NorthResearch Group

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 

NorthResearch Group

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