Abstract
Photolysis rate constant of HNO3 on the surface (HNO3(s)) has been found to be enhanced by 1–4 orders of magnitude from that of gaseous HNO3, with HONO and NO2 as the main products. Such Re-NOx-ification pathway extends the apparent lifetime of reactive nitrogen species and modifies the atmospheric oxidative capacity along its long-rang transport. Despite of its importance, the detailed kinetics and mechanisms of HNO3(s) photolysis are still not clear. Surface film of HNO3 and organic compounds is ubiquitous in the environment and imposes matrix effect on HNO3(s) photolysis. Here we studied photolysis of HNO3 on Pyrex glass in a photochemical flow reactor over a wide range of HNO3 surface density (DHNO3) with or without the presence of model organic compounds. The photolysis rate constant of HNO3(s) varied with DHNO3 and surface-catalysis mechanism was proposed. Organic compounds further enhance the photolysis rate constant by up to one order of magnitude via both photosensitization and H-donating reaction. The H-donating reaction enhances as well the secondary HONO yield from reaction between the primary product NO2 and adjacent H-donor, and thus increases the HONO/NO2 production ratio. Finally, detailed mechanisms involving surface-catalyisis, photosensitization and H-donating reactions was integrated.
Highlights
Background level and correctionsSeveral corrections are made when calculating HONO and NO2 production rates and photolysis rate constants
We have studied various natural and artificial surfaces in a previous paper and measured the photolysis rate constant of HNO3(s) in the range from 9 × 10−6 s−1 to 3.7 × 10−4 s−1, depending on the types of surfaces[3]
It was evidential that the immediate increase HNO3(s)
Summary
Several corrections are made when calculating HONO and NO2 production rates and photolysis rate constants. Background signals from dark experiment were used as experiment baseline and were subtracted from the light exposure signals. Blank signals contributed from photolysis of deposited HNO3 on reactor and window surface were corrected by subtracting one half of the blank signals from the light exposure signals, since the bottom of the flow reactor was covered by the Pyrex glass surface in light exposure experiment. Photolytic losses of HONO and NO2, the products from HNO3(s) photolysis, in the flow reactor during light exposure experiment were considered. The production rate (nmol s−1) of HONO, PHONO, and production rate of NO2, PNO2, were calculated by: PHONO = With a residence time of about 30 seconds in the flow reactor, ~5% HONO loss was calculated, and about 25% NO2 loss was observed when a gaseous NO2 standard (Matheson Tri-Gas Inc., CP) was introduced into the flow reactor.
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