Vertical profiles of aerosol black carbon in the atmospheric boundary layer over a tropical coastal station: Perturbations during an annular solar eclipse

Altitude profiles of aerosol black carbon (BC) in the atmospheric boundary layer (ABL) over a tropical coastal station, Trivandrum have been examined on two days using an aethalometer attached to a tethered balloon. One of these days (15th January, 2010) coincided with a (annular) solar eclipse, the longest of this century at this location, commenced at 11:05 local time and ended by 15:05, lasting for 7 min and 15 s (from 13:10:42), with its maximum contact occurring at ~ 13:14 IST with ~ 92% annularity, thereby providing an opportunity to understand the eclipse induced perturbations. Concurrent measurements of the ABL parameters such as air temperature, relative humidity and pressure were also made on these days to describe the response of the ABL to the eclipse. BC profiles, in general, depicted similar features up to an altitude of ~ 200 m on the eclipse day and control day, above which it differed conspicuously with profiles on eclipse day showing increasingly lower concentration as we moved to higher altitudes. Examination of the meteorological profiles showed that the altitude of maximum convection rapidly fell down during the eclipse period compared to that on control day indicating a rather shallow convection on eclipse day. Comparison of diurnal variations of BC at the surface level showed that the rate of decrease in BC during daytime on the eclipse day was smaller than that on the control day due to the reduced convection, shallow ABL and consequent reduction in the ventilation coefficient. Moreover the time of the nocturnal increase has advanced by ~ 1:30 h on the eclipse day, occurred at around 19:30 IST in contrast to all the other days of January 2010, where this increase usually occur well after 20:30 IST, with a mean value of 21:00 IST. This is attributed to the weak sea-breeze penetration during the eclipse day, which led to an early onset of the land breeze.