given to the relation of these plumes to coastal transport patterns in lower tropospheric layers throughout the Gulf of Maine (GOM) and their contribution to large scale pollution outflow from the North American continent. Using measurements obtained during a series of flights of the NOAA WP-3D and the NASA DC-8, a unique quasi-lagrangian case study is made of a freshly emitted plume emanating from the New York City source region in late July, 2004. Weak synoptic forcing triggers the release of the plume into well defined layers in the shallow troposphere in which it is advected by mean southwesterly flow. Further analysis indicates that the plume layers exhibit similar properties to an internal boundary layer, with signatures of steep vertical gradients in temperature, moisture and wind speed often resulting in periodic turbulence. This layer structure remains well defined during the three day period of the plumes transit to the northern extent of the GOM and the outflow region over western North Atlantic. Airborne trace gas observations additionally indicate minimal exchange with the surface with small levels of marine tracers appearing within the plume layer and mixing ratios of reactive nitrogen and sulfate compounds remaining much higher in coastal and outflow regions than anticipated. The impact of the plume inland was assessed using observations from the AIRMAP air quality network. This impact is noticeably detected as a contribution to poor surface ozone conditions and significant elevations of other major pollutants to levels equaling the highest observed that summer.