Abstract
Background: Ambient particulate matter (PM) smaller than 2.5 µm in diameter (PM2.5) undergoes diurnal changes in chemical composition due to photochemical oxidation. In this study we examine the relationships between oxidative activity and inflammatory responses associated with these diurnal chemical changes. Because secondary PM contains a higher fraction of oxidized PM species, we hypothesized that PM2.5 collected during afternoon hours would induce a greater inflammatory response than primary, morning PM2.5. Methods: Time-integrated aqueous slurry samples of ambient PM2.5 were collected using a direct aerosol-into-liquid collection system during defined morning and afternoon time periods. PM2.5 samples were collected for 5 weeks in the late summer (August-September) of 2016 at a central Los Angeles site. Morning samples, largely consisting of fresh primary traffic emissions (primary PM), were collected from 6-9am (am-PM2.5), and afternoon samples were collected from 12-4pm (pm-PM2.5), when PM composition is dominated by products of photochemical oxidation (secondary PM). The two diurnally phased PM2.5 slurries (am- and pm-PM2.5) were characterized for chemical composition and BV-2 microglia were assayed in vitro for oxidative and inflammatory gene responses. Results: Contrary to expectations, the am-PM2.5 slurry had more proinflammatory activity than the pm-PM2.5 slurry as revealed by nitric oxide (NO) induction, as well as the upregulation of proinflammatory cytokines IL-1β, IL-6, and CCL2 (MCP-1), as assessed by messenger RNA production. Conclusions: The diurnal differences observed in this study may be in part attributed to the greater content of transition metals and water-insoluble organic carbon (WIOC) of am-PM2.5 (primary PM) vs. pm-PM2.5 (secondary PM), as these two classes of compounds can increase PM2.5 toxicity.
Highlights
Particulate matter (PM) with an aerodynamic diameter less than 2.5 μm, is associated with diverse health problems and chronic diseases, including asthma, chronic obstructive pulmonary disease (COPD), lung cancer, and coronary heart disease (Delfino et al, 2005; Delfino et al, 2011; Dockery et al, 1993; Dominici et al, 2006; Kaufman et al, 2016; Kim et al, 2013; Landrigan et al, 2018; Shah et al, 2013)
The diurnal differences observed in this study may be in part attributed to the greater content of transition metals and waterinsoluble organic carbon (WIOC) of am-PM2.5 vs. pm-particulate matter (PM) 2.5, as these two classes of compounds can increase PM2.5 toxicity
A dose-dependent nitric oxide (NO) response to PM2.5 treatments relative to control was observed at all timepoints (30 min., 60 min., 24 hr.), which was greater for am-PM than pm-PM exposures
Summary
Particulate matter (PM) with an aerodynamic diameter less than 2.5 μm (fine PM or PM2.5), is associated with diverse health problems and chronic diseases, including asthma, chronic obstructive pulmonary disease (COPD), lung cancer, and coronary heart disease (Delfino et al, 2005; Delfino et al, 2011; Dockery et al, 1993; Dominici et al, 2006; Kaufman et al, 2016; Kim et al, 2013; Landrigan et al, 2018; Shah et al, 2013). Emitted primary PM undergoes photochemical oxidation reactions over the course of the day, catalyzed by ultraviolet (UV) sunlight, which results in diverse oxidized organic and inorganic products (secondary PM) (Forstner et al, 1997; Grosjean & Seinfeld, 1989), along with concomitant changes in PM toxicity. These diurnal changes in PM2.5 composition and associated toxicity are relevant to and may inform future long-term epidemiological studies of primary and secondary particulate matter. Largely consisting of fresh primary traffic emissions (primary PM), were collected from 6-
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