Abstract. The heterogeneous conversion of NO 2 on different secondary organic aerosols (SOA) was investigated with the focus on a possible formation of nitrous acid (HONO). In one set of experiments different organic aerosols were produced in the reactions of O 3 with α-pinene, limonene or catechol and OH radicals with toluene or limonene, respectively. The aerosols were sampled on filters and exposed to humidified NO 2 mixtures under atmospheric conditions. The estimated upper limits for the uptake coefficients of NO 2 and the reactive uptake coefficients NO 2 →HONO are in the range of 10 −6 and 10 −7 , respectively. The integrated HONO formation for 1 h reaction time was < 10 13 cm −2 geometrical surface and < 10 17 g −1 particle mass. In a second set of experiments the conversion of NO 2 into HONO in the presence of organic particles was carried out in an aerosol flow tube under atmospheric conditions. In this case the aerosols were produced in the reaction of O 3 with β-pinene, limonene or catechol, respectively. The upper limits for the reactive uptake coefficients NO 2 →HONO were in the range of 7 × 10 −7 − 9 × 10 −6 . The results from the present study show that heterogeneous formation of nitrous acid on secondary organic aerosols (SOA) is unimportant for the atmosphere.

ClOOCl was prepared in situ in a temperature controlled photoreactor (v = 420 L) by photolyzing OClO/N2 mixtures in the wavelength range 300-500 nm at temperatures between 242 and 261 K and total pressures between 2 and 480 mbar. After switching off the lights, excess NO2 was added, and IR and UV spectra were monitored simultaneously as a function of time. By spectral stripping of all other known UV absorbers (in particular, other chlorine oxides and chlorine nitrate), we determined rate constants k-1 of the reaction ClOOCl (+M) --> ClO + ClO (+M) from the first-order decay of the residual UV absorption of ClOOCl at 246 and 255 nm. k-1,0 = [N2] x 7.6 x 10(-9) exp[(-53.6 +/- 6.0) kJ mol(-1)/RT] cm3 molecule(-1) s(-1) (2sigma) was derived for the low-pressure limiting rate constant. Application of Troe's expression for the limiting low-pressure rate constants of unimolecular decomposition reactions leads to E0 = Delta(r)H0(0)(ClOOCl-->ClO+ClO) = 66.4 +/- 3.0 kJ mol(-1). k-1,0 started to fall off from the pressure proportional low pressure behavior at p approximately 30 mbar; however, reliable extrapolation to the high pressure limit was not possible. The decomposition rate constants of ClOOCl were directly measured for the first time, and they are higher, depending on temperature and pressure, by factors between 1.5 and 4.2 as compared to experimental data on k-1 by Nickolaisen et al. [J. Phys. Chem. 1994, 98, 155] which were derived from the approach of ClO to thermal equilibrium with its dimer ClOOCl. Combination of the present dissociation rate constants with recommended temperature and pressure dependent data on the reverse reaction (k1) demonstrate inconsistencies between the dissociation and recombination rate constants. Summarizing laboratory data on k1 and k-1 above 250 K and field measurements on the ClO + ClO <= => ClOOCl equilibrium in the nighttime polar stratosphere close to 200 K, the expression Kc = k1/k-1 = 3.0 x 10(-27) exp(8433 K/T) cm3 molecule(-1) is derived for the temperature range 200-300 K.

BrNO2 was prepared in situ in a static reactor (v = 420 L) by photolyzing Br2/NO2/N2 mixtures in the wavelength range 500−700 nm at temperatures between 263 and 294 K. After the lights were switched off, the excess NO was added, and IR and UV spectra were monitored simultaneously as a function of time. From the pseudo-first-order decay of the IR absorption of BrNO2 in the presence of a large excess of NO, the second-order rate constant for reaction 4, BrNO2 + NO ⇒ BrNO + NO2, was determined to be k 4 = 2.3 × 10-12 exp[(−17.8 ± 2.1) kJ mol-1/RT] cm3molecule-1s-1 (2σ). The measured yields of BrNO were close to 100% (98 ± 5%). These results suggest that reaction 4 is unimportant as a loss process of BrNO2 under most tropospheric conditions. Additional experiments on the thermal stability of BrNO2 led to an upper limit of 4.0 × 10-4 s-1 for its thermal gas-phase decomposition rate constant at 298 K in 1 atm of synthetic air. Finally, the mechanism of the Br + NO2 reaction and the thermochemistry of BrNO2 and BrONO are discussed in light of the results of the present experiments and of previous work from the literature.

The solubility of HBr in H 2 SO 4 /H 2 O and HNO 3 /H 2 SO 4 /H 2 O solutions was determined by measuring the HBr vapor pressure over stirred bulk solutions using tunable diode laser spectrometry. The experimental results for the solubility of HBr in sulfuric acid solutions show good agreement with experimental literature data. However, there is a factor 2-6 discrepancy between experimental and model values. The solubility of HBr in sulfuric acid was parameterized as a function of the H 2 SO 4 concentration and temperature in the range 53-75 wt % H 2 SO 4 and 195-250 K, respectively. The solubility of HBr in ternary HNO 3 /H 2 SO 4 /H 2 O solutions was determined for the first time. An increase in the solubility was observed on exchanging H 2 SO 4 by HNO 3 at constant water weight fraction. This observation is in qualitative agreement with model calculations, however, the observed solubility change was much larger than predicted by the model calculations. The solubility of HBr in ternary solutions was parameterized as a function of both the concentration of HNO 3 /H 2 SO 4 and the temperature. The relations derived can be used for atmospheric modeling of the influence of heterogeneous HBr reactions on atmospheric ozone destruction.

Pages 8492 and 8494. In a recent paper we published the HBr solubility in H 2 SO 4 /H 2 O and HNO 3 /H 2 SO 4 /H 2 O solutions. In response to an inquiry by F. Lefevre in which he pointed to unrealistic values for the Henry's law constants calculated by eq 7, we have carefully checked again all the numbers in the publication. Two mistakes of the sign for two numbers given in eqs 7 and 8 were found:(1) In eq 7, the correct value for m 3 is +(!) 4.445 (K).(2) In eq 8, the correct value for m 1 is -(!) 4.726 × 10 -5 (wt % -2 K).We are grateful to F. Lefevre for drawing our attention to the unrealistic values of the calculated solubility with the numbers given in the paper.