We compare in situ measurements of ozone (O3) and carbon monoxide (CO) profiles from the CARIBIC program with the results from the regional chemistry transport model (WRF-Chem) to investigate the role of local and regional emissions and long-range transport over southern India during the summer monsoon of 2008. WRF-Chem successfully reproduces the general features of O3 and CO distributions over the South Asian region. However, absolute CO concentrations in the lower troposphere are typically underestimated. Here we investigate the influence of local relative to remote emissions through sensitivity simulations. The influence of 50?% increased CO emissions over South Asia leads to a significant enhancement (upto 20?% in July) in upper tropospheric CO in the northern and central Indian regions. Over Chennai in southern India, this causes a 33?% increase in surface CO during June. However, the influence of enhanced local and regional emissions is found to be smaller (5?%) in the free troposphere over Chennai, except during September. Local to regional emissions are therefore suggested to play a?minor role in the underestimation of CO by WRF-Chem during June–August. In the lower troposphere, a?high pollution (O3: 146.4?±?12.8, CO: 136.4?±?12.2?nmol mol?1) event (15?July?2008), not reproduced by the model, is shown to be due to transport of photochemically processed air masses from the boundary layer in southern India. A sensitivity simulation combined with backward trajectories indicates that long-range transport of CO to southern India is significantly underestimated, particularly in air masses from the west, i.e., from Central Africa. This study highlights the need for more aircraft-based measurements over India and adjacent regions and the improvement of global emission inventories.
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