Abstract
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 a long-path absorption photometer (LPAP). A Na2CO3-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 1-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 × 10−8 + 0.15 FHONO) mol m−2 h−1, 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 × 10−6 moles m−2 h−1. The HONO flux reached a maximal mean of ~0.7 × 10−6 moles m−2 h−1 around solar noon, contributing a major fraction to the HONO source strength required to sustain the observed ambient concentration of ~70 pptv. There were no significant correlations between [NOx] and daytime HONO flux and between JNO2 × [NO2] and HONO flux, suggesting that NOx was not an important precursor responsible for HONO daytime production on the forest canopy surface in this low-NOx rural environment. Evidence supports the hypothesis that photolysis of HNO3 deposited on the forest canopy surface is a major daytime HONO source.
Highlights
Nitrous acid (HONO) is a trace gas in the atmosphere, but it is an important component of NOy
Two major modifications were made to the original method of Huang et al (2002), including: (1) HPLC/UV-vis detection technique was replaced by the long-path absorption photometer (LPAP) technique using a miniature fiber optic spectrometer with a 1-m flow cell, and (2) a 3-way solenoid valve was used to alternately introduce ambient air or “zero-HONO” air to a coil sampler
The sensitivity of the LPAP technique is greatly enhanced compared to the conventional spectrophotometric method, by two orders of magnitude (He et al, 2006)
Summary
Nitrous acid (HONO) is a trace gas in the atmosphere, but it is an important component of NOy. N. Zhang et al.: Measurements of ambient HONO concentrations and vertical HONO flux. To assess quantitatively the source strength on HONO on ground surface, measurement of exchange flux of HONO between the ground and the atmosphere is essential. There are only a few HONO flux measurements reported in literature (Beine et al, 2005, 2006; Harrison and Kitto, 1994; Honrath et al, 2002; Ren et al, 2011; Stutz et al, 2002; Zhou et al, 2001, 2011); most of these HONO flux measurements were made based on an indirect gradient method, which is less accurate due to the large scatter of the concentration measurements and the influence of chemical transformation during the turbulent transport process (Galmarini et al, 1997). The results quantitatively reveal the role of canopy surfaces on HONO heterogeneous formation
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