Exploring the Influence of Land Use on the Urban Carbonyl Sulfide Budget: A Case Study of the Metropolitan Area of Barcelona

Carbonyl sulfide (OCS) is used to quantify the carbon capture potential of the biosphere because of its direct correlation with CO2 uptake during photosynthesis. However, to constrain the urban biosphere signal, it is necessary to evaluate potential anthropogenic sources. We conducted two sampling campaigns in the Metropolitan Area of Barcelona (AMB), Spain, during May (full COVID lockdown) and October 2020 to measure the spatial distribution and variability of OCS in four urban land uses as follows: built, urban forest, urban park, and peri-urban agriculture. The OCS background levels determined at Tibidabo (442 m asl) were approximately 484 +/- 20 ppt and 407 +/- 8 ppt for May and October 2020, respectively, and agreed with other seasonal surveys conducted in Europe during that same period. Averaged emissions were in the range of +Delta 12 +/- 40 ppt for the city and were +Delta 9.4 +/- 40.9 ppt for urban +Delta 22.1 +/- 48 ppt for urban green +Delta 20.7 +/- 42.9 ppt for agricultural and -Delta 4.8 +/- 19.6 ppt for forest. The urban values ranged from neutral to above background, suggesting nearby anthropogenic and marine emissions such as +Delta 150 ppt in Montjuic, which is downwind of Barcelona's harbor. During the crop-growing season in May, the agricultural areas consistently showed values below the background (up to -Delta 76 ppt in Gava, uptake) at 7:00 UTC when the land breezes were dominant, while later in the morning, when the sea breeze are developed, the plant sink is masked by the transport of marine emissions. Urban forests located north of Tibidabo showed OCS values up to -Delta 70 ppt, suggesting significant uptake by urban forests. We conclude that determining the urban biosphere signal using OCS as a tracer is more complex than expected because the marine and anthropogenic emissions from the port strongly impact the spatial-temporal distribution of OCS. Carbonyl sulfide (OCS) is a gas used to quantify the carbon capture potential of the biosphere. However, cities have potential anthropogenic OCS sources that can impede a clear constraint of the urban biosphere signal. We conducted two sampling campaigns in the Metropolitan Area of Barcelona (AMB), Spain, during May (full COVID lockdown) and October 2020 to measure the variability of OCS in four urban land uses: built, urban forest, urban park, and peri-urban agriculture. The OCS background levels determined at Tibidabo (were approximately 484 +/- 20 ppt and 407 +/- 8 ppt for May and October 2020, respectively). The urban built sites were typically above background, suggesting nearby anthropogenic and marine emissions. During the crop-growing season, the agricultural areas consistently showed values below the background suggesting plant OCS uptake when the land breezes were dominant, while later in the morning, when the sea breezes developed, the influence of marine emission resulted in values above the background. Urban forests showed significant OCS uptake. We conclude that determining the urban biosphere signal using OCS as a tracer is more complex than expected because the marine and anthropogenic emissions from the port strongly impact the spatial-temporal distribution of OCS. The urban built sites were typically above background, suggesting nearby anthropogenic and marine emissionsAgricultural areas consistently showed values below the background suggesting plant Carbonyl sulfide (OCS) uptake when the land breezes were dominantDetermining urban biosphere signal using OCS is complex because marine and anthropogenic emissions impact its spatial-temporal distribution