Abstract
Abstract. Light-absorbing and high-molecular-weight secondary organic products were observed to result from the reaction of glyoxal in mildly acidic (pH=4) aqueous inorganic salt solutions mimicking aqueous tropospheric aerosol particles. High-molecular-weight (500–600 amu) products were observed when ammonium sulfate ((NH4)2SO4) or sodium chloride (NaCl) was present in the aqueous phase. The products formed in (NH4)2SO4 or ammonium nitrate (NH4NO3) solutions absorb light at UV and visible wavelengths. Substantial absorption at 300–400 nm develops within two hours, and absorption between 400–600 nm develops within days. Pendant drop tensiometry measurements show that the products are not surface-active. The experimental results along with ab initio predictions of the UV/Vis absorption of potential products suggest a mechanism involving the participation of the ammonium ion. If similar products are formed in atmospheric aerosol particles, they could change the optical properties of the seed aerosol over its lifetime.
Highlights
Secondary organic aerosol (SOA) formation has been the subject of study for many years, recent field measurements have revealed significantly more secondary organic aerosol (SOA) than predicted by state-of-the-art atmospheric chemistry models (Heald et al, 2005; Volkamer et al, 2006)
The abundant biogenic VOC isoprene was recently identified as a major SOA source which was previously not included in models
It has been suggested that SOA formation in urban areas from anthropogenic VOCs is underrepresented in models (Volkamer et al, 2006), and that the heterogeneous uptake of glyoxal (CHOCHO) by aerosols could be a significant source of unaccounted-for OA (Volkamer et al, 2007)
Summary
Secondary organic aerosol (SOA) formation has been the subject of study for many years, recent field measurements have revealed significantly more SOA than predicted by state-of-the-art atmospheric chemistry models (Heald et al, 2005; Volkamer et al, 2006) This suggests the existence of SOA precursors or mechanisms which have not yet been identified, or pathways which are known but not currently represented well in models. After testing three scenarios with their model, they concluded that the missing sink of glyoxal was most likely irreversible heterogeneous uptake to aqueous inorganic aerosols with a reactive uptake coefficient of γ ≈0.0037 They linked this missing sink to the 15–25% of observed SOA mass which was unaccounted for by models during the same Mexico City case study (Volkamer et al, 2006). The irreversible uptake of glyoxal by cloud droplets followed by dehydration is another likely pathway for SOA formation (Ervens et al, 2004, 2008; Carlton et al, 2007; Fu et al, 2008)
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