Insights into the compositional differences of PM1 and PM2.5 from aerosol mass spectrometer measurements in Beijing, China

Aerosol mass spectrometers (AMS) have been widely used for characterization of submicron aerosol (PM1) species in the world. However, the chemical differences between PM1 and fine particles (PM2.5) in megacities are poorly understood. Here a PM1 high-resolution AMS with standard vaporizer (SV) and a PM2.5 time-of-flight aerosol chemical speciation monitor (ToF-ACSM) with capture vaporizer (CV) were deployed simultaneously in summer and winter in Beijing, China, and the chemical differences of non-refractory aerosol species between PM1 and PM2.5 were characterized. Our results showed that aerosol species in PM2.5 were overall highly correlated with those in PM1 (r2 > 0.8), yet the PM1/PM2.5 ratios varied largely for different aerosol species. Nitrate existed dominantly in PM1 (80-90%), while a considerable fraction of sulfate (30-45%) was observed in the range of 1-2.5 mu m (PM1-2.5). Comparatively, PM1 organics showed higher fractions in PM2.5 in summer than winter (similar to 70%). The changes in PM1/PM2.5 ratios of secondary inorganic aerosol (SIA) depended strongly on relative humidity (RH) in summer with considerable decreases as the increase of RH, while such a RH depen-dence of PM1/PM2.5 ratios was not significant in winter due to the absence of high RH data. We further compared the source apportionment results of organic aerosol (OA) between PM1 and PM2.5. Similar to previous studies, the SV AMS reported ubiquitously higher primary OA than the CV ToF-ACSM, nearly by a factor of 2 for cooking OA, and the difference in secondary OA composition can be substantial, particularly in winter. With the simultaneous measurements of AMS and ACSM, a parameterized relationship between oxygen-to-carbon (O/C) ratio and CV f44 (fraction of m/z 44 in OA) in summer and winter was developed, which has a potential implication for studying the oxidation and hygroscopicity properties of OA considering the rapid increases in CV ToF-ACSM measure-ments worldwide.