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Airborne particulate matter (PM) is a major health risk in urban settings. Ammonia (NH₃) from vehicle exhaust is an under-recognised ingredient in the formation of inorganic PM and there remains a shortage of data to properly quantify the role of NH₃ from vehicles in PM formation. An Open-path Fourier transform infra-red (OP-FTIR) spectrometer measured atmospheric NH₃, carbon monoxide (CO) and carbon dioxide (CO₂) at high temporal resolution (5 min) in Western Sydney over 11 months. The oxides of nitrogen (NO₂ and NO; NO_x) and sulphur dioxide (SO₂) were measured at an adjacent air quality monitoring station. NH₃ levels were maxima in the morning and evening coincident with peak traffic. During peak traffic NH₃:CO ratio ranged from 0.018 to 0.022 ppbv:ppbv. Results were compared with the Greater Metropolitan Region 2008 (GMR2008) emissions inventory. Measured NH₃:CO was higher during peak traffic times than the GMR2008 emissions estimates, indicating an underestimation of vehicle NH₃ emissions in the inventory. Measurements also indicated the urban atmosphere was NH₃ rich for the formation of ammonium sulphate ((NH₄)₂SO₄) particulate was SO₂ limited while the formation of ammonium nitrate (NH₄NO₃) was NH₃ limited. Any reduction in NO_x emissions with improved catalytic converter efficiency will be accompanied by an increase in NH₃ production and potentially with an increase in NH₄NO₃ particulate.