Sulfate and Carboxylate Suppress the Formation of ClNO₂ at Atmospheric Interfaces

Abstract

We report measurements of the nitryl chloride (ClNO 2 ) branching fraction following reactive uptake of N 2 O 5 to mixed organic and inorganic solutions representative of atmospheric interfaces. For sodium chloride containing solutions, mixed with either sodium sulfate (Na 2 SO 4 ) or sodium acetate (NaAc), the ClNO 2 branching fraction (Φ ClNO 2 ) is suppressed relative to a sodium chloride only solution. In the case of the sulfate-chloride solution, Φ ClNO 2 is reduced from 0.85 ± 0.03 (0.5 M NaCl) to 0.32 ± 0.14 upon the addition of 2.0 M Na 2 SO 4 . In the case of the acetate-chloride solution, Φ ClNO 2 is reduced to 0.18 ± 0.03 upon the addition of 0.5 M NaAc. In contrast, no statistically significant suppression in Φ ClNO 2 was observed for the addition of sodium perchlorate up to 3.0 M, implying that an increase in ionic strength of the solution does not necessitate a reduction in Φ ClNO 2 . We suggest that the reduction in Φ ClNO 2 may result from a direct reaction between SO 4 2− (and Ac − ) with NO 2 + (or NO 2 + NO 3 − ) which competes with the NO 2 + + Cl − reaction that produces ClNO 2 . The dependence of Φ ClNO 2 on SO 4 2− and Ac − is compared with both a time-dependent reaction-diffusion model and recent field observations, suggesting that the reaction rate of SO 4 2−(or Ac − ) with NO 2 + would need to be similar in magnitude to the rate of the NO 2 + + Cl − reaction to explain the observed suppression in Φ ClNO 2 . We show that the dependence of Φ ClNO 2 on particulate sulfate and carboxylate can be readily incorporated into existing parametrizations of ClNO 2 heterogeneous chemistry. The results presented here indicate that anions which are ubiquitous in atmospheric aerosol, yet commonly considered to be unreactive, may regulate the production of reactive gases such as ClNO 2 .

Heterogeneous N<sub>2</sub>O<sub>5</sub> reactions on atmospheric aerosols at four Chinese sites: improving model representation of uptake parameters

“…5 . A recent laboratory study (Staudt et al, 2019) has reported that sulfate and acetate can suppress ClNO 2 for Cl − -containing solutions, but such a sulfate suppression effect was not observed in our results. Further studies are needed to identify and quantify these effects for better parameterizing the heterogeneous ClNO 2 production.…”

“…The two cities of Tai'an and Jinan (the capital of Shandong Province) are 15 and 60 km to the south and north, respectively. N 2 O 5 and ClNO 2 were measured using an iodide-adduct chemical ionization mass spectrometer (CIMS; THS Instru-ment, Atlanta), which has been deployed in several field campaigns Tham et al, 2016;Yun et al, 2018). The related trace gases (O 3 ,NO,and NO 2 ,etc.…”

“…All the sites are regionally representative sites, as they are situated in an area with limited anthropogenic influences Yun et al, 2018;. Detailed information on the sampling sites, instrumentation, and the γ N 2 O 5 determination approach have been described in previous publications Tham et al, 2016;, and site descriptions are briefly summarized in the Supplement. The locations of all the measurement sites are shown on the map in Fig.…”

“…The BT09 parameterization excluding chloride effects (orange markers) gives much better agreement, with more values located in the range within a factor of 2, though the γ N 2 O 5 was still overpredicted in most of the studies in North America (Bertram et al, 2009b;Riedel et al, 2012;McDuffie et al, 2018a) except for Boulder (Wagner et al, 2013). The improvement indicates that the efficiency of chloride in competing for the H 2 ONO + 2 intermediate and the effects on N 2 O 5 uptake on ambient aerosols might be overestimated, possibly due to the existence of other nucleophiles competing with Cl − ( McDuffie et al, 2018b;Staudt et al, 2019) or different mixing states of particle and nonuniform distribution of available chlorine in the aerosols.…”

Heterogeneous N<sub>2</sub>O<sub>5</sub> reactions on atmospheric aerosols at four Chinese sites: Improving model representation of uptake parameters

“…The BT09 parameterization excluding 190 chloride effects (yellow markers) gives much better agreement, with more values located in the range within a factor of 2, though the γ N2O5 was still overpredicted in most of the studies in North America (Bertram et al, 2009b;Riedel et al,2012;McDuffie et al, 2018a) except for Boulder (Wagner et al, 2013). The improvement indicates that the efficiency of chloride in competing for the H 2 ONO 2 + intermediate and the effects on N 2 O 5 uptake on ambient aerosols might be overestimated, possibly due to the existence of other nucleophiles competing with Cl - (McDuffie et al, 2018b;Staudt et al, 2019), or different mixing 195 states of particle and non-uniform distribution of available chlorine in the aerosols.…”