Particulate matter emissions from a commercially available closed fireplace were determined in terms of particle number and the size distribution, employing a dilution sampling scheme capable of providing near-real-world emission situation. The time profile of the emissions was also traced together with the main gaseous species providing insight onto the particle formation mechanisms in relation with the varying combustion conditions in the batch-wise system. An average emission factor of 1.4 x 10(15) # kg(-1) (7.1 x 10(13) # MJ (1)) was found for total number of particles. Nanoparticles (geometric mean diameter GMD = 28 nm) account for the 32% of the emissions, whereas larger particles (GMD = 127 nm) constitute the 68%. Number emissions were in strict relationship with the combustion conditions in the fireplace. The ignition phase is responsible for the 46% of the nanoparticle burden of the whole burning cycle. Larger particles are thought to be primary soot particles emitted during the flaming combustion phase and coated by condensing semivolatile organic species in the diluted and cooled flue gas. (C) 2013 Elsevier Ltd. All rights reserved.