Seasonal variations in composition and sources of atmospheric ultraﬁne particles in urban Beijing based on near-continuous measurements

. Understanding the composition and sources of atmospheric ultraﬁne particles (UFPs) is essential in evaluating their exposure risks. It requires long-term measurements with high time resolution, which are scarce to date. We performed near-continuous measurements of UFP composition during four seasons in urban Beijing using a thermal desorption chemical ionization mass spectrometer, accompanied by real-time size distribution measurements. We found that UFPs in urban Beijing are dominated by organic components, varying seasonally from 68 % to 81 %. CHO organics (i.e., molecules containing carbon, hydrogen, and oxygen) are the most abundant in summer, while sulfur-containing organics, some nitrogen-containing organics, nitrate, and chloride are the most abundant in winter. With the increase of particle diameter, the contribution of CHO organics decreases, while that of sulfur-containing and nitrogen-containing organics, nitrate, and chloride increases. Source apportionment analysis of the UFP organics indicates contributions from cooking and vehicle sources, pho-tooxidation sources enriched in CHO organics, and aqueous/heterogeneous sources enriched in nitrogen-and sulfur-containing organics. The increased contributions of cooking, vehicle, and photooxidation components are usually accompanied by simultaneous increases in UFP number concentrations related to cooking emission, vehicle emission, and new particle formation, respectively, while the increased contribution of the aqueous/heterogeneous composition is usually accompanied by the growth of UFP mode diameters. The highest UFP number concentrations in winter are due to the strongest new particle formation, the strongest local primary particle number emissions, and the slowest condensational growth of UFPs to larger sizes. This study provides a comprehensive understanding of urban UFP composition and sources and offers valuable datasets for the evaluation of UFP exposure risks.

The calculated emission rates for (b) 3-30 nm particles and (c) 30-50 nm particles.The number concentration valley at ~16:00 is possibly caused by the highest MLH, which increases from 8:00 in the morning to the highest in 16:00, then gradually decreases to the lowest until ~18:00.The number concentration peak at ~8:00 is possibly introduced by the emissions of 3-30 nm particles because that the emission of 30-50 nm particles does not increase simultaneously with 3-30 nm particles.The number concentration peak at ~18:00 is accompanied by both the emission of 3-30 nm particles and the 30-50 nm particles, they could be caused by a combined effect of the decrease of MLH, as well as primary emissions, or transportation.As a result, the morning peak at ~8:00 is chosen to study the primary emission rates in the main text.

Figure
Figure S1.(a) Average diurnal variations of the particle size distributions during all the non-NPF days.The calculated emission rates for (b) 3-30 nm particles and (c) 30-50 nm particles.The number concentration valley at ~16:00 is possibly caused by the highest MLH, which increases from 8:00 in the morning to the highest in 16:00, then gradually decreases to the lowest until ~18:00.The number concentration peak at ~8:00 is possibly introduced by the emissions of 3-30 nm particles because that the emission of 30-50 nm particles does not increase simultaneously with 3-30 nm particles.The number concentration peak at ~18:00 is accompanied by both the emission of 3-30 nm particles and the 30-50 nm particles, they could be caused by a combined effect of the decrease of MLH, as well as primary emissions, or transportation.As a result, the morning peak at ~8:00 is chosen to study the primary emission rates in the main text.

Figure S3 .
Figure S3.Details of the measured UFP composition over four seasons.(a) Mass spectra and (b) mass defect of compounds with the top 100 highest signals measured by the TDCIMS.

Figure S4 .
Figure S4.Seasonal variation of UFP mass (a) integrated from TDCIMS signals in negative ion mode; and (b) its comparison with collected UFP masses integrated from size distributions assuming spherical particles with a density of 1.4 g cm -3 .

Figure
Figure S5.Averaged, normalized thermal desorption profiles of (a) the slowly desorbed compounds and (b) the quickly desorbed compounds.The signals are normalized to the corresponding highest signal of the thermal desorption curves.

Figure S6 .
Figure S6.Diurnal variations of CHO organics and the corresponding O3 variation over four seasons.

Figure S8 .
Figure S8.The PMF results for spring.

Figure S11 .
Figure S11.Summary of PMF factors during the four seasons.(a) PMF factor contribution for each season; (b) correlation of 5 factors with the measured key species, trace gas, and PM2.5 for each season.

Figure S12 .
Figure S12.Contribution of different factors as a function of particle size.dp,50 corresponds to 50% volume mean diameter of particles collected on the TDCIMS filament.