## 4 Discussion  
In our prior AP studies on PM2.5 and VOCs, we characterized chemical species that can participate in two reactions associated with acute toxicity, (1) prooxidants that catalyze the formation of hydrogen peroxide and hydroxyl radical, and (2) electrophiles that form covalent bonds with nucleophilic groups such as amino and thiolate found in proteins. Furthermore, we showed that VOC electrophiles associated with AP can activate cellular signaling pathways relevant to adverse health effects [23]. However, VOC biological assessment has received minimal attention in conjunction with PM research despite its apparent contributions to AP toxicology, with the exception of adductomics studies by [PERSON] and his associates (e.g., [24]). For this study, analytical methods we previously developed and validated [14] were used to characterize chemical and biological properties of SB, CM and LB AP samples. The samples were obtained from (1) an aqueous suspension of the particulates, PM2.5, and (2) an organic extract of the VOCs trapped by polystyrene resins placed downstream of the PM2.5 filters which have been shown to contain the majority of the total electrophiles present in ambient air [14]. Based on our previous analyses of seasonal effects on AP properties [14; 16], the samples for this study collected in late spring/early summer were likely have relatively higher prooxidant and VOC electrophile content compared to winter collections. Nonetheless, despite such quantitative differences, predominantly PM--TNF-\(\alpha\)--and VOC electrophile responses were consistently observed for both seasonal collections. The quantitative nature of the regional data allowed for multiple comparisons between different parameters of AP chemical reactivities in terms of pro- and anti-inflammatory properties, and their distribution between PM 2.5 and VOC phases. Prooxidant reactivity and proinflammatory responses (TNF-\(\alpha\) increases) were predominantly associated with PM samples while electrophile reactivity and anti-inflammatory responses (HO-1 increases and ARE activation)  
Figure 9: Activation of the antioxidant response element (ARE) in RAW 264.7 macrophages by air samples from CM, LB and SB. Cells transfected with ARE-luciferase cDNA were exposed to either PM2.5 and VOC from CM, LB, SB at 1 m\({}^{3}\)/mL for 16 h and ARE activation assessed; (see Methods for ARE procedures). Each value is the mean \(\pm\) SD, n = 4/sample site; site values normalized to corresponding blanks; CM VOC vs. CM PM2.5, \"\" \(p\) < 0.005; LB VOC vs. LB PM 2.5, SB VOC vs. SB PM2.5, \"\" \(p\) < 0.00001.  
were predominantly associated with VOCs. Future studies, both in vitro and in vivo, are needed to determine whether the interactions of these responses are synergistic or antagonistic following concurrent exposure of PM and VOCs. Presently, evidence for a potential interaction is suggested from the results of the two-phase studies we conducted with RAW 264.7 macrophages in which the anti-inflammatory effects of the VOC suppressed the pro-inflammatory effects of the PM2.5. The following analysis illustrates how the data we generated can be used to explain differences in the response profiles of the two most reactive and potent samples, SB and CM2. Of particular note, the SB and CM2 samples have comparable chemical reactivities in the DTT assay (0 65, 0.50; Table 2), but the SB pro- and anti-inflammatory potencies as derived from the biological assays were significantly greater (slope values from linear regression analysis; Table 2B). Namely, the SB TNF-\(\alpha\) and HO-1 responses were 14 and four times more potent, respectively, than those generated by CM2. We interpret these differences in terms of the chemical content in the samples being collected at different regions of the LA Basin. The CM2 sample consists mostly of railroad traffic emissions, whereas the SB sample has been subjected to atmospheric chemical reactions on the air mass as it moves approximately 80 miles across the Basin.