Characteristics of black carbon aerosol mixing state over an urban region deduced using single particle soot photometer (SP2) and differential mobility analyzer (DMA)

Özet

The present study, first over tropics, presents a quantitative approach to characterize aerosol mixing state over an urban location. A differential mobility analyzer (DMA)-single particle soot photometer (SP2) setup was designed to quantify size selected refractory black carbon (rBC) mixing state. Aerosols of size 70, 100, 200, 300, 400 and 500 nm were selected using DMA after which rBC mass concentration, and mixing state quantification were deduced using SP2. Strong correlation exists between rBC mass concentrations and particle number concentrations suggesting that most possible source of rBC is roadways traffic. Observations revealed that refractory externally mixed and internally core-shell mixed BC aerosol population decrease with increase in particle size, while the trend reversed for nonrefractory aerosols. Mixing state information shows that particles at lower size (70 nm) are mostly externally mixed rBC (78.7%), whereas particles at higher size (500 nm) are nonrefractory (74.2%). Further aerosols at lower sizes are more (7.5%) internally core-shell mixed BC particles, whereas higher size particles are less (2.9%) internally core-shell mixed. Particle population near 100 nm has relatively equal number of refractory and nonrefractory particles, however, populations differ below and above 100 nm. These results are important and highlight that aerosols near 100 nm are neutral, and act as barrier for equilibrium mixing of aerosol population. This study offers new insights to connect aerosol physical and chemical processes to evolution of aerosol mixing state as aerosol populations near, below and above 100 nm have different implications to optical and hygroscopic properties of aerosols, and climate.