A study of HONO chemistry in the forested environment during PROPHET2016

Project Overview
The PROPHET’2016 summer field measurement intensive is an atmospheric research initiative to improve our understanding of radical chemistry in a forested environment (PROPHET’2016 Whitepaper). Four major science questions have been raised and are to be addressed by this collaborative field campaign: (1) How well do we understand the sources and sinks of HOx radicals both above and below the forest canopy? (2) How important is the contribution of enhanced cycling of radicals by BVOCs and NOx to the observed discrepancies in the HOx budget? (3) How well do we understand the termination of radicals, including the production of hydroperoxides and organic nitrates, and the fate of nitrogen oxides in forest environments? (4) What is the source and composition of the observed missing OH reactivity above and below the forest canopy? We are requesting a supplemental fund through the current NSF grant (AGS-1216166) to support our participation in this major collaborative field study. Our previous measurement results at the PROPHET site have shown substantial levels of nitrous acid (HONO), in the range of 20-150 ppt, exist in and above the forest canopy during the day. Photolysis of HONO is an important HOx precursor upon photolysis, accounting for 18-40% of the daily photolytic HOx budget above the forest canopy. Furthermore, photolysis of HNO3 on the forest canopy surface have been found to generate the majority of HONO observed over the forest, and represents a potentially important pathway to remobilize HNO3 deposited on canopy surface back into the atmosphere in the photochemical reactive forms such as HONO and NOx. However, formation mechanisms and transport of HONO below the forest canopy is still well understood. Since HONO is an important OH precursor and a major intermediate in the renoxifiaction process in this low-NOx rural forested site, it is an important species to measure to fully address the first three science questions in the PROPHET’2016 field study. As a component of the PROPHET’2016 collaborative measurement efforts, we propose to: (1) Measure HONO concentrations continuously at four heights above and below the level of forest canopy, i.e., at 2 m, ~12 m, 22 m (canopy height) and 32 m above the ground, using two 2-channel HONO systems. (2) Determine HONO emission flux from the forest ground soil, using an additional 2-channle HONO measurement system and an open-bottom Teflon chamber. We will determine the contribution of HONO photolysis to the total photolytic HOx source within and above the forest canopy, based on the measurement results of HONO concentrations and UV intensity, to answer the first science question in the PROPHET’2016 Whitepaper. HONO absorbs light in much longer wavelengths into the UV-A range than other photolytic HOx precursors such as ozone and formaldehyde, and thus HONO photolysis is likely to be the most important photolytic HOx source in the morning hours and within the forest canopy. Based on the soil HONO emission flux and the HONO concentration gradient information, we will examine relative strengths of different HONO sources within the forest environment, including photolysis of HNO3 on the canopy surface, heterogeneous NO2-H2O reaction on the forest surfaces, and emission from the soil. Although biochemical/microbial process in the soil has been found to be an important HONO source in the agriculture areas, it is expected to be relatively minor due to the low soil nitrate content in this nutrient-poor forest. We will quantitatively examine how the environmental factors, such as light intensity, temperature/relative humidity in the air and the soil, and wind speed/direction, affect the behaviors of HONO, such as sources, sinks, vertical distribution and transport, in and above the forest. Since HONO is an intermediate in the reactive nitrogen cycling, detailed understanding of HONO chemistry is essential to answer the second and the third science questions in the PROPHET’2016 Whitepaper. The supplemental fund requested will provide training and research opportunities for one Ph.D. student and one summer student intern (undergraduate or MPH student).
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