Date of Award

1-1-2011

Language

English

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

College/School/Department

Department of Environmental Health Sciences

Content Description

1 online resource (viii, 147 pages) : illustrations (chiefly color), color maps.

Dissertation/Thesis Chair

Xianliang Zhou

Committee Members

Chris Walcek, Haider Khwaja, Michael Kitto, Ying Wang

Keywords

flux, HONO, nitrous acid, photochemistry, rural troposphere, vertical profile, Atmospheric nitrous oxide, Tropospheric chemistry, Tropospheric aerosols

Subject Categories

Other Chemistry

Abstract

Nitrous acid (HONO) is an important precursor of hydroxyl radical and plays an important role in the terrestrial boundary layer photochemistry. However, there are still many unknowns regarding HONO chemistry such as formation mechanisms and distributions in the troposphere, especially in rural and remote environments. In this research, we presented the first vertical concentration profiles of HONO in the troposphere, measured on a small aircraft platform. The HONO mixing ratios ranged from 4 to 17 pptv in the free troposphere and from 8 to 74 pptv in the rural boundary layer. The HONO distribution patterns were strongly correlated to the air column stability at the surface, suggesting that ground surfaces was a major source of HONO in the boundary layer. Photolysis of HNO3/nitrate on and in aerosol particles is likely to be the major in situ HONO source, sustaining the large fraction of the observed HONO levels over water bodies and in the free troposphere. A technique based on relaxed eddy accumulation was developed for direct measurement of vertical HONO flux. Mean HONO flux at 10 m above a forest canopy is 0.37×10-6 moles m-2 hr-1, accounting for ~ 60% of the observed daytime HONO concentrations (~70 pptv). Photolysis of HNO3 deposited on canopy surface is likely to be the major mechanism responsible for the observed daytime HONO flux. Laboratory photochemical experiments indicated that organics enhance surface HNO3 photolysis by up to ~10 times compared to pure HNO3 condition, with HONO as the major product and NO2 as the minor product at 50% RH. A mechanism is proposed for HONO formation from surface HNO3/nitrate photolysis, involving NO2* as the primary product, followed by abstraction of H atoms from organic compounds and/or water on surfaces. Derived HNO3 photolysis rate constants on surfaces and in aerosols well explained field observations in aircraft measurement and flux measurement and confirmed that photolysis of aerosol nitrate and HNO3/nitrate on surfaces is a major daytime HONO source and an important re-NOx-ification pathway in the troposphere.

Download

Share

COinS