Role of transport in elevated CO levels over Delhi during onset phase of monsoon

While local anthropogenic emission sources contribute significantly to pollutant levels in a megacity Delhi, meteorology can significantly modulate carbon monoxide (CO) levels. We hereby demonstrate that the concentration of CO during the unusual onset of monsoon 2013 was dramatically elevated and crossed the permissible limit (1.7 ppmv) for about a month long-period as against normal onset of monsoon where CO concentration generally drops down significantly. We compare level of CO in 2013 with normal monsoon year 2012. We provide a hypothesis to interpret this long lasting high concentration CO episode and validate using interactive high resolution WRF-Chem model. The model indicates that the high CO episode was caused by an unusually active Bay of Bengal branch of monsoon which bounces off from the east and transports CO rich air from the eastern part of the Indo Gangetic Plains to Delhi.