Many aromatic compounds (e.g., polycyclic aromatic hydrocarbons (PAHs)) found in atmospheric aerosols are toxic and exist in both unsubstituted and substituted forms. Previous studies have mainly concentrated on investigating unsubstituted PAHs, leaving the substituted compounds largely uncharacterized. This study focuses on detection of both unsubstituted and substituted aromatics in ambient aerosol samples using ultrahigh-resolution mass spectrometry. Aerosol samples collected from roadside, urban, and suburban sites in Hong Kong were characterized by Fourier-transform ion cyclotron resonance mass spectrometry (FT-ICR-MS) coupled with atmospheric pressure photoionization (APPI) or electrospray ionization (ESI). In the APPI+ mode, 166 aromatic CH formulas (i.e., formulas containing C and H only and with a double bond equivalent (DBE) of 4 or higher) were determined through molecular formula calculations based on an accurate m/z determination. Among the determined aromatic CH formulas, 141 are possible alkylated monocyclic aromatic hydrocarbon (MAH) or PAH formulas, and account for ≥ 45% of the total intensity by aromatic CH+ formulas. Both APPI+ and ESI+ are effective in detecting nitroaromatics (i.e., CHO2N1 formulas and DBE ≥ 5). The two ionization modes provide complementary formula coverage, with formulas determined by APPI+ extending to higher DBE and those by ESI+ covering higher carbon numbers. Alkylated nitrobenzene compounds are the most abundant among nitroaromatics, and they, together with alkylated nitro-PAHs, account for > 80% of the total intensity of aromatic CHO2N+ formulas, indicating the importance of these compounds in real aerosol samples. Aromatic CHN+ and CHO+ formulas are also determined, confirming the atmospheric presence of some previously reported O- and N-containing aromatic compounds and revealing new possible formulas. The determination of aromatic organic formulas in this study provides useful guidance for future quantitative analysis of hazardous aromatic compounds. Future work is needed to determine the abundance and to study the toxicity of alkylated MAHs and PAHs outside the 16 US EPA priority PAH compounds. [Figure not available: see fulltext.].