Modeling of daytime HONO vertical gradients during SHARP 2009

K W Wong,B Lefer,C Tsai,N Grossberg,J Stutz

doi:10.5194/acp-13-3587-2013

Abstract

Abstract. Nitrous acid (HONO) acts as a major precursor of the hydroxyl radical (OH) in the urban atmospheric boundary layer in the morning and throughout the day. Despite its importance, HONO formation mechanisms are not yet completely understood. It is generally accepted that conversion of NO2 on surfaces in the presence of water is responsible for the formation of HONO in the nocturnal boundary layer, although the type of surface on which the mechanism occurs is still under debate. Recent observations of higher than expected daytime HONO concentrations in both urban and rural areas indicate the presence of unknown daytime HONO source(s). Various formation pathways in the gas phase, and on aerosol and ground surfaces have been proposed to explain the presence of daytime HONO. However, it is unclear which mechanism dominates and, in the cases of heterogeneous mechanisms, on which surfaces they occur. Vertical concentration profiles of HONO and its precursors can help in identifying the dominant HONO formation pathways. In this study, daytime HONO and NO2 vertical profiles, measured in three different height intervals (20–70, 70–130, and 130–300 m) in Houston, TX, during the 2009 Study of Houston Atmospheric Radical Precursors (SHARP) are analyzed using a one-dimensional (1-D) chemistry and transport model. Model results with various HONO formation pathways suggested in the literature are compared to the the daytime HONO and HONO/NO2 ratios observed during SHARP. The best agreement of HONO and HONO/NO2 ratios between model and observations is achieved by including both a photolytic source of HONO at the ground and on the aerosol. Model sensitivity studies show that the observed diurnal variations of the HONO/NO2 ratio are not reproduced by the model if there is only a photolytic HONO source on aerosol or in the gas phase from NO2* + H2O. Further analysis of the formation and loss pathways of HONO shows a vertical dependence of HONO chemistry during the day. Photolytic HONO formation at the ground is the major formation pathway in the lowest 20 m, while a combination of gas-phase, photolytic formation on aerosol, and vertical transport is responsible for daytime HONO between 200–300 m a.g.l. HONO removal is dominated by vertical transport below 20 m and photolysis between 200–300 m a.g.l.

Highlights

Data Systems on aerosol and ground surfaces have been proposed to explain the presence of daytime HONO
While the model results of other trace gases have been compared to observations to ensure that we capture daytime chemistry, we will not show these comparisons here, as the vertical profiles of NO2 and HONO are most relevant for this study
A 1-D chemistry and transport model was used to reproduce the vertical gradients of HONO, NO2 and the HONO/NO2 ratios observed during the Study of Houston Atmospheric Radical Precursors (SHARP) 2009 experiment in Houston, TX, and to investigate potential daytime HONO formation pathways

Summary

Introduction

Data Systems on aerosol and ground surfaces have been proposed to explain the presence of daytime HONO. It is unclear which mechanism dominates and, in the cases of heteroge-. Observations of ban atmosphere were nmitardoeuGsbaeycdoidifsf(eHcrOieenNtniOatl)iofiinpcttihcealpaobllsuotrepdtiuornspectroscopy (DMOAoSd)ebyl DPlaettveet allo. M neous mechanisms, on which surfaces they occur. Its role as a hydroxyl radical (OH) precursor in the morning. Vertical concentration profiles of HONO and its precur- It has since been recognized that HONO photolysis Daytime HONO and NO2 vertical tion R1) plays chemistry. 70–130, and 130–300 m) in Houston, TX, during the 2009 Study of Houston Atmospheric Radical Precursors (SHARP). An imporHtanyt drorloe ilnoigniytiaatinngddaytime photoprofiles, measured in three different height intervals

Results

Discussion

Conclusion