Notice of Public Lecture: Kristin Julianne Wall

 Faculty of Graduate Studies 

Graduate Programme in Chemistry 
ORAL EXAMINATION PROSPECTUS 

Kristin Julianne Wall

A Candidate for the Degree of 
Doctor of Philosophy

Title of Thesis:
Investigation of the photoenhanced reduction of nitrogen dioxide (NO2) on organic films and above soils as the missing source of daytime tropospheric nitrous acid (HONO)

Abstract:  It has long been recognized that the photolysis of nitrous acid (HONO), is a potentially important production mechanism for the hydroxyl radical (•OH), the most important oxidizing species in the daytime atmosphere, in the polluted urban atmosphere, with the newest tropospheric chemistry models specify that ozone mixing ratios are about three times more sensitive to HONO than to NO2 inputs. Although it is ordinarily proposed that HONO is generated by the heterogeneous conversion of NO2 on humid surfaces, it is still under debate whether HONO formation is controlled by the surface of aerosol particles or by ground surfaces. Recent observed unpredictably high HONO daytime concentrations (over snow, ground and vegetation sources), demanding such a heterogeneous conversion to proceed greater than 60 times faster at noon than during the night, likely due to unknown surface photochemistry, have prompted the study at hand, concerning the effect of ultraviolet-A radiation on the uptake kinetics of gas-phase NO2 on various phenolic-containing organic films (taken as proxies for surfaces encountered in the troposphere) using our in-house wetted-wall flow tube (WWFT) photoreactor. In assessing the contribution of HONO to the HOx (OH+HO2+RO2) budget, such ambiguities are key, and therefore critical to the entire VOC/NOx/O3 chemistry involved in photochemical pollution.

The WWFT film parameters (flow dynamics, uniformity and evaporative cooling), along with the gas-phase HONO and NO2 detectors, are discussed in detail, justifying the correctness of its design for this project. The kinetics of the photoenhanced NO2 uptake reaction under acidic conditions are examined separately for the tested non-humic compounds, including water and selected ortho-nitrophenols (2-NP and 4M2NP); and humic compounds, namely humic acid (HA), with the emphasis of the discussion placed on the latter, ubiquitous group of environmental compounds.

Our experimentally determined first-order kinetics (k1,NO2 = 6.65(± 0.86) ×10-3 s-1) with respect to NO2 concentration (18-72 ppb, typical urban levels), for a concentrated (1 g L-1) HA film, dismisses an elementary photochemical mechanism. For the photolysis reaction (NO2 = 0 ppb), no HONO(g) was observed. Based on our HONO efficiency data for a wide variety of HA samples, and their available composition data, we conclude that NO2 uptake increases in kinetic favourability for HA samples having increasing degrees of humification and diagenetic changes, (and thus refractivity); and aliphatic character. Over an environmentally relevant range of HA concentration (0.5-0.84 g L-1), first-order kinetics were observed, with a non-zero intercept likely corresponding to a measurable surface adsorption delay. Above 0.84 g L-1, a quadratic dependence was observed, likely due to a subsequent geometry optimization under these solvation conditions. A relatively simple, ‘well-mixed, equilibrium conditions’ partitioning model allowed computations of the absolute measure, yieldHONO, for a wide range of tested pH values (1.5 to 4.3). The modeled behaviour shows that the largest kinetic yield, corresponds to the most acidic film where the aqueous HA matrix is highly coiled, and decreases as the HA matrix slowly expands and, as such, gradually favours complexation over NO2 uptake. For our usual gas-liquid reaction time (1 s), we estimate that the reactant particles (on average) have diffused into only about 2% of the film depth, suggesting that the NO2 uptake under study is, indeed, diffusion-limited, with a surface coverage of ≥ 1.5 HA monolayers.

Our simulated HONO emission rate profiles, computed using our HONO efficiency rates and geometric considerations, while rather approximate, demonstrate that the complex pH-dependent kinetics of the surface chemistry studied is sufficient for producing the typical range of daytime HONO emission rates observed in the field for both rural (170-500 ppt h-1) and moderately to heavily polluted conditions (up to 2 ppb h-1).