Formation and Stability of Atmospherically Relevant Isoprene-Derived Organosulfates and Organonitrates

Abstract

Isoprene is the precursor for number of alcohol, organosulfate, and organonitrate species observed in ambient secondary organic aerosol (SOA). Recent laboratory and field work has suggested that isoprene-derived epoxides may be crucial intermediates that can explain the existence of these compounds in SOA. To confirm this hypothesis, the specific hydroxy epoxides observed in gas phase isoprene photooxidation experiments (as well as several other related species) were synthesized and the bulk phase aqueous reactions of these species in the presence of sulfate and nitrate were studied via nuclear magnetic resonance (NMR) techniques. The results indicate that both primary and tertiary organosulfates and organonitrates are efficiently formed from the potential SOA reactions of isoprene-derived epoxides. However, the tertiary organonitrates are shown to undergo rapid nucleophilic substitution reactions (in which nitrate is substituted for by water or sulfate) over the whole range of SOA pH, while the tertiary organosulfates are found to undergo a much slower acid-dependent hydrolysis reaction. The primary organonitrates and organosulfates under study were found to be stable against nucleophilic substitution reactions, even at low pH. This finding provides a potential explanation for the fact that organosulfates are more commonly detected in ambient SOA than are organonitrates.

Thermodynamics and kinetics of the hydrolysis of atmospherically relevant organonitrates and organosulfates

“…1-nitrato-2-butanol, 2-nitrato-1-butanol, 3-nitrato-2-butanol, and 2-methyl-2-nitrato-1-butanol were prepared according to a previous procedure (Muthuramu et al, 1993). All other organonitrates were prepared in situ in the reaction solutions by nucleophilic attack of nitrate on epoxide precursors (Darer et al, 2011). For the thermodynamics experiments, the difunctional organosulfates were prepared in situ in the reaction solutions via alcohol esterification reactions with sulfuric acid (Minerath et al, 2008).…”

“…2 that is used to determine k N n . Table 2 contains the lifetimes (τ hyd = 1 k N n ) for hydrolysis of the tertiary organonitrates studied in the present work, and Table 3 includes the hydrolysis lifetime for the isoprene-derived tertiary organonitrate studied in our previous work (Darer et al, 2011).…”

“…Formal acid dependent measurements were carried out for 2-methyl-2-sulfato-1-propanol, and k S H+ was determined to be 2.8 × 10 −4 m −1 s −1 (the statistical uncertainty in this value is on the order of 20 %). The presence of an acid-dependent mechanism for tertiary organosulfate hydrolysis was previously attributed to the fact that a sulfate group will be a more efficient leaving group for hydrolysis if the organosulfate species has an overall neutral charge (singly protonated sulfate group) (Darer et al, 2011). Table 2 contains the neutral mechanism lifetimes (τ hyd = 1 k S n ) for the tertiary organosulfates under study in the present work, and Table 3 includes the hydrolysis lifetime for the isoprene-derived tertiary organosulfate studied in our previous work (Darer et al, 2011).…”

“…Some of these compounds are undoubtedly formed from gas phase processing (Perring et al, 2009a, b;Lockwood et al, 2010;Ng et al, 2008), of isoprene, while other compounds (such as organosulfates) clearly indicate that additional chemical processing is occurring on ambient SOA particles themselves (Cole-Filipiak et al, 2010;Paulot et al, 2009;Surratt et al, 2010;Chan et al, 2010;Minerath et al, 2008Eddingsaas et al, 2010;Szmigielski et al, 2010;Nozière et al, 2010;Rudzinski et al, 2009) We recently investigated some of the potential SOA-phase chemistry for isoprene-related compounds, and proposed an overall mechanism (shown in Fig. 1) that can rationalize the existence of five of the seven observed isoprene-derived SOA components (Darer et al, 2011). In particular, our study showed that tertiary organonitrates are unstable under typical SOA conditions (for typical acidities and atmospheric lifetimes), and can undergo nucleophilic substitution reactions with water and sulfate to form alcohols and organosulfates: …”

“…We were also motivated to investigate how the presence of adjacent functional groups affects the thermodynamics and kinetics of the hydrolysis reactions. Therefore, we chose to study a number of monofunctional and difunctional organonitrates and organosulfates with varying carbon substitution properties in order to supplement the structure-activity relationships that were revealed in our previous study of the isoprene-derived species (Darer et al, 2011). Nuclear magnetic resonance (NMR) techniques were used to study the bulk phase aqueous reactions of these organonitrates and organosulfates in order to determine hydrolysis reaction rate and, in some cases, thermodynamics information.…”