## 1 Introduction  
Epidemiological and clinical studies have firmly established associations between ambient air pollution (AP) levels and adverse health effects (see for examples, [1; 2; 3; 4]). To advance our understanding of the relationships between AP exposure and disease processes, chemical and biological studies are needed to identify and characterize underlying molecular and cellular mechanisms. An essential element of those studies is the use of quantitative methodologies that allow for the comparison of results across other AP studies conducted in different local environments and geographical regions. Our prior studies have been focused on characterizing reactive chemicals in AP that interact with cellular targets to elicit responses such as inflammation. In ambient air, these chemicals are differentially distributed between particle and vapor phases as obtained in AP sample collections. The particulate matter phase (typically PM2.5) contains mostly prooxidants such as redox active metals together with higher molecular weight quinones and humic-like substances (HULIS) [5; 6] together with inorganic electrophiles such as arsenic and zinc. The vapor phase contains mostly volatile organic compounds (VOCs) that include quinones [7; 8; 9] and electrophilic carbonyls [10]. At the functional level, prooxidants catalyze the reduction of oxygen to reactive oxygen species and electrophiles react with nucleophilic functions, such as thiolate and amino groups, to form covalent bonds (see Graphical Abstract). Surprisingly, in recent years, research on biological assessments of AP has largely focused only on the consequences of PM exposure without addressing the potential biological contributions of corresponding VOCs. To address this deficiency, the present studies were designed to generate quantitative data on both PM2.5 and VOCs of ambient AP samples for evaluating their distinct chemical reactivities and biological effect profiles. In AP biological studies, the consequences of chemical exposure upon cellular components involves induction of oxidative stress and/or protein modifications that at low levels activate cytoprotective mechanisms but at higher levels initiate proinflammatory responses [11]. To understand the relationship between levels of reactive chemicals and this continuum of cellular responses, in this study we determined in PM2.5 and VOC fractions of ambient AP samples the prooxidant content based on their ability to transfer electrons from dithiothreitol to oxygen [12] and electrophile content, based on their ability to inactivate glyceraldehyde 3-phosphate dehydrogenase (GAPDH) [13]. The same fractions were then subjected to cellular assays designed to determine relative potencies of their biological responses. The AP samples for this study were collected at three regions that neighbor railyards (Long Beach (LB), Commerce (CM), and San Bernardino (SB)) in the Los Angeles Basin with the objective of assessing regional differences in the chemical and biological actions of AP [14]. The chemical study results show distinct distributions of reactive chemicals: prooxidants in the PM2.5 and the electrophiles in the VOCs. The biological study results show that the samples with high reactive chemical content were the most potent in eliciting biological responses: PM2.5 promoting an inflammatory response and VOCs promoting an anti-inflammatory response. Additionally, a second set of AP samples were collected at CM at sites 0.03-1 mile from its railyards to assess their chemical and biological actions as a function of proximity to the emission source. Those CM local samples exhibited profiles similar to the three regions' data, i.e., high chemical reactivity was associated with potent biological effects; furthermore, distance-dependent chemical and biological reactivities were also observed. The pro- and anti-inflammatory biological responses to the PM2.5 and VOC samples raised the question of whether additive or antagonistic interactions occur following ambient AP exposures (i.e., PM2.5 + VOCs).