An MDE Funded Project
In air quality monitoring, studies have been carried out mostly during day-time thanks to passive ground-based, airborne or satellite measurements. Theory related to turbulent transport in the convective boundary layer is robust. However, the vertical and horizontal transport processes operating at night have been given relatively less attention. The structure of the nocturnal boundary layer (NBL) is primarily determined by complex interactions between the static stability of the atmosphere and processes that govern mechanical generation of turbulence. Turbulence in the NBL is generated as a result of shear associated with changes in wind velocity with height.
Low-level jets (LLJ) are considered a mechanism of turbulence in the NBL. A LLJ refers to a thin stream of fast moving air with a wind speed maximum within the range of 10 to 20 m s-1 between 100 and 1500 m above the ground level, and it is primarily a nocturnal phenomenon that occurs more frequently during spring and summer. The base of the jet is typically 100 to 300 m above the surface. LLJ can extend to a few 100 km in width and ~ 1000 km in length. In the Eastern United States, the nocturnal LLJ (NLLJ) that presents a South-Southwesterly directional wind, typically develops around 00:00 to 07:00 Eastern Standard Time (EST) between the Appalachian Mountains to the Eastern coastline. The NLLJs have the capability to transport air pollutants (gazes and particulates) that most of the time adds up to the pre-existing local pollution sources (natural and/or anthropogenic).
The Atmospheric Lidar Group (ALG) at the University of Maryland, Baltimore County (UMBC) has been contributing to the Maryland Department of the Environment (MDE) effort to localize pollution sources in the Mid-Atlantic region. Upon identification of meteorological conditions favoring the formation of NLLJ during multiday air quality events, LIDAR measurements are carried out to measure aerosol vertical profiles in the atmosphere. The comparison between LIDAR and wind field measurements shows the importance of using remote sensing techniques to study the formation and development of urban NBL.
Example cases from the USAQ Smog Blog of Low Level Jet discussions: