Direct Measurements of HO_x Radicals in the Marine Boundary Layer: Testing the Current Tropospheric Chemistry Mechanism

Abstract

OH and HO(2) radicals, atmospheric detergents, and the reservoir thereof, play central roles in tropospheric chemistry. In spite of their importance, we had no choice but to trust their concentrations predicted by modeling studies based on known chemical processes. However, recent direct measurements of these radicals have enabled us to test and revise our knowledge of the processes by comparing the predicted and observed values of the radical concentrations. We developed a laser-induced fluorescence (LIF) instrument and successfully observed OH and HO(2) at three remote islands of Japan (Oki Island, Okinawa Island, and Rishiri Island). At Okinawa Island, the observed daytime level of HO(2) agreed closely with the model estimates, suggesting that the photochemistry at Okinawa is well described by the current chemistry mechanism. At Rishiri Island, in contrast, the observed daytime level of HO(2) was consistently much lower than the calculated values. We proposed that iodine chemistry, usually not incorporated into the mechanism, is at least partly responsible for the discrepancy in the results. At night, HO(2) was detected at levels greater than 1 pptv at all three islands, suggesting the presence of processes in the dark that produce radicals. We showed that ozone reactions with unsaturated hydrocarbons, including monoterpenes, could significantly contribute to radical production.

OH and HO2 concentrations, sources, and loss rates during the Southern Oxidants Study in Nashville, Tennessee, summer 1999

“…Unexpectedly high HO 2 concentrations and increasing ratios of observed to modeled HO 2 , and thus P(O 3 ) HO2 , at greater NO have been observed in the free troposphere [ Folkins et al , 1997; Faloona et al , 2000]. There is some evidence for this effect at Okinawa Island [ Kanaya and Akimoto , 2002]. Although the authors of this study state that the HO 2 dependence is quantitatively reproduced by the model, above 1 ppbv of NO a plot of their HO 2 observations and model calculations [ Kanaya and Akimoto , 2002, Figure 4] shows that observed HO 2 does not decrease to the expected extent, with mixing ratios more than 10 times larger than expected for 2.5 × 10 −6 s −1 < J(O 1 D) < 7.5 × 10 −6 s −1 at ∼10 ppbv NO.…”

“…There is some evidence for this effect at Okinawa Island [ Kanaya and Akimoto , 2002]. Although the authors of this study state that the HO 2 dependence is quantitatively reproduced by the model, above 1 ppbv of NO a plot of their HO 2 observations and model calculations [ Kanaya and Akimoto , 2002, Figure 4] shows that observed HO 2 does not decrease to the expected extent, with mixing ratios more than 10 times larger than expected for 2.5 × 10 −6 s −1 < J(O 1 D) < 7.5 × 10 −6 s −1 at ∼10 ppbv NO. We have now observed unexpectedly high HO 2 at high NO in the metropolitan environment in Nashville.…”

Comparisons of observed and modeled OH and HO&lt;sub&gt;2&lt;/sub&gt; concentrations during the ambient measurement period of the HO&lt;sub&gt;x&lt;/sub&gt;Comp field campaign

“…Although they may not lead to an immediate clarification of processes, the accumulation of these comparisons at multiple sites is important for the identification of common tendencies. For example, HO 2 concentrations are overestimated in clean coastal regions (Sommariva et al, 2004;Kanaya et al, 2002Kanaya et al, , 2007a, and this assists systematic surveying of the processes responsible for the discrepancies. Recent observations revealed that OH concentrations under high volatile organic compound (VOC) and low NO x conditions were systematically larger than the modeled concentrations.…”

Comparisons of observed and modeled OH and HO&lt;sub&gt;2&lt;/sub&gt; concentrations during the ambient measurement period of the HO&lt;sub&gt;x&lt;/sub&gt;Comp field campaign

“…Although they may not lead to an immediate clarification of processes, the accumulation of these comparisons at multiple sites is important for the identification of common tendencies. For example, HO 2 concentrations are overestimated in clean coastal regions (Sommariva et al, 2004;Kanaya et al, 2002Kanaya et al, , 2007a, and this assists systematic surveying of the processes responsible for the discrepancies. Recent observations revealed that OH concentrations under high volatile organic compound (VOC) and low NO x conditions were systematically larger than the modeled concentrations.…”