Optical properties of aerosols over a tropical rain forest in Xishuangbanna, South Asia

Abstract

Observation and analysis of the optical properties of atmospheric aerosols in a South Asian tropical rain forest showed that the annual mean aerosol optical depth (AOD) and aerosol Ångström exponent (α) at 500nm were 0.47±0.30 (± value represents the standard deviation) and 1.35±0.32, respectively, from 2012 to 2014, similar with that of Amazon region. Aerosol optical properties in this region varied significantly between the dry and wet seasons. The mean AOD and α were 0.50±0.32 and 1.41±0.28, respectively, in the dry season and 0.41±0.20 and 1.13±0.41 in the wet season. Because of the combustion of the rich biomass in the dry season, fine modal smoke aerosols increased, which led to a higher AOD and smaller aerosol control mode than in the wet season. The average atmospheric humidity in the wet season was 85.50%, higher than the 79.67% during the dry season. In the very damp conditions of the wet season, the aerosol control mode was relatively larger, while AOD appeared to be lower because of the effect of aerosol hygroscopic growth and wet deposition. The trajectories were similar both in dry and wet, but with different effects on the aerosol concentration. The highest AOD values 0.66±0.34 (in dry) and 0.45±0.21 (in wet) both occurred in continental air masses, while smaller (0.38–0.48 in dry and 0.30–0.35 in wet) in oceanic air masses. The range of AOD values during the wet season was relatively narrow (0.30–0.45), but the dry season range was wider (0.38–0.66). For the Ångström exponent, the range in the wet season (0.74–1.34) was much greater than that in the dry season (1.33–1.54).