Hygroscopic behavior of water-soluble matter extracted from biomass burning aerosols collected at a rural site in Tanzania, East Africa

In this study, we present the hygroscopic behavior of water-soluble matter (WSM) extracted from biomass burning derived particulate matter 2.5 (PM2.5) aerosols collected at a rural background site in Tanzania during June–August 2011. Hygroscopic growth factors, g(RH), of WSM were measured by hygroscopic tandem differential mobility analyzer (H-TDMA) with an initial dry particle diameter of 100 nm. We observed that the g(RH) of WSM at 90% relative humidity (RH), g(90%)WSM, ranged from 1.10 to 1.47 with an average of 1.25 ± 0.12. The H-TDMA retrieved hygroscopicity parameter of WSM, κWSM, ranged from 0.04 to 0.24 with a mean of 0.11 ± 0.07. We found that the observed g(90%)WSM is positively correlated with PM2.5 mass fractions of K+ (R2 = 0.61), Cl- (0.54), and organic carbon (0.58). Moreover, it well correlates with levoglucosan (0.67) and total diacids (0.76), implying that although the inorganic fraction may be the most important factor to control the hygroscopicity; biomass burning organics play a significant role in the hygroscopicity of Tanzanian aerosols. The lower growth factors obtained over the sampling site are probably due to the formation of less water-soluble potassium oxalate (K2C2O4) or less hygroscopic K2SO4 particles during atmospheric aging. We observed a moderate correlation (R2 = 0.33) between PM2.5 mass fraction of WSOC and g(90%)WSM. The retrieved g(90%)WSOM values ranged from 1.0 to 1.25 with a mean of 1.16 ± 0.05. This study demonstrates that the hygroscopicity of Tanzanian aerosols is largely controlled by the emission of biomass burning products and the subsequent chemical aging during atmospheric transport.