No abstract

No abstract

The formation yields of OH radical, propanal, and ethane from the reactions of ozone with Z-3-hexenal (3HA) and Z-3-hexen-1-ol (3HO) were measured under atmospheric conditions. These OH yields were determined to be 0.32 « 0.07 for 3HA and 0.28 « 0.06 for 3HO. The relative formation yield of the C 2 H 5 CHOO intermediate was estimated based upon the formation yields of propanal and ethane, which consistently suggested that C 2 H 5 CHOO is formed more efficiently from the O 3 + 3HA reaction than from the 3HO reaction.The green leaf volatiles (GLVs), Z-3-hexenal (3HA) and Z-3-hexen-1-ol (3HO) are emitted when plants are wounded. 1,2In lawn mowing activity, emission of 3HA and 3HO has been observed not only immediately following lawn mowing, but also from drying lawn clippings. Emissions from drying lawn clippings last for several hours, which suggests the emission of 3HA and 3HO into the atmosphere would typically continue into late afternoon.3,4 3HA and 3HO contain a C=C double bond and hence reactions with O 3 are an important atmospheric fate of these GLVs. Using the reported rate constants, 5,6 the lifetimes of 3HA and 3HO due to reactions with O 3 can be estimated to be 8 and 4 h ([O 3 ] = 40 ppb), respectively, which are comparable to the lifetimes with respect to OH reactions (a few hours). Recently, it has been recognized that the release of GLVs would have an impact on atmospheric chemistry, for example through secondary organic aerosol formation following their O 3 and OH reactions. 7,8The gas-phase reactions of O 3 with alkenes can initiate the atmospheric oxidation of alkenes. Furthermore, the ozone reactions are known to produce OH radicals, the most important atmospheric oxidant. OH formation from O 3 + alkene reactions is thought to be a potential source of OH not only during the nighttime but also in the daytime. 9 The initial step of O 3 + alkene reactions is believed to be the formation of an energized primary ozonide, followed by its rapid decomposition into a carbonyl oxide (Criegee intermediate, CI) and a carbonyl compound. The CI contains excess energy and can either be stabilized or decompose into other products, including OH radicals. The expected initial stage of the reaction of O 3 with 3HA/3HO is schematically shown in Figure 1.In this study, we measured the yields of OH radical formation from the reactions of O 3 with 3HA and 3HO. Furthermore, the formation yields of propanal and ethane were also measured in order to estimate the branching ratio of decomposition of the primary ozonide (º a and º b in Figure 1).All experiments were conducted using a 6 m 3 evacuable chamber at 1 atm and 298 « 2 K. Details of this chamber have been reported elsewhere. 5 The concentrations of gaseous species were monitored using a Fourier transform infrared (FTIR) spectrometer equipped with multireflection mirror optics (optical path length: 221.5 m). The initial concentrations of O 3 and substituted Z-3-hexenes (3HX, which are 3HA or 3HO) were 15 and 0.21 ppm, respectively.The OH formation yields from O 3 +...

During two days in late December 1994, total ozone measured at the Mauna Loa Observatory on the island of Hawaii, dropped below 200 Dobson Units (DU) for the first time since ozone measurements began at this site over 30 years ago. Total ozone values this low have not previously occurred over populated areas except on rare occasions when the edges of the springtime Antarctic ozone hole temporarily pass over the southern tip of Argentina. The monthly total ozone average for January 1995 was 216 DU, about 14% below the 1964–1981 baseline value. Ultraviolet radiation measured at Mauna Loa on clear days during this period increased inversely with ozone as expected, more than tripling in relative intensity at 295 nm. The normal annual minimum in total ozone occurs in winter at Mauna Loa; however, some winters experience considerably lower values than others. This was the case during the winter of 1994–1995. Although the general decline in global ozone, which began about 1980 and also appears in the Mauna Loa record, may be related to chemical ozone depletion, the unusually low values during some winters at Mauna Loa appear to be related to ozone transport from the tropics, and the timing of phase transitions of the QBO. This analysis provides an accurate method of forecasting low‐ozone, high‐UV winters in Hawaii.