On the interpretation of in situ HONO observations via photochemical steady state.

Leigh R Crilley,James D Lee,William J Bloss,Dwayne E Heard,Christopher Reed,Danny R Cryer,Louisa Kramer,Francis D Pope,Lisa K Whalley

doi:10.1039/c5fd00224a

Abstract

A substantial body of recent literature has shown that boundary layer HONO levels are higher than can be explained by simple, established gas-phase chemistry, to an extent that implies that additional HONO sources represent a major, or the dominant, precursor to OH radicals in such environments. This conclusion may be reached by analysis of point observations of (for example) OH, NO and HONO, alongside photochemical parameters; however both NO and HONO have non-negligible atmospheric lifetimes, so these approaches may be problematic if substantial spatial heterogeneity exists. We report a new dataset of HONO, NOx and HOx observations recorded at an urban background location, which support the existence of additional HONO sources as determined elsewhere. We qualitatively evaluate the possible impacts of local heterogeneity using a series of idealised numerical model simulations, building upon the work of Lee et al. (J. Geophys. Res., 2013, DOI: 10.1002/2013JD020341). The simulations illustrate the time required for photostationary state approaches to yield accurate results following substantial perturbations in the HOx/NOx/NOy chemistry, and the scope for bias to an inferred HONO source from NOx and VOC emissions in either a positive or negative sense, depending upon the air mass age following emission. To assess the extent to which these impacts may be present in actual measurements, we present exploratory spatially resolved measurements of HONO and NOx abundance obtained using a mobile instrumented laboratory. Measurements of the spatial variability of HONO in urban, suburban and rural environments show pronounced changes in abundance are found in proximity to major roads within urban areas, indicating that photo-stationary steady state (PSS) analyses in such areas are likely to be problematic. The measurements also show areas of very homogeneous HONO and NOx abundance in rural, and some suburban, regions, where the PSS approach is likely to be valid. Implications for future exploration of HONO production mechanisms are discussed.

Highlights

Chemical processes affect atmospheric composition, leading to the removal of primary species, including many air pollutants, and the formation of secondary products
The question which arises is, how spatially variable is HONO in the real environment – in particular in nonremote environments where there is substantial landscape and built environment heterogeneity – are the challenges illustrated in Scenarios 1–4 evident in actual measurements? To explore this issue, we present exploratory spatially resolved measurements of HONO and nitrogen oxides (NOx) abundance, performed using a mobile instrumented laboratory in Birmingham (UK) and the surrounding areas
A substantial body of literature has shown that HONO levels are higher than can be explained by simple/established gas-phase chemistry (reactions (1)–(3)) supporting the existence of additional sources, and the role of HONO as an important net OH precursor in the boundary layer

Summary

Introduction

Chemical processes affect atmospheric composition, leading to the removal of primary species, including many air pollutants, and the formation of secondary products. The observed levels of HONO are such that it forms an important – sometimes dominant – component of the OH budget within the boundary layer (where air pollutants immediately impact upon health), e.g. accounting for 48–56% of OH production in New York City,14,15 33% of OH production in a deciduous forest near Julich (Germany),16 35% of the OH source at a suburban site near Paris,17 80.4% of the OH source at a semi-rural site in Colorado[18] and 40% in central London.[19] We can identify two contrasting HONO formation mechanisms: within-atmosphere gas-phase HONO formation (with no net effect upon OH abundance) – through reaction (1), and other processes and heterogeneous formation and/or direct emissions (net HONO production, leading to increased OH – reaction (4)). The focus of this discussion paper is not to identify the source(s) responsible for the net HONO formation frequently observed in the eld, but rather to explore the strengths – and limitations – of one method used to identify the magnitude and suggest the identity of additional HONO sources, including in a number of the studies cited above – the application of photostationary steady state to the HONO–OH–NO system

Photostationary steady state

Illustrative model simulations

Findings

Conclusions