Sofia-Orlov most

The Sofia-Orlov most traffic monitoring station in Sofia recorded PM2.5 at 22.8 μg/m³ annual mean (data capture 93.7%), NO2 at 24.8 μg/m³, and ozone at 57.4 μg/m³. The station is operated by MoE Bulgaria and reports under EEA Dataflow E1a as part of Bulgaria's contribution to Directive 2008/50/EC.

Because this is a traffic station (sited within 10 metres of a road kerb), readings here typically run 20–40% higher than at co-located urban background stations for NO2 and PM2.5. Ozone is generally lower than at background sites because NOx emissions titrate ozone away. Station altitude: 555 m above sea level. Coordinates: 42.6929, 23.3344.

The European Environment Agency classifies fixed monitoring stations by dominant emission source (traffic, industrial, or background) and by surrounding land use (urban, suburban, or rural). Siting criteria in Annex III of Directive 2008/50/EC determine whether a station qualifies as a regulatory reference instrument or as informational. Stations within ten metres of a busy road kerb are flagged traffic; stations on the outskirts of urban areas without major near-field sources are flagged urban background; stations more than twenty kilometres from significant sources are rural background. Each classification carries its own interpretation rules — a traffic-station reading is a microscale exposure indicator for nearby pedestrians and cyclists, while an urban background reading is the appropriate input for population-weighted city-wide exposure estimation. Mixing the two without adjustment is the most common analytical error in city-level air quality comparisons.