Scientific Theme: Advanced Modeling and Observing Systems

Scientific Theme: Regional Processes
study appeared in Atmospheric Chemistry and Physics in January 2007, and results from NEAQS 2004 are currently
in press in the Journal of Geophysical Research. Results from TexAQS 2006 and Erie 2007 are anticipated in the
coming year.
The figure below shows an example of a vertical profile from the TexAQS 2006 study. The left graph is NO3 and
N2O5 along the bottom axis and potential temperature along the top axis against altitude on the vertical axis. The
structure in the potential temperature profiles shows the low-level stratification of the nighttime atmosphere, with
distinct regions of stability near 200 and 600 m. Associated with each feature is a nitrogen oxide plume. The two
right graphs show the formation rate of NO3, P(NO3), the lifetime of NO3 and N2O5, or their concentrations divided
by their formation rate, and the partitioning among nitrogen oxides, F(NOx), a measure of the amount of NOx
present as NO3 and N2O5. The two
nitrogen oxide plumes are less than
100 m in depth, and have distinctly
different chemistry, as measured by
their lifetimes and partitioning.
Because this chemistry is an
important factor in determining the
atmospheric residence time of emitted
NOx pollution and the nocturnal loss
rates for O3 pollution, and because it
varies on small spatial scales at night,
detailed studies of the vertical
distributions of these compounds are
critical to understanding their role in
chemical transformations of pollutants
and regional air quality.
MILESTONE CSD08.7:
Measure the rate coefficient for the reaction of hydroxyl radical with acetaldehyde, CH3C(O)H,
over the range of temperatures common to the troposphere and lower stratosphere to better
elucidate the role of oxygenated hydrocarbons in radical (HOx) and ozone production.
ACCOMPLISHMENTS FOR CSD08.7:
CIRES researchers measured the rate coefficient for the reaction of hydroxyl radical with acetaldehyde, CH3C(O)H,
over the range of temperatures common to the troposphere and lower stratosphere (between 204 and 373 K) to better
elucidate the role of oxygenated hydrocarbons in radical (HOx) and ozone production. This research provided rate
coefficient data for the accurate determination of the atmospheric loss of acetaldehyde by reaction with the OH
radical. Knowledge of atmospheric lifetimes and chemical destruction pathways for acetaldehyde (an oxygenated
hydrocarbon) is important for characterizing climate-chemistry coupling.
The temperature dependence of the rate coefficients for the reaction of OH with CH3CHO was measured using
pulsed laser photolytic production of OH coupled with its detection via laser-induced fluorescence. Acetaldehyde
was purified and its concentrations were measured using three methods: FTIR spectroscopy, UV absorption at 184.9
nm and gas flow rates. The rate coefficients for the title reaction between 204 and 373 K are reported in the
Arrhenius format: (5.32±0.74)×10 -12 exp[(314±37)/T], which gives k1(298 K) = (1.52±0.15)×10 -11 , in units of cm 3
molecule -1 s -1 . The rate coefficients for the reaction of OH(v = 1) with CH3CHO between 204 and 296 K were
determined and is expressed in the Arrhenius format: (3.5±1.4)×10 -12 exp[(500 ± 90)/T], which gives a value of
(1.9±0.6)×10 -11 cm 3 molecule -1 s -1 at 298 K. The obtained results are compared with previous data. Atmospheric
implications of the data are discussed.
94