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Jakobstad, or Pietarsaari in Finnish, occupies a unique coastal position in the Ostrobothnia region of Western Finland. Situated at 63.6667° N, 22.7000° E, the city sprawls along the Gulf of Bothnia, a relatively shallow arm of the Baltic Sea. The terrain is generally flat, characteristic of the coastal plains formed during the last ice age, with elevations rarely exceeding 30 meters. This low-lying landscape significantly influences air quality, limiting natural dispersion of pollutants. Historically, Jakobstad developed as a trading port and a centre for wood processing and, later, the metal industry. While heavy industry has declined, remnants contribute to localised pollution sources. Surrounding the city is a predominantly agricultural landscape, dominated by fields of grain and root vegetables, with some forested areas further inland. The urban-rural gradient is relatively sharp, transitioning quickly from the town centre to open farmland. The Baltic Sea’s influence is strong; maritime winds play a crucial role in pollutant transport, sometimes bringing in pollutants from further afield, while also offering some degree of dilution. The proximity to the sea also means potential for salt spray and related aerosol formation, impacting particulate matter composition. The city’s layout, with a compact centre and expanding residential areas, affects local wind patterns and pollutant concentrations.

Jakobstad’s air quality follows a distinct seasonal pattern dictated by meteorological conditions and human activity. Winter, from December to February, sees the lowest sunlight hours and frequent temperature inversions, trapping pollutants near the ground. Wood burning for heating becomes prevalent, contributing to elevated particulate matter (PM) concentrations. While overall emissions may be lower due to reduced industrial activity, the lack of atmospheric mixing worsens air quality. Spring, March to May, brings increasing sunlight and winds, gradually improving conditions, but agricultural activities like soil preparation can release dust. Summer, June to August, generally offers the cleanest air, with prevailing winds dispersing pollutants and higher temperatures promoting atmospheric mixing. However, occasional heatwaves can lead to localised ozone formation. Autumn, September to November, witnesses a return to cooler temperatures and increased wood burning, alongside agricultural harvesting, leading to a gradual decline in air quality. Fog is common during autumn mornings, potentially exacerbating particulate matter issues. Sensitive individuals – children, the elderly, and those with respiratory conditions – should limit strenuous outdoor activity during prolonged temperature inversions in winter and periods of high pollen counts in spring/summer. Monitoring wind direction is advisable, as pollutants can be transported from nearby agricultural areas.