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Abstract. Biomass burning and primary biological aerosol particlesaccount for an important part of urban aerosols. Floods of studies have beenconducted on the chemical compositions of fine aerosols (PM2.5) inmegacities where the haze pollution is one of the severe environmentalissues in China. However, little is known about their size distributions inatmospheric aerosols in the urban boundary layer. Here, size-segregatedaerosol samples were collected in Beijing during haze and clear days fromApril 2017 to January 2018. Three anhydrosugars, six primary saccharides andfour sugar alcohols in these samples were identified and quantified by gaschromatography/mass spectrometry (GC/MS). Higher concentrations of a biomassburning tracer, levoglucosan, were detected in autumn and winter than in otherseasons. Sucrose, glucose, fructose, mannitol and arabitol were moreabundant in the bloom and glowing seasons. A particularly high level oftrehalose was found in spring, which was largely associated with the Asiandust outflows. Anhydrosugars, xylose, maltose, inositol and erythritolare mainly present in the fine mode (&lt;2.1 µm), while the othersshowed the coarse-mode preference. The concentrations of measured tracers ofbiomass burning particles and primary biological particles in the hazeevents were higher than those in the non-hazy days, with enrichment factorsof 2–10. Geometric mean diameters (GMDs) of molecular markers of biomassburning and primary biological aerosols showed that there was no significantdifference in the coarse mode (&gt;2.1 µm) between the hazeand non-haze samples, while a size shift towards large particles and largeGMDs in the fine fraction (&lt;2.1 µm) was detected during thehazy days, which highlights that the stable meteorological conditions withhigh relative humidity in urban Beijing may favor the condensation oforganics onto coarse particles.The contributions of reconstructed primaryorganic carbon (POC) by tracer-based methods from plant debris, fungalspores and biomass burning to aerosol OC in the total-mode particles werein the ranges of 0.09 %–0.30 % (on average 0.21 %), 0.13 %–1.0 %(0.38 %) and 1.2 %–7.5 % (4.5 %), respectively. This study demonstratesthat the contribution of biomass burning was significant in Beijingthroughout the whole year with the predominance in the fine mode, while thecontributions of plant debris and fungal spores dominated in spring andsummer in the coarse mode, especially in sizes &gt;5.8 µm. Our observations demonstrate that the sources, abundance and chemicalcomposition of urban aerosol particles are strongly size dependent inBeijing, which is important to better understand the environmental andhealth effects of urban aerosols and should be considered in air quality and climate models.

Molecular markers of biomass burning and primary biological aerosols in urban Beijing: size distribution and seasonal variation