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Impact of the heterogeneous hydrolysis of BrONO2 on calculated ozone changes due to HSCT aircraft and increased sulphate aerosol levels

Impact of the heterogeneous hydrolysis of BrONO2 on calculated ozone changes due to HSCT aircraft and increased sulphate aerosol levels
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  GEOPHYSICAL RESEARCH LETTERS, VOL. 23, NO. 4, PAGES 343-346, FEBRUARY 15, 1996 Impact of the heterogeneous hydrolysis of BrONO on calculated ozone changes due to HSCT aircraft and increased sulphate aerosol levels. L.K. Randeniya, .F. Vohralik, .C. Plumb, and K.R. Ryan '• CSIRO Division of Applied Physics, Lindfield Australia S.L. Baughcum The Boeing Company, Scuttle, Washington Abstract. The heterogeneous ydrolysis of BrONO2 via sulphate aerosols has been included in 2-D model calculations of the impact of projected fleets of super- sonic aircraft on atmospheric ozone. Calculations have been performed or aerosol evels ranging from zero to 16x the lower imit in WMO [1992]. The results show that the addition of this reaction has a major effect when the heterogeneous ydrolysis of N205 has reached saturation in regions where the night length is short. At 4x the lower limit of aerosols, he additional calcu- lated change n aircraft impact due to the inclusion of BrONO2 hydrolysis s of the same order as the impact calculated when this reaction is not included. Calcula- tions for background atmospheres t high aerosol ev- els show that the inclusion of this reaction significantly increases he predicted ozone depletion resulting from volcanicMly-enhanced aerosol evels. For conditions f high aerosol evels, eaction 2) can have a significant effect on ozone concentrations, s dis- cussed y Prather [1992], but, for what are expected to be more normal conditions, eaction 1) has a much greater impact on the chemistry of the lower strato- sphere and upper troposphere. The combination of field measurements, aboratory experiments and model calculations has resulted in a reappraisal of the importance of the catalytic cycles that remove ozone Wennberg t al., 1994]. The dom- inant influence of reactions uch as (1) and (2) is to repartition NOr into HNOa. This has the effect of in- creasing HO•], [ClOt] and [BrOr] and decreasing Ca] in the lower stratosphere. Thus, as shown by Wetsen- stein et al. [1993], he ozone columns or background atmospheres decrease as the aerosol evels ncrease. Recently Hanson and Ravishankara 1995] have re- ported rneasurements or the reaction Introduction Emissions rom projected fleets of high speed civil transports HSCTs) would have a significant ffect on the NOy and H20 budgets of the stratosphere. The effect of these emissions on ozone has been the sub- ject of study for more han 25 years Johnston, 971]. The inclusion n the models of heterogeneous eactions thought to occur on sulphate aerosols as had a signifi- cant effect on the calculated mpact of HSCTs on ozone [Weisenstein t al., 1993, and references herein]. The two heterogeneous eactions most often consid- ered in aircraft impact assessments re' N205 + H20•o,o• -• C1ONO2 + H2•o,o• -• 2HNOa (1) HNOa+HOC1 (2) BrONO2 + H20,ero•ot -• HOBr + HNO3 (3) on sulphate erosols ith a value or 7 of 0.4 This process rovides an additional mechanism or repar- titioning NOy and ncreasing HO•]. The effect f reac- tion (3) is expected o differ rom that of reaction 1) because N205 is predominantly ormed at night and, at high aerosol evels, the repartitioning of NO• through reaction 1) is usually ate-limited by the reaction of O3 with NO• and the length of the night. In contrast, BrONO2 s formed niainly during he day. Photoly- sis of HOBr produced n (3) results n further daytime processing f NOr to HNOs and an increase f [HOwl HOBr+hy -• OH+Br (4) Br+O3 -• BrO+Oa (5) BrO+NO2 -• BrONO• (6) 1 Also associated with Cooperative Research Centre for South- ern Hemisphere Meteorology, Monash University, Clayton, Vic 3168, Australia 2 Address orrespondence o this author Copyright 1996 by the American Geophysical nion. Paper number 96GL00263 0094-8534/96/96 GL-00263 $03.00 Accordingly, it is to be expected that the effects of the inclusion f reaction 3) on the calculated mpact of HSCTs would be most noticeable for aerosol levels for which he effect of reaction 1) was near saturation and for high latitudes near summer. Dani in and Mc- Cormell 1995] have ecently ncluded eaction 3) in a box-model calculation to simulate conditions for 60øS and ~20 km and for aerosol evels equal to the lower limit given n WMO [1992] nd approximately 0 times 343  344 RANDENIYA ET AL: BRONO2 AEROSOL REACTION; EFFECT ON O 3 4ø 2O 10 9os 60 30 o 30 60 90N Latitude deg) Figure 1. % difference n 03 mixing ratio due to inclu- sion of reaction 3) as a function of altitude and atitude for September, 4x aerosols. the lower limit. For the higher aerosol evel, they con- cluded that this reaction was responsible or an addi- tional ~30% ozone depletion in winter. This work examines the effect on the calculated im- pact of HSCTs on ozone of the inclusion f reaction 3) in a 2-D chemical transport model. Calculations are performed or aerosol evels hat cover he range from no aerosols o 16 x the lower imit in WMO [1992], which also allows conclusions o be drawn about the impact of reaction 3) on the chemistry t volcanically-enhanced aerosol levels. Model Description The model domain extends from 90øS to 90øN in 50 steps and from the ground to 80 km in steps of ~2 km (log pressure). Zonal-mean iabatic circulation o- gether with self-consistent ddy diffusion provide the transport terms. The chemistry module s family-based and employs ull diurnal averaging methods. The so- lar radiation module includes fully interactive multiple scattering calculations nd allowance or the curvature of the Earth. Cross-sections and rate coefficients are from DeMote et al. [1994], except or JHosr, which s from Orlando and Burkholder 1995]. A detailed de- scription f the model appears n Stolarski t al. [1995]. The scenarios sed n these calculations are for a pro- jected 2015 atmosphere as described n Stolarski and Wesoky 1993]. The present calculations efer to Ex- periment IV in that publication, which compares he effects of fleets of HSCTs cruising between 18 and 20 km with an NO• EI = 15 and an H20 EI = 1230 on a background tmosphere ith total Cly and Bry mixing ratios of 3.7x10 9 and 23x10 12 respectively. erosol levels are multiples of the lower limit case described n WMO [1992]; .g., 8x aerosols eans times he lower limit level. 4x aerosols is believed to be the median level over he past two decades WMO, 1992]. Results and Discussion Background Atmosphere Figure 1 shows he % difference n ozone mixing atio as a function of altitude and latitude with 4x aerosols for September hen eaction 3) is included ompared to the case when it is not. The result of including reac- tion (3) is to deplete 03] below 30 km, mainly at mid to high latitudes, giving an annually-averaged lobal reduction of ozone of 0.9% and a reduction of 1.3% be- tween 40øN and 60øN. For lx aerosols, hese depletions are reduced by a factor of about 3. Repartitioning of the BrOx species ccurs uch hat [HOBr] doubles ver most of the aerosol egion at the expense f [BrONO•]. Photolysis of HOBr results n a substantial ncrease n [HO•], which s the main cause f the reduction n [03]. More nsight nto the importance f reaction 3) can be obtained by examining he effects of aerosol evels on the calculations. igure 2(a) shows he HNO3 produc- tion rate for July at 47.5øN and 17 km. The calculations have been performed.for aerosol evels anging rom 0x to 16x. Contributions to HNO3 production from: NO2 + OH --, HN03 (7) as well as rom reactions 1), (3) and (2) are shown. The contribution rom reaction 1) saturates t about 4x aerosols at this altitude whereas the contribution from reaction 3) continues o increase nd the effect of re- 10 ß ß ß i ß ß ß i ß ß ß i ß ß 13 ..... 13 Reaction 7) O --O Reactions 7) + (1) A .... A Reactions 7) + (1) + (3) ß O -- • Reactions 7) + (1) + (3) + (2) (a) . ß , ß i . , , i . , , i . . . No BrON% ydrolysis With rON% ydrolysis /.--"' HO .. ---• _ NO (b) ß . . i . , . i . , . i . . 1.5 1.0 1:).5 0.0 0 4 8 12 16 Aerosol level Figure 2. (a) HNO3 production ate and b) NO• and HO• mixing ratios as functions of aerosol evel for July, 17 km, 47.5øN.  RANDENIYA ET AL: BRONO2 EROSOL EACTION; EFFECT ON O 3 345 action 2) is negligible hroughout he range of aerosols examined. The impact of reaction (3) on [NOel and [HO•] is shown n Figure 2(b). Figure 2 provides nformation on the way in which reaction 3) modifies he chemistry. Fig. 2(a) shows that this process accounts for a substantial fraction of the additional HNO3 production rate for aerosol evels of 4x and above. The relatively small additional de- crease n [NOel seen n Fig. 2(b) when the effects of reaction 3) are included s a result of the increase n [OH] brought about by the photolysis f HOBr during the recycling f bromine hrough eactions 5), (6), (3) and (4). Thus, although nclusion f reaction 3) leads to an increase in the rate of formation of HNO3, this is offset by reaction with increased OH from reaction 4) OH + HNOa -+ NO3 + H20 (8) Because f this, at 4x aerosols HNOa] remains argely unchanged hroughout much of the atmosphere when reaction 3) is included. Some of the NO• is converted to C1ONO2 as a result of OH attack on HC1 to yield C1. The effect of reaction 3) on [HO•] is most marked in summer months. In September, for example, the in- crease n [HO.•] is about a factor of 3 lower than in July and the effect on NO• reduction is also much less. The effect on [HO•] change s even more pronounced at higher atitudes han shown n Fig. 2(b). However, the concentration of ozone n a given region of the at- mosphere depends upon the relative magnitudes of the local chemical and transport lifetimes of O•. Effects of HSCT l*leets Figure 3 shows he results of calculations f the im- pact of emissions rom a fleet of Much 2.4 HSCTs, NOx EI = 15. Fig. 3(a) considers nly the heterogeneous reactions 1) and (2) whereas ig. 3(b) also ncludes reaction 3). Both calculations sed 4 x aerosols. he diffe,'ences etween igs. 3(a) and (b) are of the same order as the calculated impact of the HSCTs. For ex- ample, between 300 and 60øN the ozone columns are calculated o increase y around 0.6% in Fig. 3(a) and around 1.1% n Fig. 3(b). Part A of Table i gives he results or HSCT impacts for a range of aerosol evels with and without the inclu- sion of reaction 3). The change n [O•] due o HSCTs is always more positive for the calculations where re- action (3) is included compared o the corresponding calculation where it is not. Weisenstein t al. [1993] have discussed he fact that increases in the rates of aerosol reactions in a back- ground atmosphere esult n increased HO•-, C1Ox- and BrOw-induced Ox losses and therefore reduced ozone. Additional NOx from aircraft emissions uppresses hese cycles n a non-linear manner through reactions such as (7) and C10 + NO2. This means hat when he effects of HSCT emissions are taken into account, the ozone columns an ncrease s shown n Fig. 3(a). The HOx-, C10•- and BrOw-induced Ox losses n the background atmosphere ,'e enhanced ue to reaction 3) (rigs. 1 30 '3• 30 (b) 90S , i 0 60 120 180 240 300 360 Day of Year •'igure 3. % change n O3 column due to HSCT fleet (NO• EI = 15) when reaction 3) (a) is not included (b)i (4x and 2). Because he suppression f these osses y air- craft NOx emissions s non-linear, arger [Ox] increases result when reaction 3)is included rig. 3(b)). These points are illustrated by reference to the re- action of H O2 with 03, which is the rate-limiting step of the dominant HO• catalytic cycle for Ox destruction below 20 kin. For 4x aerosols, he rates of this cycle in July, integrated between 45øN and 50øN and between 12 and 18 km (the altitude range where most of the change n column occurs) n the background tmosphere are 0.087 %/day and 0.101%/day without and with re- action (3), respectively. When aircraft emissions re included, he corresponding ates are 0.065 %/day and 0.075 %/day. Hence, or the background tmosphere, inclusion f reaction 3) increases he rate of this cycle Table 1. % differences n O3 contained in specified latitude bands A: due to fleets of HSCTs (NO• E1 = 15) with and without reaction 3) B: due to specified changes n aerosol evels with and without reaction 3). Part A R(3) Latitude 1 x 4 x 8 x 16 x off -90 to 90 -0.19 0.25 0.41 0.55 on -90 to 90 -0.07 0.48 0.72 0.92 off 40 to 60 -0.21 0.71 1.03 1.29 on 40 to 60 0.06 1.23 1.72 2.15 Part B R(3) Latitude 1-4x 1-8x 4-8x 4-16x off -90 to 90 -0.88 -1.42 -0.54 -1.17 on -90 to 90 -1.43 -2.39 -0.98 -2.17 off 40 to 60 -1.24 -1.90 -0.67 -1.37 on 40 to 60 -2.01 -3.28 -1.29 -2.82  346 RANDENIYA ET AL: BRONO2 EROSOL REACTION; EFFECT ON O 3 by 0.014 %/day, but with HSCTs present, he corre- sponding ncrease s 0.010 %/day. The overall esult of including eaction 3) on the HSCT impact s therefore a small reduction n the rate of destruction f ozone by this cycle. Similar non-linear effects are found for the C10• and BrO• cycles lthough he contributions rom these cycles are smaller. The differences ue to reaction 3) become much more marked at higher northern atitudes, the most ob- vious being a reversal n sign or ozone change round the autumn equinox polewards of 650 N. Because of the constant daylight at high northern atitudes n summer, reaction 1)is ineffective hereas eaction 3) contin- ues to be effective eading to the calculated ncrease in /HOwl n summer. Transport of NO• from the air- craft to this region hen leads o a decrease n the loss rate for O•. Because the chemical lifetime for ozone at high latitudes is of the order of a year, the effects of these summer-based hanges re not evident on the local ozone until late summer. Volcanic Episodes Part B of Table demonstrates he potential mpor- tance of reaction 3) when considering he impact of volcanic vents n ozone. Here he % change n ozone s calculated etween tmospheres hat contain 1x and 4x), (1x and 8x), (4x and 8x) and 4x and 16x) aerosols. n all cases, hen he aerosol evel s ncreased, the decrease n ozone s greater or the calculations hat include eaction 3). For example, or 40øN o 60øN he decrease s 1.2% without eaction 3) and 2.0% when t is ncluded hen he aerosol evel s changed rom 1x to 4 x. When the aerosol evel s changed rom 4x to 16 x the corresponding zone decreases re 1.4% and 2.8% respectively. onsequently, he nclusion f reaction 3) will cause ubstantial hanges o the calculated ffects of sudden ncreases n sulphate aerosol evels such as those arising rom the eruption of Mt Pinatubo. The box-model calculations of Danilin and McConnell showed hat reaction 3) was an important rocess or the depletion of O3 at 20 km and 60øS when the aerosol levels were about 40x. Their results indicate that the greatest depletion f ozone occurs n September, .e., late winter/early spring. However, most of the addi- tional ozone epletion ttributable o reaction 3) oc- curs elow 0 km and, or our 2-D calculation, he great- est additional depletion n the ozone column occurs n late summer and early autumn. Conclusions Inclusion f reaction 3) has been shown o produce significant hanges o the calculated zone columns n the background tmosphere nd to the calculated m- pact of a fleet of HSCTs n ozone. Although nclusion f this reaction s important to the chemistry of the lower stratosphere or any level of sulphate aerosol, he effects are greatest or conditions here eaction 1) is close o saturation and the day ength s long. These conditions exist in periods of volcanically-enhanced erosol evels and at high latitudes in summer. References Danilin, M. Y. and J. C. McConnell, Stratospheric ef- fects of bromine activation on/in sulfate aerosol, J. Geophys. Res., 100, 11237-11243, 1995. DeMote, W. B. et al., Chemical Kinetics and Photo- chemical Data for Use in Stratospheric odeling, JPL Publication 94-26, 1994. Hanson, D. R. and A. R. Ravishankara, eterogeneous chemistry of bromine species n sulfuric acid under stratospheric onditions, Geophys. Res. Lett., 22, 385-388, 1995. Johnston, H., Reduction of stratospheric zone by ni- trogen oxide catalysts rom supersonic ransport ex- haust, Science, 173,517-522, 1971. Orlando, . J. and J. B. Burkholder, as-phase V/visible absorption spectra of HOBr and Br20, J. Phys. Chem., 99, 1143-1150, 1995. Prather, M., Catastrophic oss of stratospheric zone n dense volcanic clouds, J. Geophys. Res., 97, 10187- 10191, 1992. Stolarski, R. S. et al., 1995 Scientific Assessment f the Atmospheric ffects f Stratospheric ircraft, NASA Reference Publication 1381, 1995. Stolarski, R. S. and H. L. Wesoky, eds., The Atmo- spheric Effects of Stratospheric Aircraft: A Third Program Report, NASA Reference ublication 313, 1993. WMO, Scientific ssessment f Ozone Depletion: 991, WMO Report No. 25, 1992. Weisenstein, . K. et al., Effects on stratospheric zone from high-speed ivil ransport: Sensitivity o strato- spheric erosol oading, J. Geophys. es., 98, 23133- 23140, 1993. Wennberg, P. O. et al., Removal of stratospheric 3 by radicals: n situ measurements f OH, HO2, NO, NO•, C10, and BrO, Science, 66,398-404, 1994. L. K. Randeniya, P. F. Vohralik, I. C. Plumb and K. R. Ryan, CSIRO Division f Applied Physics, O Box 218, Lindfield, NSW 2070, Australia S. L. Baughcum, he Boeing Company, O Box 3707, MS 6H-FC, Seattle, WA 98124 (received August 995; evised November 995; accepted 5 December 995.)
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