Identifying Criegee Intermediates As Potential Oxidants In The Troposphere

Abstract:

Criegee intermediates (CI) are formed during the ozonolysis of unsaturated compounds and have been intensively studied in the last few years due to their possible role as oxidants in the troposphere. Stabilised CI (SCI) are now known to react very rapidly, k(298 K) = 10^-12 to 10^-10 cm³ molecule^-1 s^-1, with a large number of trace gases (SO₂, NO₂, organic acids, water dimers). An assessment of their effective oxidative capacity remain challenging as, CI chemistry is complex, it spans a large range of rate coefficients for different SCI conformers reacting with water dimers and trace gases, and in addition, no reliable measurement technique able to detect ambient SCI concentrations is currently available.

In this study, we examine the extensive dataset from the HUMPPA-COPEC 2010 and the HOPE 2012 field campaigns, aided by literature data, to estimate the abundance of SCI in the lower troposphere. The budget of SCI is analyzed using four different approaches: 1) based on an observed yet unexplained H₂SO₄ production; 2) from the measured concentrations of unsaturated volatile organic compounds (VOC); 3) from OH reactivity measurements; 4) from the unexplained production rate of OH. A SCI concentration range between 5 x 10³ and 2 x 10⁶ molecule cm^-3 is calculated for the two environments. The weighted mean estimate of the SCI concentration over the boreal forest of ~ 5 x 10⁴ molecules cm^-3 implies a significant impact on the conversion of SO₂ into H₂SO₄.

In addition, we present measurements obtained using our inlet pre-injector laser-induced fluorescence assay by gas expansion technique (IPI-LIF-FAGE) for the above-mentioned campaigns. A recent laboratory study performed with the same instrumental setup showed that the IPI-LIF-FAGE system is sensitive to the detection of the OH formed from unimolecular decomposition of SCI. In order to investigate the applicability of the laboratory findings to the ambient data, measurement of the background OH (OH_bg), the signal detected by the IPI-LIF-FAGE after the removal of ambient OH, measured during the two field campaigns are represented in comparison with many other trace gases to assess if the observations in controlled conditions are transferable to ambient conditions.