Abstract:
Collocated measurements of the boundary layer evolution and surface ozone, made for the first time at a
tropical rural site (Gadanki 13.5◦N, 79.2◦E, 375 m amsl) in India, are presented here. The boundary layer
related observations were made utilizing a lower atmospheric wind profiler and surface ozone observations
were made using a UV analyzer simultaneously in April month. Daytime average boundary layer height
varied from 1.5 km (on a rainy day) to a maximum of 2.5 km (on a sunny day). Correlated day-to-day
variability in the daytime boundary layer height and ozone mixing ratios is observed. Days of higher
ozone mixing ratios are associated with the higher boundary layer height and vice versa. It is shown
that higher height of the boundary layer can lead to the mixing of near surface air with the ozone rich
air aloft, resulting in the observed enhancements in surface ozone. A chemical box model simulation
indicates about 17% reduction in the daytime ozone levels during the conditions of suppressed PBL in
comparison with those of higher PBL conditions. On a few occasions, substantially elevated ozone levels
(as high as 90 ppbv) were observed during late evening hours, when photochemistry is not intense. These
events are shown to be due to southwesterly wind with uplifting and northeasterly winds with downward
motions bringing ozone rich air from nearby urban centers. This was further corroborated by backward
trajectory simulations.
