LAUSR

PM2.5 filter samples were collected in July and October 2014 and January and April 2015 in urban Shanghai, and analyzed using ultra-high-performance liquid chromatography (UHPLC) coupled to Orbitrap mass spectrometry (MS). The measured chromatogram-mass spectra were processed by a non-target screening approach to identify significant signals. In total, 810-1510 chemical formulas of organic compounds in the negative polarity (ESI-) and 860-1790 in the positive polarity (ESI+), respectively, were determined. The chemical characteristics of organic aerosols (OAs) in Shanghai varied among different months and between daytime and nighttime. In the January samples, organics were generally richer in terms of both number and abundance, whereas those in the July samples were far lower. More CHO- (compounds containing only carbon, hydrogen, and oxygen and detected in ESI-) and CHOS- (sulfur-containing organics) were found in the daytime samples, suggesting a photochemical source, whereas CHONS- (nitrogen- and sulfur-containing organics) were more abundant in the nighttime samples, due to nocturnal nitrate radical chemistry. A significant number of mono- and polycyclic aromatic compounds, and nitrogen- and sulfur-containing heterocyclic compounds were detected in all samples, indicating that biomass burning and fossil fuel combustion made important contributions to the OAs in urban Shanghai. Additionally, precursor-product pair analysis indicates that the epoxide pathway is an important formation route for organosulfates observed in Shanghai. Moreover, a similar analysis suggests that 35-57% of nitrogen-containing compounds detected in ESI+ could be formed through reactions between ammonia and carbonyls. Our study presents a comprehensive overview of OAs in urban Shanghai, which helps to understand their characteristics and sources.