Atmos. Chem. Phys. Discuss., 12, 7273-7304, 2012
www.atmos-chem-phys-discuss.net/12/7273/2012/
doi:10.5194/acpd-12-7273-2012
© Author(s) 2012. This work is distributed
under the Creative Commons Attribution 3.0 License.
Review Status
This discussion paper has been under review for the journal Atmospheric Chemistry and Physics (ACP). Please refer to the corresponding final paper in ACP.
Measurements of ambient HONO concentrations and vertical HONO flux above a northern Michigan forest canopy
N. Zhang1, X. Zhou1,2, S. Bertman3, D. Tang4, M. Alaghmand5, P. B. Shepson5, and M. A. Carroll6
1Department of Environmental Health Sciences, State University of New York, Albany, NY 12201, USA
2Wadsworth Center, New York State Department of Health, Albany, NY 12201, USA
3Department of Chemistry, Western Michigan University, Kalamazoo, MI 49008, USA
4Department of Psychology, Texas A{&}M University, College Station, TX 77843, USA
5Departments of Chemistry, Purdue University, West Lafayette, IN 47907, USA
6Department of Atmospheric, Oceanic and Space Science, University of Michigan, Ann Arbor, MI 48109, USA

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 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 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 × 10−8 + 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 maximum mean of ~0.7 × 10−6 moles m−2 h−1 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 NOx and daytime HONO flux, suggesting that NOx was not an important precursor responsible for HONO daytime production on the forest canopy surface in the low-NOx rural environment. Evidence suggests that photolysis of HNO3 deposited on the forest canopy surface is a major daytime HONO source.

Citation: Zhang, N., Zhou, X., Bertman, S., Tang, D., Alaghmand, M., Shepson, P. B., and Carroll, M. A.: Measurements of ambient HONO concentrations and vertical HONO flux above a northern Michigan forest canopy, Atmos. Chem. Phys. Discuss., 12, 7273-7304, doi:10.5194/acpd-12-7273-2012, 2012.
 
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