Explaining air pollution exceedance days through land use densities

Air pollution is one of the most critical sustainability and public health issues facing megacities in the Global South, and Tehran illustrates this challenge with more than 100 polluted days each year. While traffic is widely acknowledged as the leading emission source, less is known about how land use patterns jointly shape exceedance-day frequencies. This study examines the influence of urban form within a 2 km radius of Tehran's 28 monitoring stations on the number of days exceeding air quality standards. Urban form is operationalized through three spatial density dimensions: green space (area density), industrial activity (count and area density), and street networks (distance density). Using GIS-derived measures and a Generalized Spatial Two-Stage Least Squares (GS2SLS) model, the analysis accounts for both spatial dependence and endogeneity. Findings reveal that connected and proximate green patches are effective in reducing exceedance days, while larger industrial areas and clusters substantially increase them. Street networks play a dual role: higher intersection connectivity raises exceedance frequencies, whereas longer continuous street segment lengths appear to facilitate pollutant dispersion and reduce exceedances. Overall, Tehran's exceedance day burden stems from a complex interaction of "source" and "sink" land uses, with fragmented greenery and concentrated industry particularly worsening conditions in southern districts. Conceptually, the study advances a density-based source-sink modeling framework; methodologically, it integrates GIS and spatial econometrics at a detailed urban scale, and substantively, it provides actionable insights for planners. Strategies such as enhancing continuous green corridors, congestion management, and industrial relocation are essential to mitigate air pollution and promote spatial justice.