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Project Number:
Implementation and evaluation of new HONO mechanisms in a 3-D Chemical Transport Model for Spring 2009 in Houston
Lead PI:
Barry Lefer
Institution(s) Represented:
University of Houston - Barry Lefer, UCLA - Jochen Stutz, ENVIRON - Greg Yarwood, UNC at Chapel Hill - Will Vizuette
AQRP Project Manager:
Elena McDonald-Buller
TCEQ Project Liaison:
Doug Boyer
Awarded Amount:

 Implementation and evaluation of new HONO mechanisms in a 3-D Chemical Transport Model for Spring 2009 in Houston
Although portions of the chemistry that lead to the formation of ozone have been understood for decades, new discoveries have revealed the need to improve scientific understanding of ozone formation chemistry.  Radical production in Houston and other urban areas appear to be underestimated by chemical mechanisms.  The roles of some radical precursors such as HONO, HCHO, and reactive VOCs in ozone formation in Houston and other Texas cities have not been well understood. Research based on both modeling and field measurements by the University of Houston, ENVIRON, University of California - Los Angeles, and the University of North Carolina - Chapel Hill has shown that nitrous acid (HONO) significantly affects the HOx budget in urban environments like Houston.  These chemical processes connect surface emissions, both anthropogenic and natural, to local and regional air quality.
From April 15th to May 30th, 2009, a team of more than 40 scientists representing more than 15 different institutions collected a relatively complete suite of atmospheric measurements, including NO, NO2, NOY, HONO, HNO3, O3, CO, SO2, HCHO, HOOH, OH, HO2, OVOCs, VOCs, actinic flux, PBL height, O3 production rates, and vertical profiles (nominally 40m, 150m, 300m) of NO2, HONO, O3, SO2, and HCHO, during the Study of Houston Atmospheric Radical Precursor (SHARP).  The SHARP dataset provides us a unique opportunity to examine and improve our understanding of atmospheric HONO formation processes and how they may be implemented into the Comprehensive Air quality Model with extensions (CAMx) 3-D chemical transport model commonly used for SIP evaluations.  The objective of the study is to develop, implement, and evaluate missing pathways for HONO formation in a photochemical model, CAMx, that is used routinely for regulatory applications in Texas and other areas. This model update is expected to improve the model's ability to simulate ozone concentrations, because HONO is a potential daytime source of the hydroxyl radical, OH, which plays an important role in the ozone formation cycle.  Measurements during the SHARP study in Houston showed that radical production in the early morning was dominated by HONO photolysis.
The modeling strategy is to take advantage of the SHARP data analysis in a previous AQRP project (Project 10-032) to develop parameterizations, based on current understanding of the important processes governing HONO formation, and implement and refine these parameterizations in CAMx using existing modeling databases for the Houston area during the SHARP period. Model performance evaluation will make use of process analysis tools to evaluate how HONO formation pathways influence radical budgets and ozone formation within CAMx simulations.

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Publications & Citations
Implementation and Refinement of a Surface Model for HONO formation in a 3-D Chemical Transport Model. Prakash Karamchandani1, Chris Emery1, Greg Yarwood1, Barry Lefer2, Jochen Stutz3, Evan Couzo4, and William Vizuete5. (1ENVIRON, 2University of Houston, 3University of California-Los Angeles, 4Massachusetts Institute of Technology, and 5University of North Carolina.) Impacts of heterogeneous HONO formation on radical sources and ozone chemistry in Houston, Texas. Evan Couzo1, Barry Lefer2, Jochen Stutz3, Greg Yarwood4, Prakash Karamchandani4, Barron Henderson5, and William Vizuete1. (1University of North Carolina (now at MIT), 2University of Houston, 3University of California-Los Angeles, 4ENVIRON, 5University of Florida.)