LABORATORY INVESTIGATIONS OF GASEOUS SULFURIC-ACID FORMATION VIA SO3-]H2SO4+M - MEASUREMENT OF THE RATE-CONSTANT AND PRODUCT IDENTIFICATION(H2O+M)

We have measured the reaction SO3+H2O+M-->H2SO4+M in a flow tube react or in the pressure range between 31 and 260 mbar of synthetic air carr ier gas. SO3 has been produced in the flow tube using a radical source for production of OH, which was converted to SO3 by reaction with SO2 . The concentration of SO3 as well as the concentration of the reactio n product has been measured by chemical ionization mass spectrometry ( CIMS). Evidence has been found for the product being H2SO4. With incre asing H2O-concentration in the flow reactor, an exponential decrease o f the concentration of SO3 accompanied by a corresponding rise of the H2SO4-concentration has been observed. A detailed flow analysis has be en carried out and the rate constants have been corrected for diffusiv e effects. The effect of heterogeneous reactions on the flow reactor w alls has been investigated and could partially be quantified. From the experiment, values for the reaction rate constant in the range betwee n 0.72 and 1.39 X 10(-15) cm(3) s(-1) have been inferred. These values are thought to be due to gas phase reactions only. Within the experim ental accuracy the reaction rate constant did not undergo a significan t pressure variation. The mean rate constant averaged over the total p ressure range is (1.2 +/- 0.2) x 10(-15) cm(3) s(-1).