The primary focus of these model evaluations are on the role of radicals in determining the level of ozone productivity. The quantification of radical budgets, as well as the source and fate of ozone precursors, and the physical processes that effect each species were determined by using the CMAQ process analysis data, post processed by UNC's net_balance codes. The data were used to investigate impacts of primary and/or secondary sources of formaldehyde and nitrous acid; two important sources of radicals that may substantially increase the ozone productivity of the Houston airshed. We investigated possible enhanced sources of primary formaldehyde that included flares, motor vehicles, and polymer facilities. Also analyzed were possible sources of nitrous acid that may include multiphase reactions on soot, soil, and manmade surfaces. In addition, nighttime reactions of ozone and NO3 radicals with olefins may contribute to radicals and formaldehyde production. The implications of these possible sources of radicals in addition to the consequences of nighttime chemistry on subsequent daytime ozone production was examined by the model.
Previous modeling of the Houston airshed prompted this focus on the link of missing radical sources and ozone productivity. In a summer of 2000 air quality simulation of Houston, the CAMx model underpredicted ozone peak concentrations despite an overprediction of VOCs and NOx . An analysis of the radical budget in these earlier modeling efforts revealed a deficient source of organically derived free radicals, HO2 and OH, in the modeling system. Atmospheric reactivity, and consequently ozone formation was limited by a lack of radical sources; regardless of the levels of additional VOCs introduced into the system. The NOx overpredictions compounded the problem by further depleting the limited pool of radicals available for ozone chemistry. Without additional sources of new radicals, most of the VOCs present in the model did not react and left the Houston airshed through transport or deposition. Therefore, the low ozone productivity in Houston may be attributed to a dearth of new radicals, as opposed to internal radicals generated from reactions of new radicals with emitted VOCs or from the decomposition of secondary species.
These finding prompted an intense radical measurement pro ject during the summer of 2006, and these results were used for this model evaluation. Model evaluations were compared with observed data of formaldehyde, nitrous acid, and radicals collected during a large scale field campaign called the Second Texas Air Quality Study (TexAQS II). TexAQS II consisted of more than $20 million worth of projects collected over the 2005 and 2006 ozone seasons. Radical measurements were made under a project named the TexAQS II Radical Measurement Project (TRAMP), conducted at the University of Houston Moody Tower. TRAMP measurements took place during the months of August and September of 2006, coinciding with this ozone modeling episode. Important measurement techniques included: long-path Differential Optical Absoprtion Spectroscopy (DOAS) measurements of HONO and formaldehyde, Hantzsch reaction fluorescence monitoring of formaldehyde, mist chamber ion chromatography measurements of HONO, and Laser-Induced Fluoresence (LIF) detection of radical species. Intercomparisons with the Moody Tower measurements was also performed with the same formaldehyde measuring Hantzsch reaction fluorescence monitor instrument deployed on the Aztec aircraft. These measurements, in addition to the VOC speciation measurements from the TexAQS II study, provided a rich data set for exploring the role of radical sources and ozone productivity in Houston.
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