ALBERT

Description: The atmospheric concentration of peroxyacetyl nitrate (PAN) was measured over the South Atlantic Ocean in the range between 37° and 70° S inMarch 1999 in order to investigate its latitudinal distribution and its role in the NOy chemistry of the remote marine boundary layer (MBL) in the high latitudes of thesouthern hemisphere. The measurements were performed using an automatic gas chromatographic system aboard RV Polarstern. Mean mixing ratios of 62 (+/- 21)pptv were obtained between 37° and 49° S. South of 55° S, PAN concentrations were very low with values of the order of 10-30 pptv.Pronounced diurnal cycles could not be observed. The concentrations over the South Atlantic were considerably lower compared with the North Atlantic where theMBL is influenced by continental air masses.

Description: Recent results from kinetic and spectroscopic laboratory studies of reactions of free radicals such as NO3 and SO4- with aromaticcompounds in aqueous solution at T = 298 K are presented. Three different experimental approaches, (i) laser photolysis-visible long-pathlaser absorption, (ii) laser photolysis-UV long-path laser absorption and (iii) time-resolved broadband diode-array spectroscopy, havebeen used to investigate the kinetics of reactions of NO3 and SO4- with aromatics, as well as the formation of oxidation intermediates.For the reactions of NO3 with benzene (1), anisole (2), p-xylene (3), p-cresol (4), toluene (5), mesitylene (6) and 1,4-dimethoxybenzene(7), rate coefficients of k(1) = (4.0 +/- 0.6) x 10(8) 1 mol(-1) s(-1), k(2) = (1.0 +/- 0.4) x 10(9) 1 mol(-1) s(-1), k(3) (1.6 +/- 0.1) x10(9) 1 mol s(-1), k(2) = (1.0 +/- 0.4) x 10(9) mol(-1) 1 s(-1), k(3) = (1.6 +/- 0.1) x 10(9) 1 mol(-1) s(-1), k(4) = (8.4 +/- 2.3) x 10(8)1 mol s(-1) k(5) = (1.2 +/- 0.3) x 10(9) 1 mol(-1) s(-1), k(6) = (1.3 +/- 0.3) x 10(9) 1 mol(-1) s(-1) and k(7) = (1.0 +/- 0.3) x 10(9) 1mol(-1) s(-1) were obtained. For the corresponding reactions of SO4- with benzene (8), toluene (9), p-xylene (10), p-cresol (11) andmesitylene (12), rate coefficients of k(8) = (6.4 +/- 2.5) x 10(8) 1 mol(-1) s(-1), k(9) = (1.3 +/- 0.6) x 10(9) 1 mol(-1) s(-1), k(10) =(2.7 +/- 0.9) x 10(9) 1 mol(-1) s(-1)k(11) = (2.8 +/- 0.8) x 10(9) 1 mol(-1) s(-1) and k(12) = (1.3 +/- 0.4) x 10(9) 1 mol(-1) s(-1) weredetermined. Reactivity correlations for both radical species with the aromatic compounds are presented and potential applications of suchcorrelations are discussed. In addition, the effect of ionic strength in the reactions of NO3 with benzene and toluene has been investigated.Transient intermediates have been spectroscopically identified in the reactions of the sulfate radical anion in aerated as well as inoxygen-free solutions. The nature of these intermediates is discussed in view of the existing literature. Finally, possible impacts offree-radical reactions with aromatic compounds on the current understanding of chemical conversion processes within troposphericmultiphase systems are considered.

Description: The rate coefficient for the reaction of Cl atoms with (1) H2O and of Cl-2(-) radical anions with (2) H2O and (3) OH- have beendetermined using laser photolysis for the generation of Cl/Cl-2(-) and long path laser absorption for the time resolved detection of Cl-2(-).The following results were obtained: k(1) = (2.3 +/- 0.6).10(5) s(-1), k(2)〈610 s(-1) and k(3) = (4.0+/-0.6).10(6) M-1 s(-1). It isconcluded that the lifetime of Cl/Cl-2(-) in aqueous solution strongly depends on the Cl- content and on pH.

Description: A laser photolysis-long path laser absorption (LP-LPLA) experiment has been used to determine the rate constants for H-atom abstraction reactions of thedichloride radical anion (Cl2-) in aqueous solution. From direct measurements of the decay of Cl2- in the presence of different reactants at pH = 4 and I = 0.1 M thefollowing rate constants at T = 298 K were derived: methanol, (5.1 ± 0.3)·104 M-1 s-1; ethanol, (1.2 ± 0.2)·105 M-1 s-1; 1-propanol, (1.01 ± 0.07)·105 M-1 s-1;2-propanol, (1.9 ± 0.3)·105 M-1 s-1; tert.-butanol, (2.6 ± 0.5)·104 M-1 s-1; formaldehyde, (3.6 ± 0.5)·104 M-1 s-1; diethylether, (4.0 ± 0.2)·105 M-1 s-1;methyl-tert.-butylether, (7 ± 1)·104 M-1 s-1; tetrahydrofuran, (4.8 ± 0.6)·105 M-1 s-1; acetone, (1.41 ± 0.09)·103 M-1 s-1. For the reactions of Cl2- with formic acidand acetic acid rate constants of (8.0 ± 1.4)·104 M-1 s-1 (pH = 0, I = 1.1 M and T = 298 K) and (1.5 ± 0.8) · 103 M-1 s-1 (pH = 0.42, I = 0.48 M and T = 298K), respectively, were derived.A correlation between the rate constants at T = 298 K for all oxygenated hydrocarbons and the bond dissociation energy (BDE) of the weakest C-H-bond of logk2nd = (32.9 ± 8.9) - (0.073 ± 0.022)·BDE/kJ mol-1 is derived.From temperature-dependent measurements the following Arrhenius expressions were derived:k (Cl2- + HCOOH) = (2.00 ± 0.05)·1010·exp(-(4500 ± 200) K/T) M-1 s-1,Ea = (37 ± 2) kJ mol-1k (Cl2- + CH3COOH) = (2.7 ± 0.5)·1010·exp(-(4900 ± 1300) K/T) M-1 s-1,Ea = (41 ± 11) kJ mol-1k (Cl2- + CH3OH) = (5.1 ± 0.9)·1012·exp(-(5500 ± 1500) K/T) M-1 s-1,Ea = (46 ± 13) kJ mol-1k (Cl2- + CH2(OH)2) = (7.9 ± 0.7)·1010·exp(-(4400 ± 700) K/T) M-1 s-1,Ea = (36 ± 5) kJ mol-1Finally, in measurements at different ionic strengths (I) a decrease of the rate constant with increasing I has been observed in the reactions of Cl2- with methanol andhydrated formaldehyde.

Description: Atmospheric concentrations of peroxyacetyl nitrate (PAN) were measured during a cruise of RV Polarstern from Cape Town (South Africa) to Bremerhaven(Germany) in May/June 1998 and at Ny-Alesund, Svalbard, in March 1998. The continuous in-situ measurements were using electron capture gas chromatographycombined with a cryogenic preconcentration technique. The time resolution of the measurements was 10 min with a detection limit of 5 parts per trillion by volume(pptv). The mixing ratios of PAN exhibited a maximum of 1100 pptv in the English Channel and values less than 5 pptv in tropical latitudes between 10 degrees Nand 10 degrees S. The latitudinal distribution of PAN showed significantly higher concentrations in the northern hemisphere compared to the southern hemisphere.North of 10 degrees N, daily maximum PAN concentrations normally exceeded 100 pptv, Mean mixing ratios decreased from 244 pptv between 50 degrees N and54 degrees N to 13 pptv between 20 degrees N and 30 degrees N. Enhanced concentrations were detected in continentally influenced air masses identified bytrajectory analysis and simultaneous measurements of black carbon and ozone. Intrusions of air masses from the upper troposphere could not be inferred fromtrajectory analysis. In temperate northern latitudes, enhanced PAN mixing ratios can be used as an indicator of long-range transport of photochemically activepollutants.

Description: Nitrate radical (NO3) reactions with benzene (R-1), toluene (R-2), p-xylene (R-3), p-cresol (R-4) and mesitylene (R-5) have beenstudied by laser photolysis/long path laser absorption (LP-LPLA) in aqueous solution. Rate constants of k(1) = (4.0 +/- 0.6) . 10(8), k(2)= (1.2 +/- 0.3) . 10(9), k(3) = (1.6 +/- 0.1) . 10(9), k(4) = (8.4 +/- 2.3) . 10(8) and k(5) = (1.3 +/- 0.3) . 10(9) lmol(-1) s(-1) wereobtained at T = 298 K. In addition, reaction rate coefficients for SO5 + Fe2+ --〉 prod. (R-6) and SO5- + Mn2+ --〉 prod. (R-7) of k(6)= (4.3 +/- 2.4) . 10(7) lmol(-1) s(-1) and k(7) = (4.6 +/- 1.0) . 10(6) lmol(-1) s(-1) (T = 298 K, I --〉 0) have been obtained by theapplication of laser photolysis/UV-VIS broadband diode array spectroscopy. A new laser photolysis/UV-long path laser absorptionexperiment has been applied to study the reaction of the Cl-2(-) radical anion with dissolved sulfur(IV). For the reactions Cl-2(-) +HSO3- --〉 2Cl(-) + H+ + SO3- (R-8) and Cl-2(-) + SO32- --〉 2Cl(-) + SO3- (R-9) rate coefficients of k(8) = (1.7 +/- 0.2) . 10(8)lmol(-1) s(-1) (T = 298 K, I --〉 0) and of k(9) = (6.2 +/- 0.3) . 10(7) lmol(-1) s(-1) (T = 279 K, I --〉 0) were obtained.

Description: In order to provide information about photolysis frequencies in the aqueous phase for chemical transport models including wet chemistry aparameterization which can be added to gaseous-phase photolysis models was developed. The actinic fluxes inside cloud droplet's arecalculated on the basis of rigorous Mie theory taking into account the effect of dissolved particulate aerosol material and 10 representativecloud droplet size distributions. The results show that the actinic flux inside cloud droplets are on the average more than twice as large ascompared to the interstitial air. The newly developed parameterization has been applied together with the model STAR (System forTransfer of Atmospheric Radiation). Apart from the parameters influencing gas-phase photolysis frequencies the radiation quantities insidethe cloud droplets and therefore the photolysis frequencies in the aqueous phase depend on the droplet size distribution, the mixing ratio ofdry aerosol particulate material to cloud droplet water, and the amount of light absorbing material in the droplets. In-droplet radical sourcestrengths have been calculated for the most important photolytic sources of OH and SO4-.

Description: A key focus of the PEAN99 campaign (Photochemical Experimentat Neumayer (70°S, 8°W) January/February 1999) was to studythe photochemistry of reactive nitrogen compounds and theirchemical and physical exchange processes with the firn layer.For the latter, two sets of controlled experiments werecarried out, designed to i) show conclusively that NOxproduction in the snowpack is photochemical, ii) determinewhether NO or NO2 is produced, iii) quantify the productionrate of NO2 and NO inside a snowblock, iv) derive fluxes ofNox from the snow into the overlying atmosphere. Throughouta diurnal cycle, measurements were made of ambient air andof air from inside a snowblock. Enhanced concentrations ofNO and NO2 (up to 15 pptv and 32 pptv, respectively) weremeasured inside the snowblock. The production rate insidethe block varied with intensity of incident radiation, andreached a maximum of 1.1*10(6) molec cm(-3) s(-1) for NO and2.1*10(6) molec cm(-3) s(-1) for NO2. A second experiment,in which the snowblock was alternately exposed to sunlightand then shaded, showed that the diurnal production wasdriven by photochemistry rather than some other diurnallyvarying factor. Gradient measurements and subsequent fluxcalculations showed that the snowpack can be a source of NOxto the overlying atmosphere, but suggest that the sourcestrength varies considerably from day to day.