Abstract. Alkyl nitrates and secondary organic aerosol (SOA) produced during the oxidation of isoprene by nitrate radicals has been observed in the SAPHIR (Simulation of Atmospheric PHotochemistry In a large Reaction Chamber) chamber. A 16 h dark experiment was conducted with temperatures at 289-301 K, and maximum concentrations of 11 ppb isoprene, 62.4 ppb O 3 and 31.1 ppb NO x . We find the yield of nitrates is 70±8% from the isoprene + NO 3 reaction, and the yield for secondary dinitrates produced in the reaction of primary isoprene nitrates with NO 3 is 40±20%. We find an effective rate constant for reaction of NO 3 with the group of first generation oxidation products to be 7×10 −14 molecule −1 cm 3 s −1 . At the low total organic aerosol concentration in the chamber (max=0.52 µg m −3 ) we observed a mass yield ( SOA mass/ isoprene mass) of 2% for the entire 16 h experiment. However a comparison of the timing of the observed SOA production to a box model simulation of first and second generation oxidation products shows that the yield from the first generation products was <0.7% while the further oxidation of the initial products leads to a yield of 14% (defined as SOA/ isoprene 2x where isoprene 2x is the mass of isoprene which reacted twice with NO 3 ). The SOA yield of 14% is consistent with equilibrium partitioning of highly functionalized C 5 products of isoprene oxidation.

Most pollutants in the Earth's atmosphere are removed by oxidation with highly reactive hydroxyl radicals. Field measurements have revealed much higher concentrations of hydroxyl radicals than expected in regions with high loads of the biogenic volatile organic compound isoprene 1-8 . Different isoprene degradation mechanisms have been proposed to explain the high levels of hydroxyl radicals observed 5,9-11 . Whether one or more of these mechanisms actually operates in the natural environment, and the potential impact on climate and air quality, has remained uncertain 12-14 . Here, we present a complete set of measurements of hydroxyl and peroxy radicals collected during isoprene-oxidation experiments carried out in an atmospheric simulation chamber, under controlled atmospheric conditions. We detected significantly higher concentrations of hydroxyl radicals than expected based on model calculations, providing direct evidence for a strong hydroxyl radical enhancement due to the additional recycling of radicals in the presence of isoprene. Specifically, our findings are consistent with the unimolecular reactions of isoprenederived peroxy radicals postulated by quantum chemical calculations 9-11 . Our experiments suggest that more than half of the hydroxyl radicals consumed in isoprene-rich regions, such as forests, are recycled by these unimolecular reactions with isoprene. Although such recycling is not sufficient to explain the high concentrations of hydroxyl radicals observed in the field, we conclude that it contributes significantly to the oxidizing capacity of the atmosphere in isoprene-rich regions.Biogenic emissions by terrestrial vegetation are responsible for 90% of the volatile organic compounds (VOCs) in the atmosphere 15,16 . Isoprene (C 5 H 8 ) being the most abundant biogenic VOC has the highest emission rates in tropical forests owing to the light-and temperature-dependent emission process. The high isoprene emission rates coincide with high production rates of the main atmospheric oxidant, the hydroxyl radical (OH), which is mainly formed by photodissociation of ozone (O 3 ) and subsequent reaction of electronically excited oxygen atoms with water vapour. Isoprene reacts readily with OH (ref. 5). This process constitutes the main sink of atmospheric isoprene, and in turn influences the OH concentration, which determines the atmospheric lifetime of greenhouse gases (for example, methane) and pollutants (for example, CO).The atmospheric oxidation efficiency of OH is greatly enhanced by nitrogen monoxide (NO)-dependent radical chain reactions, which regenerate OH. The reaction of OH with isoprene, for example, forms hydroxy-peroxy isoprene radicals (RO 2 ), which can be converted to hydroperoxy radicals (HO 2 ) through their reaction with NO followed by the reaction of HO 2 with NO reforming OH ROOH IEPOX+OH +OH 17% 25% +HO 2 Isoprene+OH+ O 2 1 ,5 -H s h if t 29% <1% MVK, MACR +HCHO+OH y(OH): 0.29 /<0.01 1 ,6 -H s h if t +NO RO 2 41% 62% 13% 13% LIM-J LIM-CS MVK, MACR +HCHO+HO 2 y(OH): 0.29/0.43 HPAL...

Abstract. The formation of organic nitrates and secondary organic aerosol (SOA) were monitored during the NO 3 + limonene reaction in the atmosphere simulation chamber SAPHIR at Research Center Jülich. The 24-h run began in a purged, dry, particle-free chamber and comprised two injections of limonene and oxidants, such that the first experiment measured SOA yield in the absence of seed aerosol, and the second experiment yields in the presence of 10 µg m −3 seed organic aerosol. After each injection, two separate increases in aerosol mass were observed, corresponding to sequential oxidation of the two limonene double bonds. Analysis of the measured NO 3 , limonene, product nitrate concentrations, and aerosol properties provides mechanistic insight and constrains rate constants, branching ratios and vapor pressures of the products. The organic nitrate yield from NO 3 + limonene is ≈30%. The SOA mass yield was observed to be 25-40%. The first injection is reproduced by a kinetic model. PMF analysis of the aerosol composition suggests that much of the aerosol mass results from combined oxidation by both O 3 and NO 3 , e.g., oxidation of NO 3 + limonene products by O 3 . Further, later aerosol nitrate mass seems to derive from heterogeneous uptake of NO 3 onto unreacted aerosol alkene.

Alkyl nitrates and secondary organic aerosol (SOA) produced during the oxidation of isoprene by nitrate radicals has been observed in the SAPHIR (Simulation of Atmospheric PHotochemistry In a large Reaction Chamber) chamber. A 16 h dark experiment was conducted with temperatures at 289-301 K, and maximum concentrations of 11 ppb isoprene, 62.4 ppb O 3 and 31.1 ppb NO x. We find the yield of nitrates is 70±8% from the isoprene + NO 3 reaction, and the yield for secondary dinitrates produced in the reaction of primary isoprene nitrates with NO 3 is 40±20%. We find an effective rate constant for reaction of NO 3 with the group of first generation oxidation products to be 7×10 −14 molecule −1 cm 3 s −1. At the low total organic aerosol concentration in the chamber (max=0.52 µg m −3) we observed a mass yield (SOA mass/isoprene mass) of 2% for the entire 16 h experiment. However a comparison of the timing of the observed SOA production to a box model simulation of first and second generation oxidation products shows that the yield from the first generation products was <0.7% while the further oxidation of the initial products leads to a yield of 14% (defined as SOA/isoprene 2x where isoprene 2x is the mass of isoprene which reacted twice with NO 3). The SOA yield of 14% is consistent with equilibrium partitioning of highly functionalized C 5 products of isoprene oxidation.

The accuracy of tropospheric hydroxyl radical measurements by long-path absorption spectroscopy is ultimately limited by the uncertainty of the effective OH absorption cross sections. The latter were determined from calculated spectra for the Q•(2), Q•(3), and P•(1) rotational lines of the OH A2E +, (v'--0) <--X21-I, (v"=0) transition at 308 nm. The calculations took into account Doppler broadening, measured data of the collisional broadening of OH by air molecules, and instrumental line broadening effects. The calculated spectra were compared with OH absorption spectra measured in a flow reactor near room temperature at 1013hPa. Excellent agreement between calculated and measured OH spectra was achieved using the collision-broadening parameters determined recently by Leonard (1990). The effective absorption cross sections at the line center (peak absorption cross sections) calculated for the Voigt line shape at 300 K and 1013 hPa are' ereff(•0)[Ql(2)] -(1.67 :k 0.1) x 10-16cm 2, •½• (P0)[Q1 (3)]-(1.4 :k 0.1) x 10-16cm 2, •½•(P0)[P1 (1)]-(1.414-0.!) x 10-16cm 2. The full width at half maximum of the spectral lines is 2.5pm, and the shape factor a is 1.41. Calculations of the effective absorption cross section under different atmospheric conditions demonstrate a strong pressure dependence for all lines. At 2800m altitude (720 hPa) the peak absorption cross section of the OH Voigt lines is about 35% higher than at sea level. The temperature dependence in an interval of :k 20 K around room temperature (at 1013 hPa) is only small (4-3% for the • lines and :k 6% for the Pi(1) line). The estimated total uncertainty of the effective absorption cross section is 8% . fluence the ambient OH concentration (i.e., trace gases, Paper number 94JD03323. 0148-0227 / 95/94 J D-03323 $05.00 photolysis rates etc.) provide a critical test of photochemical reaction models of the atmosphere [Ehhelt et el., 1991; Poppe et el., 1993]. Among several different spectroscopic and nonspectroscopic OH detection techniques that are in use or will be soon available for OH field measurements [Crosley, 1994] laser long-path absorption spectroscopy has unique advantages [ Hiibler et el., 1984; Pernet et el., 1987; Platt et el., 1988; Cellies, 1988; Dom et el., 1988; Mount, 1992; Armerding et el., 1994]' (1) It is a direct and absolute OH detection method. (2) The absorption light path is directed through the free troposphere •nu nu no surface effects like wall reactions or losses at the sample inlet system can falsify the measurements. (3) No calibration is needed, because this technique is based on Lambert-Beer's law of absorption.