A study of the Interaction Between Sea Breezes and the Urban Boundary Layer from the ClearfLo Project

Urban air pollution concentrations are controlled by a complex combination of chemical reactions, local and regional emissions, and urban boundary layer processes, particularly for cities near to coastlines. The ClearfLo project (www.clearflo.ac.uk) was a large multi-institution UK project that established integrated measurements of the meteorology, composition and particulate loading of London's urban atmosphere, accompanied by modelling of urban meteorology and air pollution. The aim was to understand the processes controlling pollutants with the most serious health impacts (O3, NOx and particulate matter) and their interactions with the urban atmospheric boundary layer. The project established a new long-term measurement infrastructure in London, encompassing measurement capabilities at street level, in the urban background, at elevated levels and in the rural surroundings, capable of determining the urban increment in meteorology and air pollution. Aside from the two-year core observation period, two intensive observation periods (IOPs) were held, in January/February 2012 and during the Olympics in summer 2012. This talk will focus on the impact of sea breezes on boundary layer mixing and urban air quality, as observed during Clearflo. Previous studies have shown that sea breezes occasionally reach London, given its relative proximity to eastern and southern coastlines, but their impact on air quality has not yet been established.

A particularly well-observed sea breeze event occurred during the summer IOP on the evening of 25th July 2012. A network of Doppler lidars and ceilometers in central London showed enhanced aerosol backscatter as the sea breeze front passed each site. Backscatter increased with height over the following hour, coinciding with strong upward motion and enhanced turbulent mixing at all heights in the boundary layer, as observed by Doppler lidar. Subsequently, over several hours, a layer of clean air associated with high directional wind shear was observed to cap a more quiescent layer at the surface. During the passage of the sea breeze accompanying peaks in NOX and Black Carbon concentrations were observed. Surface ozone concentrations, which usually decreased significantly overnight, were observed to remain high after the sea breeze passage. It is hypothesized that the evening sea breeze front caused rapid downward mixing of ozone-rich air from the residual layer aloft, thus worsening nocturnal air quality. Numerical simulations were made using the UK Met Office Unified Model at 100 m horizontal resolution with a 3D Smagorinsky turbulence closure scheme. Simulations compared well with the observations in simulating the timing, depth and wind characteristics of the sea breeze, and broadly capturing the change in particulate structure in the boundary layer due to its passage. A fuzzy logic algorithm was developed that allowed automatic detection of sea breezes from the meteorological observations, and showed that multiple sea breezes were observed during the Clearflo study, allowing the case study to be put into context.