2 methyl 1 4 nq

1,2-NQ (97.4% purity determined by high-performance liquid chromatography (HPLC)), 1,4-NQ (98% purity determined by gas chromatography (GC)), 5,8-dihydroxy-1,4-NQ (79.8% purity determined by HPLC), 1,4-BQ (98% purity determined by iodometric titration), tertbutyl-1,4-benzoquinone (TBQ, 98% purity determined by GC), 2-hydroxy-1,4-NQ (99.9% purity determined by HPLC), and APAP were purchased from Tokyo Chemical Industry (Tokyo, Japan). 2-Methyl-1,4-NQ (98% purity determined by HPLC) was obtained from Nacalai (Kyoto, Japan). 5-Hydroxy-1,4-NQ (95% purity determined by HPLC), NAPQI, BPM, and the HRP-linked anti-biotin antibody were purchased from Sigma-Aldrich (St. Louis, MO, USA), Invitrogen (Carlsbad, CA, USA) and Cell Signaling Technology (Beverly, MA, USA), respectively. Vapor phase samples were collected in the mid-town area of Los Angeles near the University of Southern California during June (from 17 th to 22 th , VP1) and July (from 1 st to 7 th , VP2) 2008 and extracted as reported previously (Eiguren-Fernandez et al., 2008 , 2010) . All other reagents were of the highest grade available.

1,2-NQ (97.4% purity determined by HPLC), 1,4-NQ (98% purity determined by GC), TBQ (98% purity determined by GC), TBHQ (98% purity determined by GC), 1,4-BQ (98% purity determined by iodometric titration), 5,8-dihydroxy-1,4-NQ (79.8% purity determined by HPLC), and 2-hydroxy-1,4-NQ (99.9% purity determined by HPLC) were purchased from Tokyo Chemical Industry (Tokyo, Japan). 5-Hydroxy-1,4-NQ (95% purity determined by HPLC), 2-chloro-1,4-BQ (95% purity determined by GC), 2,3,5,6-tetramethyl-1,4-BQ (97% purity determined by GC), 2-methyl-1,4-BQ (98% purity determined by HPLC) and 2-hydroxy-3-(3-methyl-2-butenyl)-1,4-NQ were obtained from Sigma-Aldrich (St. Louis, MO, USA). 2-Methyl-1,4-NQ (98% purity determined by HPLC) and BHA (98% purity determined by GC) were obtained from Nacalai (Kyoto, Japan). AhR antagonist, 1-methyl-N- [2-methyl-4-[2-(2methylphenyl) diazenyl]phenyl-1H-pyrazole-5-carboxamide (CH223191), was perchased from ChemBridge (San Diego, CA, USA). 1,2-NQ antibody was prepared as reported previously (Miura and Kumagai, 2010) . All other reagents were of the highest grade available.