The temporal evolution of UFPs, HCHO, HONO and changes in atmospheric composition in the southeast of UK

This study investigates the temporal evolution of ultrafine particles (UFPs, Dp < 100 nm), sub-micron particles (100 < Dp < 800 nm), and reactive gases including formaldehyde (HCHO) and nitrous acid (HONO) in southeast UK urban air. Data were collected at the Brighton Atmospheric Observatory (BAO) from July 2015 to June 2023 using high-resolution instrumentation, including scanning mobility particle sizers and condensation particle counters, alongside gas analysers and meteorological sensors. UFP number concentrations displayed distinct seasonal and diurnal patterns, with smaller particles (20–70 nm) influenced by local emissions and larger ones (70–800 nm) showing evidence of regional transport. Strong correlations (e.g., R² = 0.79 between N30_50 and N50_70) indicate sequential growth likely driven by coagulation and shared sources. HCHO peaked around midday during summer due to enhanced photochemistry, while HONO levels were higher in colder months, likely due to reduced dispersion and surface-mediated formation. Polar plot analysis revealed direction-specific pollutant enhancements, with elevated levels of NO₂, SO₂, HCHO, and HONO associated with distinct wind sectors, suggesting both local and transported source contributions. These patterns underscore the interplay between emissions, atmospheric processing, and meteorological factors. Hence, the study provides new insights into UFP behaviour and secondary pollutant dynamics in an urban coastal setting. The findings highlight the need for seasonally adaptive air quality strategies and contribute valuable evidence to support public health and regulatory decision-making.