Atmospheric Chemistry
Tropospheric Oxidation of Hydrocarbons
The photochemical oxidation of hydrocarbons plays a central role in
atmospheric chemistry and thus detailed chemical mechanisms for this chemistry is necessary for predictive air quality modeling. To date, however, there remain significant uncertainties in reactivity and chemical branching of the intermediate radical species in these models. The photochemical oxidation of unsaturated hydrocarbons is inherently complex, involving numerous chemical reactions and intermediate species. Much of this complexity arises from isomeric branching in the initial steps of the oxidation process which is
kinetically, rather than thermodynamically, driven. Since different isomers ultimately lead to distinct end products and chemical transformations, studies which can isolate these isomeric-selective pathways will provide valuable information to interpret end product yields and assess the validity of lumping approaches in chemical mechanisms. The current state of kinetics research involves lumped, non-isomeric selective measurements. Our specific objectives of the proposed work were to 1) develop and refine a novel approach to studying the isomeric selective oxidation of unsaturated and aromatic hydrocarbons relevant to air quality modeling and 2) to provide the critical knowledge for quantitative evaluation and validation of current condensed chemical oxidation models.
Our experimental approach exploited the novel photolytic preparation of energy-selected single radical isomers corresponding to the initial oxidation step. The methodology involves the photodissociation of a suitable, photochemically labile precursor as a route to the formation of a single isomer. To our knowledge, this work was unique and represented an important
advance in addressing critical issues in model validation. Our initial proof-of-principle studies are very encouraging. The overall approach utilized a combined theoretical and experimental approach. The experimental methodology included a slow flow reaction cell coupled to laser photolysis/laser induced fluorescence and laser photolysis/cavity ring-down spectroscopy to follow the kinetics of short lived transient species and molecular beam ion imaging to study the precursor photolysis. One of the key results
from our work was identifying and confirming new mechanistic pathways in the oxidation of isoprene (left)Our group is also active in the Center for
Atmospheric Chemistry and Environment (CACE). In addition, we have been collaborating with Sandia National Laboratory to study our systems using the Advanced Light Source in Berkeley.
Atmospheric Monitoring
We have developed field-based instruments for measuring free radicals to study outstanding issues in regional and urban air quality. In particular, we constructed and calibrated an
instrument based on the work of Brown
and Ravishankara for simultaneous detection of NO3 and N2O5 in the field. We also constructed an instrument to measure ambient OH and HO2 using the fluorescence assay by gas expansion
(FAGE) technique in collaboration with Conoco Phillips.
Urban Nocturnal NOx
Surface-induced heterogeneous reactions play a central role in
partitioning of trace species in the troposphere, but the detailed
kinetics and mechanism of the heterogeneous processes remain
poorly understood. In the urban atmosphere, heterogeneous
conversion of NOx on soot surfaces has been suggested to occur
efficiently to form nitrous acid (HONO). This proceed may lead to
accumulation of elevated levels of HONO at night and subsequent
photolysis of HONO during the morning leads to a sudden rise in
the hydroxyl radical (OH) leading to enhanced oxidation of
volatile organic compounds (VOCs) and ozone production.
Model calculations have demonstrated that inclusion of the
heterogeneous conversion of NO2 to HONO on the surfaces of
soot aerosols accelerates the O3 production by about 1 hour in
the morning and leads to a noticeable increase of about 7 ppb on
average in the daytime O3 level over the Houston area.
We are currently studying NOx processes at night in downtown
Houston to assess the importance of radical sources and sinks.
The hydrolysis of N2O5 on aqueous aerosols is essential to the nighttime nitrogen chemistry. Nitrate radical (NO3)
formed from the oxidation of NO2 by O3 represents the dominant nighttime free radicals. Similar to OH, NO3 reacts with
VOCs through H-atom abstraction or addition reactions, leading to nighttime peroxy radical formation. NO3 also reacts
with NO2 to form N2O5. Although N2O5 is not very reactive in the gas phase, it is taken up efficiently by aqueous droplets
to form HNO3. N2O5 also thermally decomposes to NO2 + NO3. Hence, N2O5 serves as either a sink or a temporary
reservoir for NO3 and impacts on the NOy budget and on ozone formation on the subsequent day. The instrument is
shown below.
OH and HO2 Measurements
The hydroxy radical (OH) and hydroperoxy radicals (HO2), or HOx, are key intermediates in atmospheric chemistry and their concentrations provide critical insight into radical sources and sinks in the urban environment. The reaction with OH represents
the primary loss mechanism for most volatile organic compounds (VOCs). The lifetime of OH varies
from approximately 1 second in clean conditions to 0.1 second in urban air and its concentration thus reflects the fast local photochemical processes.
Although monitoring OH is important to understanding atmospheric chemistry, the low concentrations of ~1x106 molecules cm-3 make its accurate detection highly demanding. We finished the construction and calibration of a FAGE instrument for urban OH/HO2 measurements. Our instrument provided in-field minimum detection limits of: [OH]min=4.45x105 molecules/cm3 and [HO2]min=3.16×106 molecules/cm3