Air Pollutant Variability in a Coastal Urban Environment: Measurement-Based Estimation of Ozone Advection Rates from Spatial Gradients During Sea-Breeze Periods

Urban coastal environments exhibit spatial heterogeneity in air pollutant distributions due to complex built environments and diverse emission sources. In these regions, land-sea breeze circulations transport ozone precursors and modulate near-surface ozone (O₃) variability. However, the limited spatial coverage in conventional air quality monitoring networks constrains the ability to resolve these coupled advection-chemistry interactions.

In this study, we conducted spatially dense, multi-point measurements of CO, NO, NO₂, O₃, PM₁₀, and PM_2.5 using a network of cost-effective air quality sensors in Ulsan, a highly industrialized coastal city in South Korea. Sensors were deployed across industrial, residential, forested, and urban background environments. Two-week intensive campaigns in both summer and winter during 2023–2025 enabled characterization of the seasonal and diurnal variability of pollutant distributions.

Pollutant concentrations and diurnal patterns differed distinctly among emission environments. CO and NO concentrations were highest at industrial and residential sites and peaked during morning and evening commuting hours, whereas PM exhibited a more spatially homogeneous distribution. In contrast, surface O₃ decreased with increasing NO_x levels, reflecting enhanced O₃ loss via NO titration during periods of elevated traffic and industrial emissions.

During sea-breeze events. The inland-bound transport of O₃-rich marine air led to pronounced spatial gradients in surface ozone. Ozone levels decreased over industrial and residential areas due to strong NO titration and subsequently increased farther inland in forested regions where NO_x concentrations remained lower. Using these spatial O₃ gradients, we estimated O₃ advection rates and outlined an observationally constrained approach for evaluating the surface O₃ chemical budget. This study are expected to advance the understanding of sea-breeze-driven surface O₃ variability in coastal cities and provide observational constraints for interpreting surface O₃ budgets.