Knowledge　of　the　molecular-level　chemistry　of　brown　carbon　(BrC)　is　important　in　reducing　the　uncertainties　in　aerosol　radiative　forcing.　Time-resolved　ambient　PM2.5　samples　were　collected　during　a　severe　pollution　episode　in　January　2017　over　Xi＇an,　China　for　a　comprehensive　nontarget　and　full　scanning　of　BrC　molecules　and　their　absorption　properties　using　electrospray　ionization　Fourier　transform–ion　cyclotron　resonance　mass　spectrometry　combined　with　partial　least　squares　regression　analysis,　which　apportioned　the　overall　ultraviolet　absorption　to　individual　molecules.　The　estimated　absorption　of　CHNO　and　CHNOS　molecules　exhibited　nighttime　prevalence,　whereas　CHOS,　CHNS,　CHN,　CHO,　CHS,　and　CH　molecules　presented　a　dynamic　trend.　Carbon　conjugation　was　positively　correlated　with　estimated　absorption　by　CHO　and　CHNO　molecules,　while　exhibiting　a　mixed　relationship　with　CHNOS.　Higher　nitrogen　content　was　associated　with　enhanced　light-absorption　properties　of　BrC　molecules,　while　higher　oxygen　and　sulfur　content　appeared　to　be　associated　with　photobleaching　during　secondary　transformation.　©2020.　American　Geophysical　Union.　All　Rights　Reserved.