Thirdly, a novel two-stage pyrolysis/combustion system is therefore designed to obtain the direct experimental evidence on the contribution of volatiles combustion to PM emission. The combustion of Na-, K- and Cl-containing volatiles, which are produced in situ from the fast pyrolysis of mallee biomass, contributes substantially to PM1 emission. The PM1 yield from volatiles combustion is 77.4 – 89.3% of total PM1 collected from the combustion of both volatiles and char. Oppositely, 97.5 – 99.7% of the yields of total PM1-10 are from char combustion. An increase in pyrolysis temperature leads to an increase in the PM0.1 yields and the mass of Na, K and Cl in PM0.1 from volatiles combustion, as results of enhanced volatilization of Na, K and Cl during pyrolysis. The mass-based PSDs of PM10 and elemental-mass-based PSDs of Na, K, and Cl (which are dominantly contained in PM1) from volatiles combustion generally show a unimodal distribution with a fine mode range from ~0.022 to ~0.043 μm. The mass-based PSDs of PM10 and elemental-mass-based PSDs of Mg and Ca (which are dominantly contained in PM1-10) from char combustion also generally show a unimodal distribution but with a coarse mode of ~6.8 μm. The results clearly demonstrate that the combustion of volatiles (therefore Na, K and Cl included) produced in situ from the fast pyrolysis of biomass is a key mechanism responsible for PM1 emission.