Production of peroxy radicals at night via reactions of ozone and the nitrate radical in the marine boundary layer

In this paper, a substantial set of simultaneous measurements of the sum of peroxy radicals, [HO2+RO2], NO3, hydrocarbons (HCs), and ozone, taken at M ace Head on the Atlantic coast of Ireland in spring 1997, is presented. Con ditions encountered during the experiment ranged from semipolluted air mass es advected from Britain and continental Europe to clean air masses off the North and mid-Atlantic, where mixing ratios of pollution tracers approache d Northern Hemispheric background mixing ratios. Average mixing ratios of p eroxy radicals varied from 2.5 to 5.5 parts per trillion by volume (pptv) a t night depending on wind sector, and were observed to decay only very slow ly from late afternoon to early morning (0.1-0.5 pptv h(-1)). Measurements of OH and HO2 on two nights using the Fluorescence Assay by Gas Expansion ( FAGE) technique give an upper limit for [OH] of 2.5x10(5) molecules cm(-3) and an average upper limit [HO2]/[HO2+RO2] ratio of 0.27. A modeling study of the (1)/e lifetimes of the peroxy radicals, assuming no radical producti on at night, yielded mean lifetimes of between similar to 8-23 min for HO2 and 3-18 min for CH3O2. Given these lifetimes, it may be concluded that the peroxy-radical mixing ratios observed could not be maintained without subs tantial production at night. No significant correlation is observed between measured [HO2+RO2] and [NO3] under any conditions. Calculation of the reac tion rates for ozone and NO, with hydrocarbons (HCs) shows that the ozone-i nitiated oxidation routes of HCs outweighed those of NO3 in the NE, SE and NW wind sectors. In the SW sector, however, the two mechanisms operated at similar rates on average, and oxidation by NO3 was the dominant route in th e westerly sector. The oxidation of alkenes at night by ozone was greater b y a factor of 4 than that by NO3 over the whole data set. The HC degradatio n rates from the three "westerly" sectors, where tracer mixing ratios were relatively low, may be representative of the nighttime oxidative capacity o f the marine boundary layer throughout the background Northern Hemisphere. Further calculations using literature values for OH yields and inferred RO2 yields from the ozone-alkene reactions show that peroxy radicals derived f rom the ozone reactions were likely to make up the major part of the peroxy -radical signal at night (mean value 66%). However, the NO3 source was of s imilar magnitude in the middle of the night, when [NO3] was generally at it s maximum. The estimated total rates of formation of peroxy radicals are mu ch higher under semipolluted conditions (mean 8.0x10(4) molecules cm(-3) s( -1) in the SE wind sector) than under cleaner conditions (mean 2.4x10(4) mo lecules cm(-3) s(-1) in the westerly wind sector). A model study using a ca mpaign-tailored box model (CTBM) for semipolluted conditions shows that the major primary sources of OH, HO2, and CH3O2 (the most abundant organic per oxy radical) were the Criegee biradical intermediates formed in the reactio ns of ozone with alkenes.