Increases and decreases in the fine mode fraction of aerosol optical depth with increasing relative humidity

• Main Author: Bose, Suchismita
• Other Authors: Di Girolamo, Larry
• Format: Thesis
• Language: English
• Published: 2012
• Subjects: geo; envir; Aerosol Robotic Network
• Online Access: http://hdl.handle.net/2142/29662

The total contribution of the small (radius <= 0.35 um) and medium (radius <= 0.7 um) sized particles as a defined by MISR (Multi-Angle Imaging SpectroRadiometer) to the Aerosol Optical Depth (AOD) is correlated to the AERONET (Aerosol Robotic Network) fine mode fraction of AOD. According to this thesis the contribution of particles having radius <= 0.7 um to the measured AOD can either increase or decrease with increasing relative humidity, depending on the initial function of aerosols. Instinctively it appears that since the aerosols swell up according to their hygroscopic properties with an increase in relative humidity, the contribution of particles having radius <= 0.7 um to the AOD (defined here as fine mode fraction of AOD) should decrease. Although this is true for certain size distributions, it is not true for all size distributions. However, often the increase in fine mode fraction of AOD as derived from AERONET/ Satellite based instruments are interpreted as an increase in fine particle pollutants of anthropogenic origin and a decrease in fine mode fraction is attributed to enhanced dust activity or influence of larger marine aerosols without considering the influence of relative humidity. Although fine mode fraction will definitely change if the emission scenario of fine particles changes (e.g. if there is increased crop waste burning or a sudden surge of wind blown dust from elsewhere), but according to the work presented in this thesis, for certain size distributions relative humidity alone is sufficient to explain large increases or decreases in fine mode fraction of AOD. Hence prior to attributing increases and decreases in particle fine mode fraction to changes in emission from different sources, the role of relative humidity and the prevalent particle size distribution in the region must both be accounted for.