Measurement of nitrous acid (HONO) and the implications to photochemical pollution

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Measurement of nitrous acid (HONO) and the implications to photochemical pollution


Author: Zha, Qiaozhi
Title: Measurement of nitrous acid (HONO) and the implications to photochemical pollution
Degree: M.Phil.
Year: 2015
Subject: Gases -- Measurement.
Air -- Pollution -- China -- Hong Kong.
Hong Kong Polytechnic University -- Dissertations
Department: Dept. of Civil and Environmental Engineering
Pages: 12, 86 pages : color illustrations
Language: English
InnoPac Record:
Abstract: Nitrous acid (HONO) plays significant roles in atmospheric chemistry;it can be the dominant source of hydroxyl radical (OH) in urban area after early morning and an important daytime source of nitric oxide (NO) in rural and remote regions.Despite its importance,the formation mechanism of HONO is not completely understood.Many field HONO measurements have been conducted in different locations of the world since 1979,which have improved the understanding of HONO formation in terrestrial environment.However,there have been very few HONO measurements in the marine boundary layer (MBL).Considering the fact that many cities are located in the coastal regions,it is of great importance to know the abundance and sources of HONO in the MBL environment and to investigate the impact of HONO on photochemistry.Information on ambient HONO concentrations are also needed to validate and improve current global and regional chemical transport models (CTMs).In this study, continuous measurements of HONO were performed using a long-path absorption photometer (LOPAP) instrument at a coastal background site (Hok Tsui) in Hong Kong during September-December 2012.Meteorological parameters and other related pollutants such as nitric oxide (NO), nitrogen dioxide (NO₂),total reactive nitrogen (NOy),ozone (O3) and carbon monoxide (CO) were also measured.Analysis of the overall data and the selected cases reveal following major findings:An averaged diurnal pattern with two HONO peaks is observed at Hok Tsui, which is different from the typical HONO pattern (with only a single nighttime peak).The first peak (with a mean concentration of 0.155 ppbv) that occurs before dawn is typical and should be due to the accumulation of nocturnal HONO;the second peak with even higher HONO level (mean concentration = 0.160 ppbv) shortly after sunrise is quite unique and should be related to strong formation of HONO during daytime.Nocturnal HONO formation is investigated by estimating and comparing the NO2-to-HONO conversion rates (CHONO) in sea-coming air masses,land-coming air masses,and other studies.The selected air masses are distinguished using back trajectory model (hybrid single particle lagrangian integrated trajectory, HYSPLIT model) and on-site wind. The CHONO in the "sea cases" (3.17-3.36 × 10-2 h-1) were significantly larger than rates determined in "land cases" (1.20-1.30 × 10-2 h-1) and other studies (0.80-1.80 × 10-2 h-1).Daytime HONO production, including both "known" and "unknown" sources, are examined.The source strength of presently known daytime sources is too small to explain the measured daytime peak,suggesting the presence of the "unknown" daytime source.The strength of the unknown source (up to 0.2 pptv/s), which is estimated using a photostationary state (PSS) method, is 4 8 times larger than the strength of the known sources (homogeneous reaction (NO+OH->HONO)and heterogeneous reaction(2NO₂+H₂O--surface--> HONO+HNO₃)).The unknown source also shows moderate correlation with NO₂, and the correlation (R = 0.57) is further enhanced by adding the term of solar radiation, implying that this unknown daytime HONO source may be a photo-enhanced heterogeneous process. HONO is found to be a more important source for OH throughout the entire daytime at Hok Tsui, compared to the contribution from ozone photolysis which is another major source of OH.

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