Transboundary haze from biomass burning is one of the most important air pollutions in Southeast Asia.The most recent serious haze episode occurred in 2015.Southern Thailand was affected by the haze during September to October when the particulate matter concentration hit a record high.We investigated physical and chemical characteristics of aerosols,including concentration and aerosol size distribution down to sub-micron sizes during haze episodes in 2013 and 2015 and,for reference,an insignificant haze period in 2017.The highest total suspended particulates and PM10 levels in Hat Yai city were 340.1 and 322.5 lμg/m3.The mass fractions were nanoparticles (＜ 100 nm) 3.1％-14.8％ and fine particles (＜ 1 pm) 54.6％-59.1％.Polycyclic aromatic hydrocarbon size distributions in haze periods peaked at 0.75 pm and the concentrations are 2-30 times higher than the normal period.High molecular weight (4-6 ring) PAHs during the haze episode contribute to about 56.7％-88.0％ for nanoparticles.The average values of benzo(a)pyrene toxic equivalency quotient were 3.34±2.54ng/m3 in the 2015 haze period but only 0.894-0.17 ng/m3 in 2017.It is clear that particles smaller than 1 pm,were highly toxic.Nanoparticles contributed 19.4％-26.0％ of total BaP-TEQ,whereas the mass fraction is 13.1％-14.8％.Thus the nanoparticles were more carcinogenic and can cause greater health effect than larger particles.The fraction of BaP-TEQ for nanoparticles during 2017 non-haze period was nearly the same,while the mass fraction was lower.This indicates that nanoparticles are the significant source of carcinogenic aerosols both during haze and non-haze periods.