Measurement and investigation of chamber radical sources in the European Photoreactor (EUPHORE)

It is essential to quantify the background reactivity of smog-chambers, since this might be the major limitation of experiments carried out at low pollutant concentrations typical of the polluted atmosphere. Detailed investigation of three chamber experiments at zero-NOx in the European Photoreactor (EUPHORE) were carried out by means of rate-of-production analysis and two uncertainty analysis tools: local uncertainty analysis and Monte Carlo simulations with Latin hypercube sampling. The chemical mechanism employed was that for methane plus the inorganic subset of the Master Chemical Mechanism (MCMv3.1). Newly installed instruments in EUPHORE allowed the measurement of nitrous acid and formaldehyde at sub-ppb concentrations with high sensitivity. The presence of HONO and HCHO during the experiments could be explained only by processes taking place on the FEP Teflon walls. The HONO production rate can be described by the empirical equation W(HONO)(EUPHORE)(dry) = a x j(NO2) x exp(- T-0/T) in the low relative humidity region (RH< 2%, a = 7.3 x 10(21) cm(-3), T-0 = 8945 K), and by the equation W(HONO)(EUPHORE)(humid) = W(HONO)(EUPHORE)(dry) + j(NO2) x b x RHq in the higher relative humidity region (2%< RH< 15%, b = 5.8 x 10(8) cm(-3) and q = 0.36, and RH is the relative humidity in percentages). For HCHO the expression W(HCHO)(EUPHORE) = c x j(NO2) exp(- T'(0)/T) is applicable (c = 3.1 x 10(17) cm(-3) and T-0' = 5686 K). In the 0 - 15% relative humidity range OH production from HONO generated at the wall is about a factor of two higher than that from the photolysis of 100 ppb ozone. Effect of added NO2 was found to be consistent with the dark HONO formation rate coefficient of MCMv3.1.