Influence of micrometeorological features on coastal boundary layer aerosol characteristics at the tropical station, Trivandrum

Characteristics of aerosols in the Atmospheric Boundary Layer (ABL) obtained from a bistatic CW lidar at Trivandrum for the last one decade are used to investigate the role of ABL micrometeorological processes in controlling the altitude distribution and size spectrum. The altitude structure of number density shows three distinct zones depending on the prevailing boundary layer feature; viz. the well-mixed region, entertainment region and upper mixing region. In the lower altitudes vertical mixing is very strong (the well-mixed region) the upper limit of which is defined as aerosol-mixing height, is closely associated with the low level inversion. The aerosol mixing height generally lies in the range 150 to 400 m showing a strong dependence on the vertical eddy mixing processes in ABL. Above this altitude, the number density decreases almost exponentially with increase in altitude with a scale height of 0.5-1.5 km. The aerosol mixing height is closely associated with the height of the Thermal Internal Boundary Layer (TIBL). Sea-spray aerosols generated as a result of the interaction of surface wind with sea surface forms an important component of mixing region aerosols at this location. This component shows a non-linear dependence on wind speed. On an average, depending on the season, the mixing region contributes about 10-30% of the columnar aerosol optical depth (AOD) at 0.5 μm wavelength. A long term increasing trend (∼ 2.8% per year) is observed in mixing region AOD from 1989 to 1997. A study on the development of the aerosols in the nocturnal mixing region shows that the convectively driven daytime altitude structure continues to persist for about 4-5 hrs. after the sunset and thereafter the altitude structure is governed by vertical structure of horizontal wind. Stratified aerosol layers associated with stratified turbulence is very common during the late night hours.