Highly　oxygenated　molecules　(HOMs)　play　an　important　role　in　the　formation　and　evolution　of　secondary　organic　aerosols　(SOA).　However,　the　abundance　of　HOMs　in　different　environments　and　their　relation　to　the　oxidative　potential　of　fine　particulate　matter　(PM)　are　largely　unknown.　Here,　we　investigated　the　relative　HOM　abundance　and　radical　yield　of　laboratory-generated　SOA　and　fine　PM　in　ambient　air　ranging　from　remote　forest　areas　to　highly　polluted　megacities.　By　electron　paramagnetic　resonance　and　mass　spectrometric　investigations,　we　found　that　the　relative　abundance　of　HOMs,　especially　the　dimeric　and　low-volatility　types,　in　ambient　fine　PM　was　positively　correlated　with　the　formation　of　radicals　in　aqueous　PM　extracts.　SOA　from　photooxidation　of　isoprene,　ozonolysis　of　α-　and　β-pinene,　and　fine　PM　from　tropical　(central　Amazon)　and　boreal　(Hyytiälä,　Finland)　forests　exhibited　a　higher　HOM　abundance　and　radical　yield　than　SOA　from　photooxidation　of　naphthalene　and　fine　PM　from　urban　sites　(Beijing,　Guangzhou,　Mainz,　Shanghai,　and　Xi＇an),　confirming　that　HOMs　are　important　constituents　of　biogenic　SOA　to　generate　radicals.　Our　study　provides　new　insights　into　the　chemical　relationship　of　HOM　abundance,　composition,　and　sources　with　the　yield　of　radicals　by　laboratory　and　ambient　aerosols,　enabling　better　quantification　of　the　component-specific　contribution　of　source-　or　site-specific　fine　PM　to　its　climate　and　health　effects.　Copyright　©　2019　American　Chemical　Society.