Supplementary Information

Transcription

1 Supplementary Information Chemical Ionization Mass Spectrometric Measurement of ClNO 2 Mixing ratios of ClNO 2 were measured with a chemical ionization mass spectrometer (CIMS), identical to that described by Slusher et al. 1 for the measurement of peroxycarboxylic nitric anhydrides (PANs). Air was sampled from a tower 6 m above the deck of R/V Brown, about 14 m above the ocean surface. The inlet consisted of 6.2 mm O.D. (4 mm I.D.) PFA Teflon or Kynar tube and was operated at a flow rate of 2.1 slpm. The inlet was split into two flows inside the instrument container, with 1.0 slpm going to the CIMS instrument. The inlet was heated to 150 C to dissociate the PAN species to NO 2 and peroxyacyl radicals (PA) which react with I - to form acetate anions. The inlet temperature was insufficient to dissociate nitryl chloride (which was detected as the {IClNO 2 } - cluster ion, see below) but was sufficient to dissociate the thermodynamically less stable O-bound isomer chlorine nitrite (ClONO) 2, which was thus not detected (discussed further below). For zero measurements, a stainless steel section heated to 200 C (0.42 s residence time) was switched inline periodically (once an hour for 2 min). In this section, PANs, PA radicals, and ClNO 2 decomposed, consistent with the thermochemistry of the ClNO 2 molecule 2 and verified by post-campaign laboratory measurements. The ionization chemistry took place in an ion flow-tube (operated at 32 torr) downstream of the ion source. Iodide ions were generated by passing a mixture of CH 3 I in N 2 through a 210 Po radioactive source. Under these conditions, the following reactions took place 3 : I - + ClNO 2 ICl - + NO 2 (S1) I - + ClNO 2 {IClNO 2 } - (S2) The ICl - ion provides the best signal-to-noise ratio for laboratory measurements 3. However, the cluster ion {IClNO 2 } - (m/z = 207.9) was chosen for ambient measurements to minimize the possibility of interferences. The instrument included a collisional dissociation chamber and octopole ion guide which served to dissociate weakly-bound clusters (e.g., with water), resulting in relatively simple and clean mass spectra. Operation of the instrument was S1

2 automated, with the mass spectrometer set to measure 10 different ions for 0.5 seconds each. Total ion scans (from 10 to 240 m/z) were conducted every few days. The instrument was calibrated post-campaign using three independent methods. The first method utilized the N 2 O 5 -NaCl reaction to produce a gas stream containing ClNO 2 and N 2 O 5. In this study, a PFA flow cell containing deliquesced NaCl was set up with a sliding injector, allowing a range of reaction times in a stream of N 2 O 5 (in air), which was synthesized as described by Fahey et al. 4. The loss of N 2 O 5 was monitored by cavity ring-down spectroscopy (see below), and the corresponding {IClNO 2 } - signal was monitored at a range of exposure areas. In this calibration, it was assumed that the yield of ClNO 2 was unity 5. For the other two methods, a sample of ClNO 2 was synthesized by the addition of HCl to a sample of N 2 O 5 at dry-ice temperature 6, and the product was vacuum distilled under several freeze-thaw cycles. This produced a ClNO 2 sample that contained a substantial amount of Cl 2, as measured by UV absorption spectroscopy 7, but no NO 2 or other N-containing impurities in the vapor at dry-ice temperatures, as indicated by UV and FTIR spectroscopy 7,8. This ClNO 2 sample was then used to calibrate the CIMS instrument, employing both the NO y technique 9 and UV-Vis spectroscopy 7 to measure the concentration of ClNO 2. While the conversion of ClNO 2 to NO via the NO y method has not been characterized independently, the measurements of ClNO 2 thermal decomposition quoted above indicated that this conversion efficiency should be essentially that of NO 2, which is measured routinely in the course of the operation of this instrument. The third calibration method involved the UV-Vis measurement of ClNO 2 in a high concentration flow stream and dilution of that stream to mixing ratios in the range measurable by the CIMS. This method relies on the well-characterized absorption cross-sections of ClNO 2 7,10. The three different methods yielded an average response factor that had a relative standard deviation of ±18%. The response factor is a function of reagent ion number density, so the measured reagent ion counts were used to calculate a response factor as a function of time for the entire campaign. The overall uncertainty was ± (30% + 50 pptv) for the ClNO 2 measurement for 5 min integrated data. The behavior of ClNO 2 on inlet surfaces and the possibility of ClNO 2 production from N 2 O 5 on such surfaces or in the instrument itself was explored in several tests both during the campaign and in the laboratory. The calibration experiments described above showed that (a) the adsorption behavior of ClNO 2 on inlet surfaces was essentially reversible, and (b) the characteristic time of inlet S2

3 equilibration for ClNO 2 in the ambient inlet was less than 1 minute. One particularly important set of tests conducted on board R/V Brown involved the repeated addition of N 2 O 5 to the CIMS inlet to examine the possibility that ClNO 2 might be produced on the inlet surfaces, for example from sea salt particles collected there over the course of the sampling period. In these tests, up to 30 ppbv of N 2 O 5 were added, from a portable calibration source, for periods up to 10 minutes. These tests indicated that generation of ClNO 2 on the inlet surfaces was not significant. Several days of post campaign experiments were carried out with N 2 O 5 samples and gasphase HCl mixing ratios up to 5 ppbv through the inlet valve and thermal decomposition tubing used during the campaign (not cleaned since the campaign). No measurable response (< 25 pptv) was observed at m/z=207.9 during any of these experiments, confirming that there was no significant inlet artifact in our measurements. These conclusions are supported by ambient measurements in which there was variation in the ratio of ClNO 2 to N 2 O 5 in short term plumes, and essentially no correlation between ClNO 2 and N 2 O 5 (e.g., main paper, Figure 2, left panel). In fact, there were cases where there were substantial N 2 O 5 mixing ratios (several 100 pptv) in fresh plumes and no observable ClNO 2, and also cases where N 2 O 5 had been essentially depleted from the air mass, and the longer-lived ClNO 2 remained. There is an alternative means of checking for possible inlet production of ClNO 2, the polluted near-urban air masses often had short-lived local sources of NO superimposed on generally high NO 2 /NO 3 /N 2 O 5 concentrations. Under these circumstances, NO 3 and N 2 O 5 were titrated to zero rapidly due to the fast reaction of NO with NO 3 and the rapid equilibrium of N 2 O 5. The corresponding ClNO 2 remained unchanged or changed only very slightly even on the 1 second time scale during those instances. Since our laboratory studies have shown ClNO 2 production from N 2 O 5 to be rapid, the lack of response to these titration events is additional strong evidence that there is not a significant inlet effect. On ClNO 2 isomers There are two isomers of ClNO 2. Reactions in the condensed phase, such as N 2 O 5 +sea salt and HCl(g)+N 2 O 5 (s), have been shown to generate the more thermodynamically stable, nitrogen-bound ClNO 2 isomer, nitryl chloride 6,11. The gas phase radical reaction of Cl atoms with NO 2 has been shown to result primarily (80%) in the oxygen bound isomer, ClONO, chlorine nitrite, which isomerizes to ClNO 2 on surfaces 12. The thermochemistry of these compounds has been the subject of some theoretical and experimental investigation, e.g., Zhu and Lin 2 and S3

4 references therein. Nitryl chloride (bond strength 32.8 kcal/mole) is considerably more stable against thermolysis and photolysis than either of the ClONO cis or trans conformers (bond strengths of 22.6 and 19.6 kcal/mole). This thermochemistry implies very short lifetimes for ClONO at temperatures in the lower troposphere and negligibly small atmospheric concentrations. That, coupled with the fact that neither ClONO conformer would survive the inlet utilized in the CIMS instrument, which was heated to 150 C, leads us to conclude that ClONO was not measured by our instrument. There is further reason to believe that reaction of I - ions with ClONO would lead virtually exclusively to ICl - and not a cluster at m/z=207.9 {IClONO} -, also based on the above bond strengths. CaRDS measurement of N 2 O 5 Measurements of N 2 O 5 were made using a multi-channel cavity ring-down spectrometer (CaRDS) for simultaneous measurements of NO 2, NO 3 and N 2 O 5 13,14. Mixing ratios of NO 3 (NO 2 ) were measured by optical absorption at 662 nm (532 nm) 14. Mixing ratios of N 2 O 5 were measured from the increase in the NO 3 absorption signal at 662 nm following thermal conversion of N 2 O 5 to NO 3 in a heated channel 13. The NO 3 and N 2 O 5 channels were zeroed through the addition of nitric oxide (NO) at the tip of the inlet, which titrated NO 3 to NO 2. NO 2 ring-down cells were placed in series with both the NO 3 and N 2 O 5 cells in order to monitor the amount of NO 2 generated in the titration of NO 3 by NO, thus providing a calibration of the NO 3 and N 2 O 5 inlet transmission efficiencies 13,14. The NO 2 channels were zeroed by overflowing the absorption cells with ultrapure ("zero") air. The CaRDS instrument sampled from a fast-flow inlet (6.4 mm or 8 mm I.D. PFA Teflon) co-located with that of the ClNO 2 -CIMS. This inlet was replaced daily. The sampled air was filtered immediately before the absorption cells with PFA Teflon filters, which removed aerosols that would lead to optical extinction in the ringdown cells. The principal uncertainty in the NO 3 (and N 2 O 5 ) measurements was due to variability in the NO 3 transmission efficiency through the sample line. In practice, the variable NO 3 transmission efficiency did not significantly affect the N 2 O 5 measurements, because the wall losses occurred for the most part before the sample flow was split between the NO 3 and the heated (N 2 O 5 ) channel. The accuracy of the N 2 O 5 measurement was ± (25% + 0.1*[NO 3 ] pptv), and the (1 second) measurement precision was ± 0.5 pptv. Measurements of N 2 O 5 in the presence of ambient NO (for example, during daytime) are lower limits, as the background NO can destroy NO 3 produced in the thermal conversion of N 2 O 5. S4

5 Descriptions of Other Instruments The aerosol surface area was derived from the number size distributions measured with two differential mobility analyzers (DMPS, submicron fraction) and with an aerodynamic particle sizer (APS, supermicron fraction) at a relative humidity of 55% - 60% with 5 min time resolution and was corrected for changes in size due to water loss or uptake when the ambient RH was different than the measurement RH. Aerosol composition reported in this paper was measured by ion chromatography of impactor samples 15. The nitrogen oxide species, NO, NO 2 and total NO y and O 3 were measured using the methods described by Williams et al. 16, although NO y data are not available for the first week of the project. The photolysis rate of ClNO 2 (j(clno 2 )) was calculated from a parameterization of photolysis rates of NO 2, and O 3 (j(no 2 ), and j(o 3 )) measured by filter radiometry 17. Total gaseous chloride was measured by the mist chamber ion chromatographic method described by Scheuer et al. 18, and Dibb et al 19. Briefly, a total flow rate of ~120 slpm air was sampled into the system through a 15m long filtered inlet and a sub-flow of ~45 slpm was sampled alternately into dual mist chambers containing de-ionized water (ph=7). The Cl - ion concentration was measured every 5 minutes. While not measured as such, gaseous chloride is presumed to be in the form of HCl. The collection of other volatile chlorine species, for example ClNO 2 or Cl 2, is not expected due to their low solubility and slow hydrolysis in neutral ph water. Numerical simulations of N 2 O 5 ClNO 2 chemistry in plumes from point sources The rate equations for nocturnal production of N 2 O 5 and ClNO 2 were integrated according to the following chemical scheme: NO + O 3 NO 2 + O 2 (S3) NO 2 + O 3 NO 3 + O 2 (S4) NO 3 + NO 2 N 2 O 5 (S5) NO 3 + NO 2 NO 2 (S6) NO 3 products N 2 O 5 + aerosol 2 HNO 3 (S7) (S8) S5

6 N 2 O 5 + aerosol ClNO 2 + HNO 3 ClNO 2 products (S9) (S10) The rate constants of S3-6 were taken from the NASA/JPL evaluation 10. Initial concentrations of NO and O 3 and the rate coefficients k(s7-s10) were adjusted to match the observed mixing ratios (of NO, NO 2, NO 3, N 2 O 5, ClNO 2, and O 3 ) at the time of the plume intercept using an estimate for the transport time since either NO x emission from the source or since sunset. Dynamics, i.e., dispersion and entrainment, was not taken into account. Table S-1 summarizes the parameters used to constrain the integrations for determination of ClNO 2 yields. Table S-2 summarizes the simulation results. Example 1 A simulation was performed for a plume observed east of Florida on July 29, 2006 (see Figure 1, main paper). In this case there was no identifiable source, so the plume transit time is not known with certainty. FLEXPART back trajectories ( show R/V Brown 225 km downwind of the Bahamas islands, although the plume could also have originated from a large marine vessel. In spite of the proximity to the Florida coastline, the trajectory analysis, VOC and NO x composition, and absence of elevated CO in the plume indicate that this plume originated from a distant marine source rather than from nearby emissions in the urban regions of Florida. An upper limit to the reaction time is the time since sunset (5 hours), since formation of N 2 O 5 and ClNO 2 is suppressed during the day due to photolysis of the precursor, NO 3. A plume age of 5 hours is consistent with the age derived from the observed partitioning of odd oxygen, i.e. the slope of a plot of O 3 versus NO In the calculation shown in Figure S-1, the ClNO 2 loss rate coefficient, k(s10), was set to 0 s -1 (i.e., no losses), the rate coefficient for NO 3 removal, k(s7), was s -1 (consistent with the loss rate coefficient of NO 3 to the observed 40 pptv DMS), k(s8) was 0 s -1, k(s9) was s -1, and k(s10) was 0 s -1. This set of parameters reproduced the measurements. The overall yield of ClNO 2 in this! k 2 ), was 61%. 3 plume, relative to the amount of NO 3 produced ( ( S4)[ NO ][ O ]dt ClNO 2 lifetimes shorter than 14 hours (i.e., k(s10) > s -1 ) gave ClNO 2 mixing ratios lower than 600 pptv, inconsistent with the observations. Example 2 Figure S-2 shows a simulation of a continental pollution plume observed on August 18, 2006, approximately 80 min after sunset (which was used as the model start time). The conditions used to reproduce the observations were: k(s7) = S6

7 s -1, k(s8) = s -1, k(s9) = s -1 (i.e., an 18% yield of ClNO 2 from N 2 O 5 ), and k(s10) = 0 s -1. The overall yield of ClNO 2 in this plume, relative! k 2, was 11%, higher than predicted from the fraction of the 3 to ( S4)[ NO ][ O ]dt aerosol surface attributed to sea salt (1.4%). This indicates either N 2 O 5 to ClNO 2 conversion on non-sea salt aerosol or the presence of another (unknown) N 2 O 5 ClNO 2 conversion pathway. Numerical Simulation Summary There are several common results to all the simulations: (1) ClNO 2 forms efficiently from N 2 O 5 with rates exceeding the predicted rate from uptake of N 2 O 5 on (measured) sea salt surface area alone, requiring N 2 O 5 to ClNO 2 conversion on nonsea salt aerosol or indicating another (unknown) N 2 O 5 => ClNO 2 conversion pathway. (2) ClNO 2 is long-lived in the marine environment. A minimum lifetime of ClNO 2 against loss (S9) of s (~14 hrs) was needed to reproduce the measurements. (3) The yields of ClNO 2 were higher in marine than in continental air masses, consistent with the presence of other NO 3 (and N 2 O 5 ) loss pathways in continental air masses that compete with ClNO 2 formation. Modeling of O 3 enhancement The impact of Cl release from ClNO 2 photolysis on the VOC-NO x photochemistry was modeled using a subset of the Master Chemical Mechanism (MCM, freely available as a community resource at [ 21,22. The model contained the oxidation mechanisms of 50 hydrocarbons, 14 oxygenated hydrocarbons, DMS, CH 4 and CO plus an inorganic chemistry mechanism taken from the IUPAC evaluation, 2005 [ Briefly, the photochemistry in such a model simulates in detail the basic steps in photochemical ozone production; initiation by reaction of OH radical (or Cl atom) with a VOC, forming peroxy radicals, peroxy radicals oxidizing NO to NO 2, NO 2 photolyzing to NO and atomic oxygen, and finally atomic oxygen reacting with O 2 to form O 3. The MCM model was initialized for conditions typical of the polluted continental boundary: a NO x mixing ratio of 12 ppbv and a S7

8 total non-methane VOC mixing ratio of 47 parts-per-billioncarbon (ppbc). The particular distribution of VOC compounds follows the observations reported by de Gouw et al. 23. The mechanism was modified to include the production of ClNO 2 from N 2 O 5 reaction with aerosol assuming an N 2 O 5 uptake coefficient of 0.03, an aerosol surface area of 400 µm 2 cm -3 and a ClNO 2 yield of 20%. These conditions reproduced the ClNO 2 mixing ratio of about 650 pptv observed on September 2, consistent with the above plume model. A higher ClNO 2 mixing ratio (1500 pptv) resulted when the ClNO 2 yield from N 2 O 5 was increase to 50%, a level only slightly higher than the highest 1 minute average observed in our study (1320 ppt). The effect of these ClNO 2 levels on ozone production during the subsequent day was given by the 2 nd day of the model. A number of cases were considered, involving the either the overnight production cases run into the next day, or the addition of 650 pptv ClNO 2 to the model, run without ClNO 2 formation chemistry, at sunrise. There were only slight differences in ozone production in the case were the same amount of ClNO 2 was added at sunrise as the amount formed overnight. The results of model runs of 650 pptv added at sunrise, and 1500 pptv ClNO 2 formed overnight were compared to the model without ClNO 2, in Figure 3b of the main paper. S8

9 References 1. Slusher, D. L., Huey, L. G., Tanner, D. J., Flocke, F. M. & Roberts, J. M. A thermal dissociation-chemical ionization mass spectrometry (TD-CIMS) technique for the simultaneous measurement of peroxyacyl nitrates and dinitrogen pentoxide. J. Geophys. Res., 109, D19315, doi: /2004jd (2004). 2. Zhu, R. S. & Lin, M. C. Ab initio studies of ClO X reactions: Prediction of the rate constants of ClO+NO for the forward and reverse processes. ChemPhysChem, 5, (2004). 3. McNeill, V. F., Patterson, J., Wolfe, G. M. & Thornton, J. A. The effect of varying levels of surfactant on the reactive uptake of N 2 O 5 to aqueous aerosol. Atmos. Chem. Phys., 6, (2006). 4. Fahey, D. W., Eubank, C. S., Hubler, G. & Fehsenfeld, F. C. A calibrated source of N 2 O 5. Atmos. Environ., 19, (1985). 5. Thornton, J. A. & Abbatt, J. P. D. N 2 O 5 reaction on submicron sea salt aerosol: Kinetics, products, and the effect of surface active organics. J. Phys. Chem. A, 109, (2005). 6. Hoffman, R. C., Gebel, M. E., Fox, B. S. & Finlayson-Pitts, B. J. Knudsen cell studies of the reactions of N 2 O 5 and ClONO 2 with NaCl: development and application of a model for estimating available surface areas and corrected uptake coefficients. Phys. Chem. Chem. Phys., 5, (2003). 7. Ganske, J. A., Berko, H. N. & Finlayson-Pitts, B. J. Absorption Cross- Sections For Gaseous ClNO 2 And Cl 2 At 298 K Potential organic oxidant source in the marine troposphere. J. Geophys. Res., 97, (1992). 8. Durig, J. R., Kim, Y. H., Guirgis, G. A. & McDonald, J. K. FT-Raman and infrared-spectra, R(0) structural parameters, ab-initio calculations and Vibrational assignment for nitryl chloride. Spectrochimica Acta Part A-Molec Biomolec. Spectr., 50, (1994). 9. Fahey, D. W., Eubank, C. S., Hubler, G. & Fehsenfeld, F. C. Evaluation of a catalytic reduction technique for the measurement of total reactive oddnitrogen, NO y, in the atmosphere. J. Atmos. Chem., 3, (1985). 10. Sander, S. P. et al. Chemical Kinetics and Photochemical Data for Use in Atmospheric Studies, Evaluation Number 15 (NASA/JPL, Pasadena, CA, 2006). 11. Rossi, M. J. Heterogeneous reactions on salts. Chem. Rev., 103, (2003). 12. Niki, H., Maker, P. D., Savage, C. M. & Breitenbach, L. P. Fourier-transform IR spectroscopic observation of chlorine nitrite, ClONO, formed via Cl + NO 2 (+M) -> ClONO (+M). Chem. Phys. Lett., 59, (1978). 13. Dubé, W. P. et al. Aircraft instrument for simultaneous, in situ measurement of NO 3 and N 2 O 5 via pulsed cavity ring-down spectroscopy. Rev. Sci. Instr., 77, (2006). 14. Osthoff, H. D. et al. Measurement of atmospheric NO 2 by pulsed cavity ringdown spectroscopy. J. Geophys. Res., 111, D12305, doi: /2005jd (2006). 15. Quinn, P. K. et al. Impacts of sources and aging on submicrometer aerosol properties in the marine boundary layer across the Gulf of Maine. J Geophys. Res., 111, D23S36, doi: /2006jd (2006). S9

10 16. Williams, E. J. et al. Intercomparison of ground-based NO y measurement techniques. J. Geophys. Res., 103, , (1998). 17. Stark, H. et al. Atmospheric in situ measurement of nitrate radical (NO 3 ) and other photolysis rates using spectroradiometry and filter radiometry. J. Geophys. Res., 112, doi: /2006jd (2007). 18. Scheuer, E., R.W. Talbot, J.E. Dibb, G.K. Seid, L. DeBell, and B. Lefer, Seasonal distributions of fine aerosol sulfate in the North Americam Arctic basin during TOPSE, J. Geophys. Res., 108, 8370, doi: /2001jd001364, (2003). 19. Dibb, J.E., E. Scheuer, S.I. Whitlow, M. Vozella, E. Williams, and B. Lefer, Ship-based nitric acid measurements in the Gulf of Maine during the New England Air Quality Study, 2002, J. Geophys. Res., 109, D20303, doi: /2004jd , (2004). 20. Brown, S. S. et al. Nocturnal odd-oxygen budget and its implications for ozone loss in the lower troposphere. Geophys. Res. Lett., 33, L08801, doi: /2006gl (2006). 21. Jenkin, M. E., Saunders, S. M., Wagner, V. & Pilling, M. J. Protocol for the development of the Master Chemical Mechanism, MCM v3 (Part B): Tropospheric degradation of aromatic volatile organic compounds. Atmos. Chem. Phys., 3, (2003). 22. Saunders, S. M., Jenkin, M. E., Derwent, R. G. & Pilling, M. J. Protocol for the development of the Master Chemical Mechanism, MCM v3 (Part A): Tropospheric degradation of non-aromatic volatile organic compounds. Atmos. Chem. Phys., 3, (2003). 23. de Gouw, J. A. et al. Budget of organic carbon in a polluted atmosphere: Results from the New England Air Quality Study in J. Geophys. Res., 110, D16305, doi: /2004jd (2005). S10

11 Figure Captions Figure S1. Model results for the July 29 plume. Observed mixing ratios are shown as squares. The NO 3 loss rate coefficient (S7) was set to the calculated instantaneous loss rate coefficient to DMS. The rate coefficient for heterogeneous conversion of N 2 O 5 to ClNO 2 (S9) was set to the instantaneous loss rate coefficient on sea salt aerosol. The rate coefficients for S10 (loss of ClNO 2 ) and S8 (heterogeneous loss of N 2 O 5 that does not result in ClNO 2 production) was set to zero. Figure S2. Model results for the August 18 plume. The N 2 O 5 heterogeneous loss rate coefficient was set to the instantaneous loss rate coefficient at the sample location (S8+S9 = ~ s -1 ), with a branching ratio of 18% for ClNO 2 production and no ClNO 2 losses (S10 = 0 s -1 ). The NO 3 loss rate coefficient (S7) was set to s -1, which is larger than the instantaneous rate coefficient at the sample location ( s -1 ). S11

12

13

14 Table S-1. Summary of observed plume characteristics used to constrain the integrations for determination of ClNO 2 yields. Example 1 Example 2 Date and time July 29, 2006, 5:10 GMT (1:10 LT) Aug 18, 2006, 01:20 GMT (20:20 LT) Location East of Florida Gulf of Mexico Plume age 5 hours (since sunset) 80 minutes (since sunset) in plume background in plume background NO (ppbv) - * - * - * - * NO 2 (ppbv) O 3 (ppbv) NO 3 (pptv) N 2 O 5 (pptv) 10 - * ClNO 2 (pptv) * * HCl (pptv) * below detection limit. Table S-2. Summary of kinetic parameters used to reproduce the observations in the numerical simulations. Date and time July 29, 2006, 5:10 Example 1 Example 2 GMT (1:10 LT) Aug 18, 2006, 01:20 GMT (20:20 LT) Location East of Florida Gulf of Mexico Plume age 5 hours (since sunset) 80 minutes (since sunset) k(s7) (10-3 s -1 ) k(s8) + k(s9) (10-4 s -1 ) Yield of ClNO 2 from N 2 O 5 100% 18% Yield of ClNO 2 w.r.t. NO 3 produced via S4 61% 11% S12