Cloud Water Chemistry Associated with Urban Aerosols: Rapid Hydroxyl Radical Formation, Soluble Metals, Fe(II), Fe(III), and Quinones

Abstract

Hydroxyl radical (OH) reactions in cloud water play a key role in secondary organic aerosol formation and sulfur oxidation. We collected aerosol samples (PM 4 ) during summer at an urban receptor site in Southern California. The site mostly receives air from the urban area in the morning, photochemically processed air arriving from the urban area and a commercial ports area in the afternoon, and a largely unpopulated mountainous area overnight. Filters were extracted in small quantities of water at pH 3.5 (adjusted with H 2 SO 4 ), simulating cloud water formation. Samples were analyzed for particle mass, OH generation in the presence of near UV light, soluble trace metals (filtered through a 0.22 μm filter, measured with inductively coupled plasma mass spectrometry, ICP−MS), soluble Fe(II) and Fe(III) (measured with the ferrozine assay, Fe fzn ), and quinones. Soluble speciated iron was about equally divided into Fe(II) fzn and Fe(III) fzn and accounted for only 22 ± 7% of the soluble Fe measured with ICP−MS. The highest concentrations of Fe fzn came from the urban area; high Fe ICP came both from city and mountains, and the mountains were the dominant source of Cu. OH formation was characterized by an initial spike in formation lasting for 1−3 min with a formation rate at ∼(0.2−1) × 10 −8 M•s −1 , followed by a second much slower phase of (0.1−10) × 10 −11 M•s −1 . OH formation activity was strongly correlated with mass and soluble (ICP) Fe and Cu; it did not correlate with ferrozine iron. Quinone concentrations were too low to contribute much to OH formation. The initial burst of OH formation is large enough to contribute substantial OH to cloud and fog droplets.

Aerosol Oxidative Potential in the Greater Los Angeles Area: Source Apportionment and Associations with Socioeconomic Position

“…The intrinsic OH activity (OP m OH ) measured in this study (averages of 0.48 and 0.63 pmol min –1 μg –1 for summer and winter, respectively) was a bit higher than 0.3 pmol min –1 μg –1 previously measured in Los Angeles in late summertime 2014 at a single site impacted either by air masses with urban aerosols containing relatively high amounts of secondary organic aerosol or by air masses from an unpopulated mountain area, depending on time of day . Our data was in the range of values measured by Li et al for an urban and a suburban site in China (0.2–1.2 pmol min –1 μg –1 ) in summer 2014.…”

Effects of Metal Ions on Aqueous-Phase Decomposition of α-Hydroxyalkyl-Hydroperoxides Derived from Terpene Alcohols

“…A model simulation study predicted that the [Fe 2+ ]/[Fe 3+ ] ratio in cloud droplets differs between day and night because solar photolysis of Fe 3+ complexes converts Fe 3+ into Fe 2+ . Zn 2+ influences the hygroscopicity of aerosol particles by forming complexes with oxalate, and Cu 2+ plays key roles in the HO x chemistry in cloud/fog droplets. , The typical concentrations of ROOHs in cloud droplets and rain water are ∼3.9 μM (marine), 20 μM (remote), and 0.5 μM (urban) . Thus, α-HHs are expected to coexist with metal ions in atmospheric aqueous phases at similar concentrations (i.e., ≤20 μM).…”

A method for liquid spectrophotometric measurement of total and water-soluble iron and copper in ambient aerosols

“…The absorption spectra of the bathocuproine-Cu(I) complex display a maximum value at 484 nm. Both techniques have been developed and used to detect Fe and Cu in natural waters (Moffett et al, 1985;Viollier et al, 2000); however, use of the Fe-ferrozine approach is much more common and has also been utilized for measurements of ambient aerosols (Kuang et al, 2019;Majestic et al, 2006;Oakes et al, 2010;Rastogi et al, 2009;Zhu et al, 1997;Zhuang et al, 1992). These analytical techniques can be applied to relatively small liquid spectrophotometers that utilize a long-path liquid waveguide capillary cell (LWCC) for high sensitivity.…”