Measurements of ambient HONO concentrations and vertical HONO flux above a northern Michigan forest canopy

Abstract

Systems have been developed and deployed at a North Michigan forested site to measure ambient HONO and vertical HONO flux. The modified HONO measurement technique is based on aqueous scrubbing of HONO using a coil sampler, followed by azo dye derivatization and detection using an optical fiber spectrometer with a 1-m long path flow cell. A Na<sub>2</sub>CO<sub>3</sub>-coated denuder is used to generate "zero HONO" air for background correction. The lower detection limit of the method, defined by 3 times of the standard deviation of the signal, is 1 pptv for 2-min averages, with an overall uncertainty of ±(1 + 0.05 [HONO]) pptv. The HONO flux measurement technique has been developed based on the relaxed eddy accumulation approach, deploying a 3-D sonic anemometer and two HONO measurement systems. The overall uncertainty is estimated to be within ±(8 × 10<sup>−8</sup> + 0.15 <i>F</i><sub>HONO</sub>) mol m<sup>−2</sup> h<sup>−1</sup>, with a 20-min averaged data point per 30 min. Ambient HONO and vertical HONO flux were measured simultaneously at the PROPHET site from 17 July to 7 August 2008. The forest canopy was found to be a net HONO source, with a mean upward flux of 0.37 × 10<sup>−6</sup> moles m<sup>−2</sup> h<sup>−1</sup>. The HONO flux reached a maximum mean of ~0.7 × 10<sup>−6</sup> moles m<sup>−2</sup> h<sup>−1</sup> around solar noon, contributing a major fraction (~60%) to the HONO source strength required to sustain the observed ambient concentration of ~70 pptv. There was no significant correlation between NO<sub>x</sub> and daytime HONO flux, suggesting that NO<sub>x</sub> was not an important precursor responsible for HONO daytime production on the forest canopy surface in the low-NO<sub>x</sub> rural environment. Evidence suggests that photolysis of HNO<sub>3</sub> deposited on the forest canopy surface is a major daytime HONO source

Diurnal fluxes of HONO above a crop rotation

“…Also the very weak correlation of the HONO flux with the NO 2 concentration above a forest canopy at the PROPHET site (Zhou et al, 2011;Zhang et al, 2012) can be attributed to an influence of the canopy. Correlations of HONO with its precursors are expected to become worse when measurements are carried out above high trees as at the PROPHET site, which are able to fully decouple the ground surface from the air above the canopy (Sörgel et al, 2011a;Foken et al, 2012).…”

“…Nowadays, eddy covariance (EC) is the most commonly applied method to measure fluxes between the surface and the atmosphere. The lack of fast and sensitive EC-HONO measurement systems, however, requires the use of indirect methods like the aerodynamic gradient (AG) method that has been described by a number of authors (Thom et al, 1975;Sutton et al, 1993) or the relaxed eddy accumulation method (REA) that was also recently used for HONO (Ren et al, 2011;Zhou et al, 2011;Zhang et al, 2012). Unfortunately, the available flux observations indicate different HONO precursors.…”

Effect of imprecise lag time and high‐frequency attenuation on surface‐atmosphere exchange fluxes determined with the relaxed eddy accumulation method

“…[] exist. It is often assumed for REA measurements with low switching frequencies, e.g., above forests, that the lag time error has no significant effect on the REA fluxes [ Park et al ., ; Zhang et al ., ].…”

“…[] and Zhang et al . [] achieved also a much higher flow rate by using a single inlet with a REA system for nitrous acid, which was necessary to avoid wall reactions. Furthermore, undesired pressure pulses, which are caused by switching of the splitter valves, can be minimized by such an inlet system.…”

Implementation and refinement of a surface model for heterogeneous HONO formation in a 3-D chemical transport model

“…A number of field studies (e.g., Kleffmann et al, 2003;Wong et al, 2011Wong et al, , 2012Wong et al, , 2013VandenBoer et al, 2013;Villena et al, 2011) and direct surface flux experiments (e.g., Ren et al, 2011;Zhang et al, 2012a) have measured large vertical gradients in HONO concentrations, with higher concentrations near the surface. This suggests the existence of surface sources of HONO, such as direct HONO emissions from combustion sources and heterogeneous reactions on ground surfaces.…”