Aerosol mass-size distributions at a tropical coastal environment: response to mesoscale and synoptic processes

Regular measurements of total mass concentration and mass-size distribution of near-surface aerosols, made using a tell-channel Quartz Crystal Microbalance (qcm) Impactor for the period October 1998-December 1999 at the tropical coastal station Trivandrum (8.5 degreesN, 77 degreesE), are used to study the response of aerosol characteristics to regional mesoscale and synoptic processes. Results reveal that aerosol mass concentrations are generally higher under land breeze conditions. The sea breeze generally has a cleansing effect, depleting the aerosol loading. The continental air (LB regime) is richer in accumulation mode (submicron) aerosols than the marine air. On a synoptic scale, aerosol mass concentration in the submicron mode decreased from an average high value of similar to 86 mug m(-3) during the dry months (January-March) to similar to 11 mug m(-3) during the monsoon season (June-September). On the contrary mass concentration in the supermicron mode increased from a low value of similar to 15 mug m(-3) during the dry months to reach a comparatively high value of similar to 35 mug m(-3) during April, May. Correspondingly, the effective radius (R-eff) increased from a low value of 0.15 0.17 mum to similar to0.3 mum indicating a seasonal change in the size distribution. The mass-size distribution shows mainly three modes, a fine mode (similar to0.1 mum); a large mode (similar to0.5 mum) and a coarse mode (similar to3 mum). The fine mode dominates in winter. In summer the large mode becomes more conspicuous and the coarse mode builds up. The fine mode is highly reduced in monsoon and the large and coarse modes continue to remain high (replenished) so that their relative dominance increases. The size distribution tends to revert to the winter pattern in the post-monsoon season. Accumulation (submicron) aerosols account for similar to 98% of the total surface area and similar to 70% of the total volume of aerosols during winter. During monsoon, even though they still account for similar to 90% of the area, their contribution to the volume is reduced to similar to 50 %; the coarse aerosols account for the rest. (C) 2001 Elsevier Science Ltd. All rights reserved.