Carbon monoxide (CO) is a toxic trace pollutant contributing indirectly to radiative forcing and global warming. It affects tropospheric greenhouse gases like methane, ozone etc. Thus CO plays a role both in air pollution and climate change. CO is detrimental to humans & other oxygen-breathing organisms. Man-made sources of CO include incomplete combustion of fossil fuels and biomass. Of the total amount of CO present in the atmosphere about 73% is from motor vehicular exhausts. As a fast developing country, India is known to be a region of high levels of CO emissions. Since continuous monitoring of air pollutants like CO help us to estimate emissions near the ground, its transport and its response to the changes in the atmosphere, ambient CO at a coastal station in the South India has been monitored continuously for a period of 7 years. As CO acts as a good tracer of vertical and horizontal atmospheric trasport, distribution of CO in the troposphere provides information on the mid and upper troposphere. This book mainly focusses on the vertical and horizontal distribution and dispersion of CO in the troposphere, its variation with changing topography, circulation & meteorology.