Biosphere-atmosphere exchange of NOx at the Sundarban mangrove forest along the northeast coast of the Bay of Bengal, India, showed uptake rates of -0.84 to -1.63 ng N m-2 s-1 during the day and both uptake and emission rates of -0.36 to 5.19 ng N m-2 s -1 during the night from September to February. However, during the period from March to August, NOx emission ranged between 0.34 and 2.13 ng N m-2 s-1 and 0.88 and 3.26 ng N m-2 s-1 in daytime and nighttime, respectively. During the postmonsoon period, NOx uptake could be attributed to mangrove stomatal activity during the day. Mangroves absorbed nitrogen from both the soil and the atmosphere. Seasonal and diurnal variability of NOx and O3 is partly due to plant growth in the postmonsoon period. In addition to the NOx-O3 photochemical cycle, stomatal uptake of NO x could also be an important process for keeping a low-ozone state at the land-ocean boundary of the northeast coast of the Bay of Bengal. Copyright 2009 by the American Geophysical Union.