BASIC MECHANISMS OF PHOTOCHEMICAL AEROSOL FORMATION*

Journal/Book: Atmospheric Environment Pergamon Press 1967. Vol. 1 pp. 287-306.. 1967;

Abstract: A. GOETZ and R. PUESCHEL California Institute of Technology Pasadena California U.S.A. (First received 17 January 1966 and in final form 5 December 1966) Present affiliation National Center for Atmospheric Research Boulder Colorado and Atmospheric Research Group Altadena California. Dept. of Civil Engineering University of Washington Seattle Washington. SUMMARY The highlights of the above results may briefly be summarized: In the absence of SO2 the aerosols generated by the photochemical reaction between the olefins and NO2 invariably produce the largest light-scattering effect (SR-level) in the low humidity range (15-30 per cent); the effect declines steadily with an increase of humidity (< 70 per cent) STEVENSON et al. (1965). In the presence of defined centers (TW latex) the aerosol formation (SR) follows the same pattern though an a substantially higher level which can exceed by an order of magnitude the scattering levels so of the particles before photolysis. An evaluation of the AS size-spectra shows for the same reactant concentrations no significant variation in the growth range (dmin- dmax) of the latex nuclei with increasing humidity; however it does show a significantly larger thermal stability factor . Humidity also tends to increase autonucleation at low concentrations of reaction centers as evident from the variation of with Ct. A striking phenomenon is the variation of SR with the sequence order by which the reactants meet in the channel flow. In the absence of particulates the effect is negligible but becomes quite significant when they are present (TW latex). Then their contact with NO2 before the other reactants (sequence a) always produces a muck larger SR-level than the combined induction of the reactants (sequence c) regardless of prevailing humidity (TABLE 2). This effect can also be seen in the AS spectra (TABLE 3) from the increased thermal stability for the largest latex concentrations which involve the least autonucleation. This indicates an activation of the nuclei by adsorption or by statistical accumulation of the NOx molecules around each particle and therefore a higher probability of photoactivation. This general pattern points in the same direction as previous findings (GOETZ and PUESCHEL 1965) by different methods. The presence of SO2 even at small fractional concentration causes drastic changes. A molar concentration ratio (SO2/NO2 approx. 10-2) reduces the aerosol formation (i.e. autonucleation) sharply when stable centers are absent. This inhibitory effect of relatively small SO2 concentrations is greater at higher relative humidities being some tenfold larger at 50 per cent r.h. than at 15 per cent. Greater SO2 concentrations gradua11y reverse this inhibition. At low relative humidity and at SO2 concentrations equal to those of NO2 the aeroso1 leve1 is about five times that in the absence of SO2 while at high humidity the level never increases above half its value without SO2. The least aerosol formation appears to occur with about 2 ppm of SO2. The presence of reaction centers (TW or latex) does not alter the effect of initial reduction and subsequent growth by SO2 nor the strong influence of humidity. However a distinct change in the sequence order is produced by the particulates. Their primary contact with SO2 causes the least reduction and the largest growth of the aerosol level-qualitatively independent from type or concentration of the reaction centers. The consistent evidence that in the absence of particulates SO2 does not alter the sequence order (a > b) which prevails for SO2 = 0 indicates that SO2 has a larger tendency to associate with the centers than has NO2. Furthermore this initial association impairs the accumulant formation an the surface least. The reason for the initial reduction caused by small concentrations of SO2 is not obvious but perhaps the interaction of SO2 with NO2 inhibits the subsequent photolysis of the reactant combination particularly at higher humidities. This Interpretation would agree with the finding that the difference of contact-sequence is almost nil at the lowest humidity increases with it and shows always the lowest aerosol level at (c) i.e. for the combined induction of all three reactants. ___MH

Zurück | Weiter