Exceeding of Henry's Law by Hydrogen Peroxide Associated with Urban Aerosols

Simultaneous measurements of gas- and aerosol-phase hydrogen peroxide (H₂O₂) have been made at two sites in Los Angeles, one near the Pacific coast at the University of California at Los Angeles (UCLA), and the other in downtown Los Angeles with close proximity to a heavily traveled freeway (freeway site). At both the freeway and UCLA sites, gas-phase H₂O₂ levels were similar, averaging 1.17 ± 1.0 and 1.05 ± 0.6 ppb, respectively. The particle-associated H₂O₂ in both fine (PM_2.5) and coarse (>PM_2.5) modes was higher at the freeway site, as compared to UCLA, by a factor of 2. However, when aerosol-phase H₂O₂ is normalized to particle mass loadings, the fine-mode H₂O₂ levels are very similar at the two sites:  0.42 ± 0.3 and 0.58 ± 0.3 ng H₂O₂/μg particle mass at the freeway and UCLA sites, respectively. The normalized coarse-mode H₂O₂ levels were significantly higher at the freeway site than at UCLA, 1.05 ± 0.3 and 0.51 ± 0.3 ng/μg, respectively. Estimating aerosol liquid water content on the basis of relative humidity and aerosol mass, a calculated equivalent H₂O₂ in aerosol liquid water averages 70 mM, more than 2 orders of magnitude higher than concentrations predicted by gas-particle partitioning (Henry's law), which averages 0.1 mM. This indicates that the sampled particles are capable of generating H₂O₂ in aqueous solution. These corresponding aqueous-phase H₂O₂ concentrations in aerosol liquid water exceed levels that have been observed to produce cellular damage to lung epithelial cells in laboratory experiments by at least 3 orders of magnitude. Although most measurements of H₂O₂ in particles were made using an extraction solution adjusted to pH 3.5, a set of measurements indicates that H₂O₂ from fine-mode particles extracted in the physiologically relevant pH range 5−7.5 also generate H₂O₂ with only slightly lowered efficiency; coarse-mode H₂O₂ production dropped by 75% at the upper end of this range. Finally, a small set of measurements was performed to investigate the degree to which the recently developed Versatile Aerosol Concentrator Enrichment System (VACES) affects H₂O₂ levels in concentrated ambient aerosols. The VACES appeared to a have minimal impact on particulate H₂O₂.