Abstract. 1 Introduction

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1 The influence of cloud chemistry on the budget of photo-oxidants G. Mauersberger Brandenberg Technical University Cottbus, Rudower Chausse 5, D Berlin, Germany Abstract The RADM II gas-phase system and a comprehensive reaction system in the liquid phase are coupled in a cloud- chemical box model in order to evaluate the great number of investigated reactions in the liquid phase and to derive a condensed mechanism for use in regional chemistry-transport models. The model results confirm that cloud chemistry can strongly affect the budget of photooxidants. The decoupled direct method (DDM) is used to perform a local sensitivity analysis. This analysis was applied by Pandis and Seinfeld [6] to determine the importance of reactions and pathways. It is shown that this way cannot be used in cases of complex structures like the oxidant dynamics. Therefore, the kinetic pathways of substructures are analysed and a non-local sensitivity analysis is performed. 1 Introduction The role of clouds in tropospheric photochemistry has been investigated by model studies ( Lelieveld and Crutzen [3] ) as well as byfieldexperiments ( Faust et al. [2], Acker et al. [1]). On the other hand, cloud chemistry in regional chemistry-transport models is directed mainly on the description of the acid formation in clouds and precipitation. The aim of the contributions to the projects SANA and EUMAC consists in the development of a cloud chemical module which is able not only to improve the description of acid formation in clouds but also to simulate the influence of cloud chemistry on the budged of oxidants. Chemical processes consume the most CPU time in a regional chemistry-transport model. Therefore, a strategy for the evaluation of liquid-

2 128 Observation and Simulation of Air Pollution phase reactions and a special fast numerical solver for the cloud chemical module are to develop and to apply. 2 Model description For the special purpose of cloud chemistry a tool ASOCC (Automatic Simulation of Cloud Chemistry) was developed. It reads the mnemonic names for the species in the gas- and liquid phase, for the reactions and reaction rate expressions from a datafile,together with the initial concentrations, the Henry constants and accommodation coefficients for the soluble gases, the equilibrium constants for grouped liquid phase compounds, and the values of other box model parameters and generates the code for the differential equation system by adding all reactions which are sinks or sources for a certain compound. The corresponding gas-liquid compounds are selected and their derivations are completed by the mass exchange rates between the phases. Also the electroneutrality equation and the code of the Jacobian are generated. On this way, a high variability with respect to the included compounds and reactions has been achieved. ASOCC has been applied to couple the RADM gas-phase chemistry and the comprehensive liquid-phase system compiled by Moller and Mauersberger [5]. It is planed to update and complete the included reactions in the liquid phase by the radical anion reactions investigated in the HALIPP project. A modification of the Hindmarsh LSODE package is used to solve the system. In connection with this modification ASOCC is able to produce simultaneously first-order sensitivity coefficients Sjj of all predicted concentrations q with respect to parameters p; where p; are reaction rate coefficients, initial values and other model parameters such as liquid water contend, temperature, and accommodation coefficients. The dimension!ess sensitivity coefficients Sjj can be interpreted by In the case of photolysis rate coefficients the parameter p; is a function of time. Generally, S expresses in this situation the Frechet's differential. The sensitivity coefficient S calculated by ASOCC is assuming that 6p/p is time-independent. 3 Results To demonstrate the possible influence of cloud chemistry on the budget of photooxidants a test case of the EUROTRAC chemical mechanism working group has been used for initial concentrations in the gas phase, photolysis rate coefficients and emission rates. In the time between 10 a.m. and 2 p.m. of the

3 Observation and Simulation of Air Pollution 129 second day a cloud event is simulated. The initial values in the liquid phase have been taken from mean concentrations measured at the Mt. Brocken cloud chemistry station by Moller et al., [4]. To separate the influence of liquid phase chemistry the photolysis rate coefficients are not changed in the cloud case. In figure 1 the total concentrations of photooxidants in this time period are compared. Since in the equilibrium case the mass ratio q between liquid and gas phase is very small for Og, HC^, OH, the total concentrations shown in figure 1 are equal to the gas-phase concentrations for these compounds. In the case of H2 2 the mass ratio q is about 1. Therefore, if cloud is raining out, the half amount will be deposited; if cloud is evaporating, the whole amount will be present in the gas phase Transactions on Ecology and the Environment vol 6, 1995 WIT Press, ISSN o.u u ^^^ A fi time [hi H H ^ Figure 1: Development of the total photooxidant concentrations; dashed: cloudless case, solid: cloud case The short-lived intermediates are reduced immediately in the presence of clouds by 75% (OH) and by 90% (HO2 ). The event starts with a small SO2

4 130 Observation and Simulation of Air Pollution concentration of 0.65 ppb(v). After the S(IV) to S(VI) conversion is mostly finished the total amount of F^C^ is growing in the cloud case stronger than in the cloudless case. The production of ozone is suppressed in the cloud case. An other view on the influence of cloud chemistry on the budget of oxidants is given by table 1. Q2 Ql Q2 S I II III ace. cloudless case gas/total -2,42 4, / / / gas / / cloud case liquid /2, total / / / Table 1: Net reaction balance [ppb(v)] in the OH - HC>2 - ^2^2 subsystem during the simulated period 10 a.m. - 2 p.m. ace.: accumulation of H^^2 > %/y : % - source, y - target Already the rough analysis which is possible by consideration of the net reaction balance in the OH - HO2 - ^O2 subsystem gives a impression how fundamental the oxidant chemistry is affected by liquid-phase reactions. The directions of the OH source balance (Ql) and of the OH -> HO2 conversion balance (I) become opposite in the cloud case. In the gas phase, the net production of OH + HO2 (Ql + Q2) is much higher in the presence of clouds. Therefore more HO2 is available produced in the gas phase (I+Q2). Although a great part of this HO2 is consumed in the liquid phase by the reduction of transition metal ions (TMI's),

5 Observation and Simulation of Air Pollution 131 the }±2 2 production is in the cloud case much more effective. It occurs only in the liquid phase whereas the gas-phase reactions are consuming I^C^. It will be shown later that the pathway II is dominated by the TMI reactions. In order to derive a condensed liquid-phase reaction mechanism for use in regional chemistry -transport models, it is necessary to develop a evaluation strategy. Pandis and Seinfeld [6] used a local sensitivity analysis of a pure liquid phase system to select the dominant kinetic pathways for a considered target species by ordering their first-order sensitivity coefficients Sy described in (1). Especially in more complex reaction systems the relation between the target concentration and a system parameter might be high non-linear. In this case the local sensitivity coefficient S will give a wrong estimation of the importance of reactions and initial values. In order to demonstrate this effect, the non-local sensitivity coefficients (2) are calculated additionally. Figure 2 shows the difference of s and S using the example of 1^02 sensitivity with respect to SC>2 and TMI initial values. 0.2 (b) time [hj Figure 2: Sensitivity coefficients of P^C^ with respect to (a) - (b) - TMI ;=iq ; solid: s (non-local); dashed: S (local) The dependence between F^C^ and SC>2 is mainly determined by the sulphate production via SIV + F^C^ in the liquid phase. In the case of such simple relation the sensitivity coefficients s and S will give similar results (fig. 2a). On the other hand, the TMI chemistry is highly complex and affects the whole oxidant budget. In this situation the coefficients s and S can differ even in the sign as figure 2b shows. In contrast to the local sensitivity coefficients S the non-local coefficients s are not additive. Therefore, s is calculated with respect to groups of reactions.

6 132 Observation and Simulation of Air Pollution The reaction group (RG) I consists of all TMI reactions; in RG II the four liquid-phase reactions HO2 + O2" + H+ > H2O2 + O2 O] + O2" > OH + 2O2 H2O2 + OH -> HO2 + ^O HSOg" + H2O2 -> SO^- +^0 + H+ are grouped. The calculation of sensitivity coefficients concerning to reactions means that the sensitivity coefficients with respect to the reaction rate coefficients are determined. The coefficient s^ infigure3 describes the variations of oxidants between the cloud case and the cloudless case. The comparison between Sj^> and SRGI+RQH shows that the reactions grouped in RG I and RG II contribute the main effect of cloud chemistry on the budget of oxidants. o.o OH time Ch] H H Figure 3: Non-local sensitivity coefficients s with respect to LWC (solid), RG I + RG II (dashed), mass transfer coefficient of HCHO (dotted)

7 Observation and Simulation of Air Pollution 133 Lelieveld and Crutzen [3] have discussed that the uptake of HCHO by the liquid phase is the main source of HO% decreasing and subsequently of reduced ozone production in the presence of clouds. However, the sensitivity coefficient SkgHCHO which gives the effect of suppressed HCHO flux between gas and liquid phase is very low for HO% and for 63 (fig. 3). Only ^2^2 is affected by the suppressed flux of HCHO. This results from the formation of S(IV)-HCHO adducts (which is not included in the reaction scheme of Lelieveld and Crutzen and even increase the solubility of HCHO) H02 H time Eh] Figure 4: Non-local sensitivity coefficients s with respect to RGI (solid), RG I (dashed), RG II (dotted) + RG II Figure 4 illustrates the fact that the non-local sensitivity coefficients s are nonadditive in contrast to S. Additionally, the coefficients S can be calculated simultaneously using the DDM method but the non-local coefficients not.

8 134 Observation and Simulation of Air Pollution Therefore, much more effort is necessary to carry out a non-local sensitivity analysis. The reaction group RG I do not affect the Og, OH, HC>2 concentrations as long as the TMI reactions are active. However, the difference between SRQ i + RG II and SRQ i shows that the influence of RG II becomes remarkable in the absence of TMI reactions. Acknowledgements This research has been funded by the BMBF of the Federal Republic of Germany. References 1. Acker, K., Wieprecht, W., Moller, D., Mauersberger, G., Naumann, S. & Oestereich, A. Evidence of ozone destruction in clouds, Naturwissenschaften, 1995, 82, Faust, B.C., Anastasio, C, Allen, J.M. & Avakaki, T. Aqueous phase photochemical formation of peroxides in authentic cloud and fog water, Science, 1993, 260, Lelieveld, J. & Crutzen, P.J. The role of clouds in tropospheric photochemistry, J. Atm. Chem., 1991,12, Moller, D., Acker, K., Wieprecht, W. & Auel, R. Study of interaction of photooxidants and acidic components between gas and liquid phase, in Annual EUROTRAC Report 1993, Part 6, (P.M. Borell, et al., eds), 1994, Moller, D. & Mauersberger, G. Cloud chemistry effects on tropospheric photooxidants in polluted atmosphere - model results, /. Atmos. Chem. 1992,14, Pandis, S.N. & Seinfeld, J.H. Sensitivity analysis of a chemical mechanism for aqueous-phase atmospheric chemistry,/. Geophys. Res., 1989, 94,

ABSTRACT INTRODUCTION

ABSTRACT INTRODUCTION The role of clouds in transformation and removal of air pollutants D. Moller, G. Mauersberger Department for Air Chemistry, Fraunhofer Institut for Atmospheric Environmental Research, D-1199 Berlin, Germany