HO_x chemistry during INTEX‐A 2004: Observation, model calculation, and comparison with previous studies

Abstract

[1] OH and HO 2 were measured with the Airborne Tropospheric Hydrogen Oxides Sensor (ATHOS) as part of a large measurement suite from the NASA DC-8 aircraft during the Intercontinental Chemical Transport Experiment-A (INTEX-A). This mission, which was conducted mainly over North America and the western Atlantic Ocean in summer 2004, was an excellent test of atmospheric oxidation chemistry. The HOx results from INTEX-A are compared to those from previous campaigns and to results for other related measurements from INTEX-A. Throughout the troposphere, observed OH was generally 0.95 of modeled OH; below 8 km, observed HO 2 was generally 1.20 of modeled HO 2 . This observed-to-modeled comparison is similar to that for TRACE-P, another midlatitude study for which the median observed-to-modeled ratio was 1.08 for OH and 1.34 for HO 2 , and to that for PEM-TB, a tropical study for which the median observed-tomodeled ratio was 1.17 for OH and 0.97 for HO 2 . HO 2 behavior above 8 km was markedly different. The observed-to-modeled HO 2 ratio increased from $1.2 at 8 km to $3 at 11 km with the observed-to-modeled ratio correlating with NO. Above 8 km, the observed-to-modeled HO 2 and observed NO were both considerably greater than observations from previous campaigns. In addition, the observed-to-modeled HO 2 /OH, which is sensitive to cycling reactions between OH and HO 2 , increased from $1.5 at 8 km to almost 3.5 at 11 km. These discrepancies suggest a large unknown HO x source and additional reactants that cycle HO x from OH to HO 2 . In the continental planetary boundary layer, the observed-to-modeled OH ratio increased from 1 when isoprene was less than 0.1 ppbv to over 4 when isoprene was greater than 2 ppbv, suggesting that forests throughout the United States are emitting unknown HO x sources. Progress in resolving these discrepancies requires a focused research activity devoted to further examination of possible unknown OH sinks and HO x sources.

Isoprene in poplar emissions: effects on new particle formation and OH concentrations

“…Observations of atmospheric OH concentrations indicate that under low NO x conditions isoprene may be involved in a so far unidentified recycling mechanism that converts HO 2 into OH 1022 A. Kiendler-Scharr et al: Effects on new particle formation and OH concentrations without involving ozone formation (Tan et al, 2001;Thornton et al, 2002;Ren et al, 2008;Lelieveld et al, 2008;Hofzumahaus et al, 2009;Whalley et al, 2011;Lu et al, 2011). Contrary, suppression of new particle formation by isoprene was ascribed to its suppression of OH concentrations (Kiendler-Scharr et al, 2009b).…”

Measurements of a potential interference with laser-induced fluorescence measurements of ambient OH from the ozonolysis of biogenic alkenes

“…However, several field campaigns have identified significant discrepancies between measured and modeled OH concentrations, especially in low-NO x forested environments (Rohrer et al, 2014). For example, Ren et al (2008) found that OH concentrations were well predicted by models to within their combined estimated uncertainty when mixing ratios of isoprene were less than approximately 500 pptv, but measurements acquired in areas with higher mixing ratios of isoprene showed observed OH concentrations that were 3-5 times larger than model predictions. Similarly, measurements in a northern Michigan forest found daytime OH concentrations approximately 3 times larger and nighttime concentrations 3-10 times larger than model predictions Faloona et al, 2001).…”

Peroxy radical isomerization in the oxidation of isoprene

“…A number of recent analyses have cast doubt on the traditional representation of peroxy radical chemistry (R2a-R3). In particular, motivated by the inability to explain measured concentrations of OH in regions with elevated levels of biogenically-derived hydrocarbons, [2][3][4][5][6] investigators have hypothesized that the peroxy radicals of the dominant VOC, isoprene, may not follow the traditional reactive pathways. Most recently, Peeters et al 7 and da Silva et al 8 have estimated from quantum mechanical calculations that intramolecular hydrogen transfer reactions of the isoprene peroxy radicals (R4) may diminish the impact of isoprene oxidation on HO x concentrations.…”