Air pollution is the most significant environmental risk in the UK, leading to economic costs of £20b/y and significant health inequalities. Quantifying the changing causes of air pollution motivated NERC investment in three fixed air quality supersites - located in urban background locations within London, Birmingham and Manchester, operated via the UKRI SPF project OSCA.

In parallel, the forthcoming (early 2021) revision of WHO guidelines will inform new national air quality targets, within the new Environment Bill, which are likely to reflect population-averaged exposure.

Poor air quality arises from the interaction of emissions, meteorology and atmospheric processes, affecting the loading and toxicity of the species present. Two key uncertainties are

1 The balance between traffic and urban emissions, and pollutants already present in the air arriving from upwind, key to the regional and national policy responsibility for improving air quality.

2 The interaction between spatially varying emissions and chemical processing affecting air quality, including the role of agricultural emissions and transport shifts for Net Zero.

Here, we will develop new UK community capability to address these uncertainties: Flexibly configurable air quality supersite triplets, spanning upwind, roadside and urban observational capability.

UK-AQST comprises the existing fixed supersites (urban), augmented by two mobile "supersites" to study (for example) upwind rural and roadside air composition. The two units will be located within sustainable mobile platforms (one electric van, one trailer for maximum flexibility) operated to national standards and producing open-access data.

The supersites are not traditional monitoring stations - they will comprise highly sophisticated instruments which monitor key species in atmospheric processes such as ammonia (key to aerosol formation), VOCs (key to ozone, secondary organic aerosol and new particle formation), as well as trace metals, nanoparticles and particle composition in near-real time, in addition to regulated gas pollutants. By using a triplet site configuration (rural, urban, roadside), not only can urban and roadside concentration increments be measured, the processing of polluted air to form key secondary pollutants such as nitrate and secondary organic particles, and freshly formed nanoparticles can be viewed in unprecedented detail to yield process understanding.

The triplet observations will generate a step-change in scientific capability for quantifying air pollution sources and processes at a fundamental level, thus consolidating UK's world-leading position in this field. It will produce policy relevant science with significant impact, particularly in informing air quality policy including the validation of approaches accounting for imported emissions, with applications across the UK and for analogous challenges globally.