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. Although there are many studies of particulate matter (PM) pollution inBeijing, the sources and processes of secondary PM species during hazeperiods remain unclear. Limited studies have investigated the PM formationin highly polluted environments under low- and high-relative-humidity (RH)conditions. Herein, we present a systematic comparison of species insubmicron particles (PM 1 ) in wintertime Beijing (29 December 2014 to 28 February 2015) for clean periods and pollution periods under low- and high-RH conditions. PM 1 species were measured with an aerosol chemical speciesmonitor (ACSM) and an Aethalometer. Sources and processes for organicaerosol (OA) were resolved by positive matrix factorization (PMF) witha multilinear engine 2 (ME-2). The comparisons for clean, low-RH pollutionand high-RH pollution periods are made from three different aspects, namely(a) mass concentration, (b) mass fraction and (c) growth rate in diurnalprofiles. OA is the dominant component of PM 1 , with an average massconcentration of 56.7  µ g m −3 (46 %) during high-RH pollution and67.7  µ g m −3 (54 %) during low-RH pollution periods. Sulfate hadhigher concentration and mass fraction during high-RH pollution periods,while nitrate had higher concentration and mass fraction during low-RHpollution periods. The diurnal variations of nitrate and oxygenated organicaerosol (OOA) showed a daytime increase in their concentrations during allthree types of periods. Nitrate had similar growth rates during low-RH (0.40  µ g m −3  h −1 ) and high-RH (0.55  µ g m −3  h −1 )pollution periods. OOA had a higher growth rate during low-RH pollutionperiods (1.0  µ g m −3  h −1 ) than during high-RH pollutionperiods (0.40  µ g m −3  h −1 ). In contrast, sulfate had adecreasing trend during low-RH pollution periods, while it increasedsignificantly with a growth rate of 0.81  µ g m −3  h −1 duringhigh-RH pollution periods. These distinctions in mass concentrations, massfractions and daytime growth rates may be explained by the difference inthe formation processes affected by meteorological conditions. Inparticular, photochemical oxidation and aqueous-phase processes may bothproduce sulfate and nitrate. The relative importance of the two pathways,however, differs under different meteorological conditions. Additional OOAformation under high-RH (> 70 %) conditions suggestsaqueous-related formation pathways. This study provides a general picture ofthe haze formation in Beijing under different meteorological conditions.

Contrasting sources and processes of particulate species in haze days with low and high relative humidity in wintertime Beijing