ISSN:
1573-0662
Keywords:
multiphase
;
modelling
;
radical chemistry
;
cloud chemistry
Source:
Springer Online Journal Archives 1860-2000
Topics:
Chemistry and Pharmacology
,
Geosciences
Notes:
Abstract A Chemical Aqueous Phase Radical Mechanism (CAPRAM) for modelling troposphericmultiphase chemistry is described. CAPRAM contains (1) a detailed treatmentof the oxidation of organic compounds with one and two carbon atoms, (2) anexplicit description of S(IV)-oxidation by radicals and iron(III), as well asby peroxides and ozone, (3) the reactions of OH, NO3,Cl- 2, Br- 2, andCO- 3 radicals, as well as reactions of the transitionmetal ions (TMI) iron, manganese and copper. A modelling study using a simplebox model was performed for three different tropospheric conditions (marine,rural and urban) using CAPRAM coupled to the RADM2-mechanism (Stockwell etal., 1990) for liquid and gas phase chemistry, respectively. In the maincalculations the droplets are assumed as monodispersed with a radius of 1μm and a liquid water content of 0.3 g m-3. In the coupledmechanism the phase transfer of 34 substances is treated by the resistancemodel of Schwartz (1989). Results are presented for the concentration levelsof the radicals in both phases under variation of cloud duration and dropletradius.The effects of the multiphase processes are shown in the loss fluxes of theradicals OH, NO3 and HO2 into the cloud droplets. Fromcalculations under urban conditions considering gas phase chemistry only theOH maximum concentration level is found to be 5.5 · 106cm-3. In the presence of the aqueous phase (r = 1 μm,LWC = 0.3 g m-3) the phase transfer constitutes the most importantsink (58%) reducing the OH level to 1.0 · 106cm-3. The significance of the phase transfer during night time ismore important for the NO3 radical (90%). Its concentrationlevel in the gas phase (1.9 · 109 cm-3) isreduced to 1.4 · 106 cm-3 with liquid waterpresent. In the case of the HO2 radical the phase transfer from thegas phase is nearly the only sink (99.8%). The concentration levelscalculated in the absence and presence of the liquid phase again differ bythree orders of magnitude, 6 · 108 cm-3 and 4.9· 105 cm-3, respectively. Effects of smallerduration of cloud occurrence and of droplet size variation are assessed.Furthermore, in the present study a detailed description of a radicaloxidation chain for sulfur is presented. The most important reaction chain isthe oxidation of (hydrogen) sulphite by OH and the subsequent conversion ofSO- 3 to SO- 5 followed by theinteraction with TMI (notably Fe2+) and chloride to producesulphate. After 36 h of simulation ([H2O2]0= 1 ppb; [SO2]0 = 10 ppb) the direct oxidation pathwayfrom sulfur(IV) by H2O2 and ozone contributes only to8% (2.9 · 10-10 M s-1) of the total lossflux of S(IV) (3.7 · 10-9 M s-1).
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1023/A:1006318622743
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