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Märsta, situated in the municipality of Sigtuna, Stockholm County, Sweden, occupies a strategic position within the Stockholm metropolitan area. Its coordinates (59.6167, 17.8500) place it on relatively flat, low-lying terrain characteristic of the Mälaren Basin. This basin, formed by glacial activity during the last ice age, is a dominant geographical feature, influencing local weather patterns and air circulation. Märsta is approximately 30-40 meters above sea level. The town’s proximity to Lake Mälaren, Sweden’s third-largest lake, introduces a moderating influence on temperature, but also potential for humidity and fog formation, which can trap pollutants. Surrounding Märsta is a mix of agricultural land, particularly to the west and south, and expanding suburban development as part of Stockholm’s growth corridor. To the north and east, more forested areas prevail. The urban-rural gradient is therefore quite pronounced, with increasing industrial and transportation activity closer to Stockholm. This proximity to the capital, and the associated Arlanda Airport, is a significant factor in Märsta’s air quality profile, contributing to emissions from road traffic, aviation, and related logistics. The prevailing winds, generally from the west, can transport pollutants from further afield, while local conditions can create pockets of stagnation.

Märsta’s air quality follows a distinct seasonal pattern, largely dictated by meteorological conditions and human activity. Winter (December-February) often sees poorer air quality due to temperature inversions – cold, dense air trapped beneath warmer layers – hindering pollutant dispersion. Reduced sunlight also limits photochemical reactions that break down some pollutants. Wood burning for heating, common in the region, adds particulate matter to the atmosphere. Spring (March-May) brings improving conditions as temperatures rise and winds become more frequent, flushing out accumulated pollutants. However, agricultural activities, such as fertilization, can contribute to ammonia emissions. Summer (June-August) generally offers the cleanest air, with strong winds and ample sunshine. However, increased road traffic during holiday periods can cause localised spikes in nitrogen dioxide. Autumn (September-November) sees a return to more stable atmospheric conditions, coupled with increased heating demand, leading to a gradual decline in air quality. Fog, common during autumn mornings, can exacerbate pollution. Sensitive individuals – children, the elderly, and those with respiratory conditions – should be particularly cautious during winter and autumn months, limiting strenuous outdoor activity on still days. Monitoring pollen levels is also crucial during spring and summer.