Using a novel two-stage pyrolysis/combustion system, volatiles produced in situ from the fast pyrolysis of a woody biomass at 800–1000 °C were combusted in a drop-tube furnace (DTF) at 1300 °C to investigate its contribution to submicrometer particle (PM1) emission. During pyrolysis, the majority of Na, K, and Cl (43.5–97.2%) was volatilized and the consequent combustion of Na-, K-, and Cl-containing volatiles contributed substantially to PM1 emission (77.4–89.3% of total PM1 collected from the combustion of volatiles and char). Oppositely, the majority (83.9–97.5%) of Mg and Ca was retained in chars, and as a result, 97.5–99.7% of the yield of total PM1–10 is from char combustion. An increase in the 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 a result of enhanced volatilization of Na, K, and Cl during pyrolysis. The mass-based particle size distributions (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 woody biomass is a key mechanism responsible for PM1 emission. There are strong correlations between the PM1 yields and the contents of (Na + K + Cl), (Na + K), and Cl in the volatiles during combustion.