Atmospheric Environment 37 (2003) 1583–1591

Tunable diode laser absorption spectrometer measurementsof ambient nitrogen dioxide, nitric acid, formaldehyde,

and hydrogen peroxide in Parlier, California

Claudia G. Sauer, John T. Pisano, Dennis R. Fitz*

College of Engineering-Center for Environmental Research and Technology (CE-CERT),

University of California Riverside, Riverside, CA 92521-0434, USA

Received 12 August 2002; accepted 22 December 2002

Abstract

Field measurements were conducted to determine atmospheric concentrations of nitrogen dioxide, nitric acid,

formaldehyde, and hydrogen peroxide gases using two dual-channel tunable diode laser absorption systems. These

measurements were made as part of the Central California Ozone Study (CCOS) and were conducted for a total of 28

days from July to September 2000 at the Kearney Agricultural Research Station in Parlier, California. All four trace

gases exhibit marked diurnal cycles. Ranges of measured concentrations: 1–38 ppb NO2, 0–26 ppb HNO3, 1–17 ppb

HCHO, and 0–1.2 ppb H2O2. Formaldehyde, nitric acid and hydrogen peroxide mixing ratios showed unusual behavior

during the last intensive measurement period in September 2000.

r 2003 Elsevier Science Ltd. All rights reserved.

Keywords: CCOS; Field study; Diurnal profile; Gas phase; Ozone episode

1. Introduction

Nitrogen dioxide (NO2), nitric acid (HNO3), formal-

dehyde (HCHO), and hydrogen peroxide (H2O2) are

important atmospheric pollutants. Although there has

been significant effort in developing techniques for the

measurement of these trace gases, few methods have

shown the capability of making the required measure-

ments with the degree of specificity and sensitivity

necessary for atmospheric assessment. Accurate mea-

surements of these species are important for assessing

the environmental impacts and devising emission con-

trol strategies to reduce these impacts. Finlayson-Pitts

and Pitts (2000) summarize the atmospheric importance,

reactions, and measurement methods for these com-

pounds.

Nitrogen dioxide is the precursor for tropospheric

ozone, which is known to be harmful to organic tissues,

including crops as well as humans (Haagen-Smit et al.,

1952). It has been shown to be measurable in ambient air

by various techniques: ozone chemiluminescence after

reduction to NO (Fontijn et al., 1970), direct luminol

chemiluminescence (Gaffney et al., 1998, 1999), tunable

diode laser absorption spectroscopy (TDLAS) (Schiff

et al., 1994; Fried et al., 1998), and laser induced

fluorescence (LIF) (Thornton et al., 2000; Matsumoto

et al., 2001). A prototype luminol-based chemilumines-

cence instrument, which measures NO2 directly, was

operated at the site. Because luminol is known to react

with ozone, PAN and SO2 (Pisano et al., 1996) this

instrument employed a capillary column to minimize

these interferences. The TDLAS technique is advanta-

geous for two major reasons. First, it is spectroscopically

specific as the line width of a typical diode laser is better

than 10 MHz, far narrower than the 200 MHz, typical of

absorption line widths (Reid et al., 1978). Second, it is a

AE International – North America

*Corresponding author. Fax: +1-909-781-5790.

E-mail address: dfitz@cert.ucr.edu (D. Fitz).

1352-2310/03/$ - see front matter r 2003 Elsevier Science Ltd. All rights reserved.

doi:10.1016/S1352-2310(03)00004-9

direct measurement technique since it does not require

conversion of NO2 into another species (e.g., NO) before

detection.

Nitric acid is an important gaseous species to measure

because it serves as the major sink for ambient airborne

nitrogen oxides as well as a significant sink for hydroxyl

radicals. The latter radical initiates many of the gas

phase reactions that degrade hydrocarbons in ambient

air. Due to its sorptive properties and equilibrium with

ammonium nitrate, nitric acid has proved to be a

difficult species to quantify in ambient air and it has

been the subject of a number of comparison studies

(Spicer et al., 1982; Anlauf et al., 1985; Fox et al., 1988;

Hering et al., 1988; Tanner et al., 1989; Fitz et al., 2003).

The present methods for measuring nitric acid include

denuder-based collection and infrared spectroscopic

techniques such as FTIR and TDLAS. Although there

has been moderate success during their use in short-term

monitoring programs, none of the methods has become

widely accepted as meeting the combined criteria of high

sensitivity, accuracy, portability, and consistency of

performance. The TDLAS was again chosen as the

measurement method of choice due to overall perfor-

mance and commercial availability.

Formaldehyde is emitted directly into the troposphere

and is also a product of the photochemical degradation

of hydrocarbons. Furthermore, HCHO in itself acts as a

photolytic radical source contributing to ozone forma-

tion. Commonly HCHO is measured via its reaction

with 2,4-dinitrophenyl hydrazine (DNPH). However,

ozone and other pollutants have been shown to interfere

with this technique (Kleindienst et al., 1998). For typical

ambient levels, DNPH cartridges must be collected for

1 h or more. Spectroscopic methods such as TDLAS,

long-path FTIR, and differential optical absorption

spectroscopy (DOAS) can provide measurements with

both high specificity and fast time resolution. A number

of inter-comparison studies (Lawson et al., 1990;

Kleindienst et al., 1988; Sirju and Shepson, 1995;

Benning and Wahner, 1998) have shown reasonable

agreement between these methods, especially if ozone is

removed prior to reaction with DNPH. TDLAS was

used in the present study because of availability and

compatibility with the simultaneous measurement of the

other trace pollutants.

Hydrogen peroxide is the product of ambient hydro-

carbon degradation in the absence of excess nitric oxide.

As a result it forms a sink for HOx radicals. At the same

time it also photolyzes to OH radicals, and, therefore,

contributes to ozone formation. The most commonly

used methods to measure atmospheric hydrogen per-

oxide use water absorption followed by reaction with a

reagent to form a colored adduct (Li and Dasgupta,

2000; Komazaki et al., 2001). Ozone and sulfur dioxide

are known to be interferents, and it is likely that other

ambient oxidants may interfere as well. TDLAS and

long-path FTIR have both shown to provide measure-

ments with high time resolution and specificity. The

TDLAS was chosen for its direct measurement cap-

ability.

2. Experimental

2.1. Measurement site

The TDLAS measurement equipment was installed at

the University of California Kearney Agricultural

Station near Parlier, California, as a part of the air-

monitoring component of the Central California Ozone

Study (CCOS). The primary objectives of CCOS were to

gather an aerometric database for modeling and to

apply air models for the attainment demonstration

portion of the State Implementation Plan (SIP) for the

Federal 8-h and State 1-h ozone standards. To meet this

objective additional temporary monitoring was added

and existing sites were supplemented with additional

equipment. Parlier was an existing site chosen because it

is rural and E25 km downwind (during the prevailing

daytime wind pattern) of Fresno, a major metropolitan

area of the San Joaquin Valley with a population of

400,000. In addition to the TDLAS measurement

system, this site was equipped to measure ozone,

JNO2; NO, NOx, NOy, NOy–HNO3, NO2, PAN, NO3

�,

CO, PM2.5 (mass), PM2.5 (light scattering), CO2, VOC

and organic carbonyl compounds. All parameters

except the last two were measured continuously. The

VOC and organic carbonyl compounds were measured

at 1- and 3-h intervals, respectively, during 14 intensive

measurement days. The days were determined by

forecasts of high levels of ozone based on meteor-

ology.

2.2. TDLAS instruments

Two Unisearch model TAMS-150 TDLAS instru-

ments were used. The first was configured to measure

NO2 and HNO3, the second HCHO and H2O2. The laser

diodes were selected to emit around 1595, 1720, 1730,

and 1280 cm�1, respectively, to measure these four

species. Fig. 1 shows a schematic of the experimental

setup for both of the TDLAS instruments. The lasers are

located in the laser source assemblies (LSA A and LSA

B), where they are cryogenically cooled. A scanner

mirror (S) was configured to switch from LSA A to LSA

B every 15 s. This allowed 30-s integration time for each

of the two gases measured every minute. Because of their

small cross-sectional area, diode lasers produce a highly

divergent beam and require f/1 optics for collimation.

The system employs two identical collimating off-axis

parabolic mirrors (OAP 1 and OAP 2), one for each of

the two channels. The signal analysis software, data

C.G. Sauer et al. / Atmospheric Environment 37 (2003) 1583–15911584

acquisition system and system control electronics are

manufactured by Unisearch Associates as part of its

TAMS/LasIR product line. A Teflon-lined 150m Horn-

Pimental version White cell, operating at 20 Torr, is the

sample cell (Horn and Pimental, 1971). A combination

of flat mirrors (M1–M6), focusing optics (lenses L1–L2

and OAP 3), and a beam splitter (BS) directs the laser

beams from the source assemblies through the sample

cell to the two Infrared Associates HgCdTe detectors.

Reference cells (RC1 and RC2), which contain high

concentrations of the target gases, are used to lock the

laser emission to the peak absorption. A 633 nm HeNe

laser is used for optical alignment.

The laser system used in this experiment employs

wavelength modulation, where a sine wave of 25 kHz is

superimposed on the laser current to generate a

frequency modulated laser output. The modulated 1f

signal inherently will have a significant zero offset due to

variations in power of the laser output as a function of

applied current (Kikuchi, 1989). To reduce this offset

and maximize the signal amplitude at the line center, the

detector output is analyzed after it passes through a

lock-in amplifier fixed at twice the modulation frequency

(Reid and Labrie, 1981). This frequency-doubling

method of detection results in a signal that is indepen-

dent of a sloping background structure as well as zero

offset variations. The 2f modulation technique has been

referred to extensively in the literature and has been

shown to have significant advantages in increasing the

signal-to-noise ratio of diode laser based spectroscopic

systems (Iguchi, 1986). A mechanical chopper at 60Hz

(C) is employed to monitor perturbations in the total

laser power output and correct the measured concentra-

tions appropriately. The former arise mostly from small

changes in the laser temperature or slight alignment

deviations.

2.3. Calibration

Since the detection technique employs wavelength

modulation and the resulting signal resembles the

second derivative after amplification through the lock-

in amplifier, a suitable calibration procedure had to be

developed for each of the gases of interest (Mackay et al.,

1990). A calibration spectrum was collected and stored

at the center of the linear concentration range for each

of the target gases, and subsequent ambient spectra were

fitted to the respective stored spectrum. Span checks

were performed at least once during each CCOS

intensive period for each measured species.

For the calibration of the nitrogen dioxide channel,

gas from a certified cylinder containing 10.11 ppm

nitrogen dioxide in nitrogen was diluted with synthetic

zero-grade air using a commercial calibrator (Columbia

Scientific model 1700) and introduced into the sample

line of the TDLAS. A multi-point calibration was

performed over the range 1–40 ppb nitrogen dioxide,

showing linear response of the TDLAS to the NO2

concentration, and the calibration spectrum was taken

at 20 ppb. The flow controllers of the calibrator itself

were calibrated beforehand in the laboratory, using

bubble meters for flows o50ml min�1 and a dry test

P = 20 torr

2f Lock-inAmplifier

D1

C

OAP3Horn-Pimental Type White Cell

Computer Data Acquisition

Vacuum

HeNeM5

M6

OAP1 M1

S

TDL TDL

OAP2M2

M3

LSA

A

LSA

B

TemperatureController

Modulation Controller

D2

L2

R C 2

R C 1

M4

2f Lock-in Amplifier

L1

TemperatureController

Current and

Inlet

Pump

ControlPressure

BS

Fig. 1. Schematic of the experimental setup for both of the TDLAS instruments. For details and explanations see text.

C.G. Sauer et al. / Atmospheric Environment 37 (2003) 1583–1591 1585

meter (referenced to the NIST) for the dilution flows.

The NO2 calibration was performed twice during the

measurement program. No deviation from linearity

(o2%) was observed in either of these calibrations.

The calibration source of nitric acid was a diffusion

tube containing 70% (w/w) nitric acid held in a water

bath at 45�C and flushed with synthetic air at about

20 cm3 min�1. This stream of span gas was diluted with

10 lmin�1 of synthetic zero-grade air to 60 ppb and

introduced into the sample line of the TDLAS. The

concentration of this calibration gas was measured by

bubbling the 20 cm3 min�1 flow through a diluted

sodium hydroxide solution of known concentration

and recording the titration curve.

The formaldehyde calibration gas was generated using

a diffusion tube held at 69�C and flushed with synthetic

air at about 20 cm3 min�1. This stream of span gas was

diluted with 10 lmin�1 of zero-grade synthetic air to

30 ppb and introduced into the sample line of the

TDLAS. To determine the concentration of formalde-

hyde in the calibration gas, the diffusion tube was

weighed before and after the study period. The

permeation rate was calculated based on the weight

loss, the elapsed time between the two mass determina-

tions and the assumption that the permeation rate was

constant during this time period.

The hydrogen peroxide calibration gas was generated

using an inverse diffusion tube held at 40�C and purged

with zero-grade synthetic air at about 20 cm3 min�1.

This stream of span gas was diluted to 15 ppb with

7 lmin�1 of ambient air, which was passed through a

charcoal filter to remove ambient hydrogen peroxide,

and subsequently introduced into the sample line of the

TDLAS. The permeation rate of the H2O2 calibration

source was measured by the colorimetric TiCl4 method

of Pilz and Johann (1974). Span checks were conducted

daily.

2.4. Data processing

The instruments were configured to record 1-min

averages for all concentrations. Each day the collected

data were compared with the operational records. Data

points that were deemed invalid due to operator

interventions (changing of the sample inlet filter,

conducting a calibration, necessary repairs, etc.) or

instrument failure were excluded from further proces-

sing. To improve the signal/noise ratio the data reported

here are 15-min averages, except for hydrogen peroxide

where, due to the low concentrations below 1 ppb,

averaging over 1 h was necessary to obtain a sufficient

signal/noise ratio to monitor ambient concentrations.

The detection limits for nitrogen dioxide, nitric acid,

and formaldehyde were 0.5 ppb, 1.0 ppb, and 1.0 ppb,

respectively. The detection limit for hydrogen peroxide

was 0.1 ppb in August and 0.25 ppb during September.

These values are calculated as three standard deviations

over the respective averaging period, i.e. 15 min for NO2,

HNO3, and HCHO, and 1 h for H2O2, respectively.

2.5. Sample train

The ambient sample was collected about 1m above

the roof of the trailer and building, respectively, where

the two instruments were located. A 5mm pore 47 mm

Teflon filter held in a PFA holder was used to remove

particles from the sampled air. Behind the filter a PFA

needle valve provided the necessary pressure drop from

ambient pressure to 20Torr. The sample line itself was a

6 m long 14

00(6mm) PFA tube; the sample rate was

7 lmin�1.

3. Results and discussion

Fig. 2 depicts the data collected during this study. All

four species measured via TDLAS show pronounced

diurnal profiles. The days shaded in gray were the

intensive measurement days when the meteorological

conditions were expected to produce episodes of high

ozone concentrations. The CCOS management declared

four such multi-day periods during the course of the

study, comprising a total of 14 days. The non-intensive

9/13 9/16 9/19 9/220

1

7/23 7/26 7/29 8/1 8/4 8/7 8/10 8/13

P D T

0

1 0

2 0

1 0

H2O2

3

0

10

20

302

mix

ing

ratio

(pp

b)

HCHO

HNO

NO

Fig. 2. Ambient gas phase concentrations for nitrogen dioxide,

nitric acid, formaldehyde, and hydrogen peroxide from July–

September 2000 in Parlier, CA. Times of intensive measurement

periods as announced by the CCOS committee are shaded in

gray.

C.G. Sauer et al. / Atmospheric Environment 37 (2003) 1583–15911586

measurement days were primarily between the second

and third intensive periods.

3.1. Nitrogen dioxide

Fig. 3 shows the composite diurnal NO2 profile for

both intensive days and non-intensive days. During

intensive days the nitrogen dioxide concentration

exhibits three maxima at 02:00, 07:30, and 21:45 with

the evening maximum being the highest. These times do

not coincide with times of typical rush hour traffic in

Fresno. The prevailing daytime wind direction at the

monitoring site is from the northwest with an observed

median wind speed of 4–6 km h�1 during the course of

the study. Using this wind velocity the evening rush hour

plume from Fresno would arrive in Parlier around

20:00–22:00 PDT, which coincides with the observed

nighttime maximum in NO2 concentrations. In the

morning the wind directions were much more variable,

making it difficult to evaluate the time delay from

Fresno’s morning rush hour. With such variable wind

conditions, elevated concentrations in the morning are

likely due to contributions from the morning traffic on

the surrounding highways and freeways as well.

Supporting this hypothesis is that the NO2 data

exhibited a high short-term, minute-to-minute variabil-

ity during the day, whereas the nighttime profiles were

much smoother on the minute scale. During non-

intensive days the maxima are less pronounced, espe-

cially the NO2 peak at night, which is lower by a factor

of 2. This result may be due to either lower emissions or

a significant increase in dilution resulting from a higher

nocturnal boundary layer. The NO2 concentrations

generally reach a minimum in the late afternoon,

coinciding with the maximum in nitric acid concentra-

tions (see Fig. 5), the major sink for nitrogen oxides in

the troposphere.

Collocated with the TDLAS was a prototype gas

chromatograph (GC) measuring nitrogen dioxide and

peroxyacetyl nitrate (PAN) by luminol detection. This

instrument is based on the chemiluminescence of the

reaction of luminol with NO2 and PAN, which are

separated by a short capillary column. Fig. 4 is a plot of

the luminol GC-NO2 data versus the TDL-NO2 data.

The slope of the least squares regression is nearly unity,

although the R2 value of 0.66 shows that there is

significant scatter. The large scatter of the data may be

attributed to local inhomogenities in the ambient air, as

the sample locations for both instruments were about

5 m apart. The GC also obtains a 2-s ‘‘snapshot’’ of

sample that is introduced once per minute, whereas the

TDLAS averages over 15 s twice each minute. The large

y-intercept for the luminol-GC data may be due to the

method originally used to process the raw signal; the

data acquisition and analysis software of the luminol

GC-instrument has since been revised, based on the

experiences during CCOS 2000.

3.2. Nitric acid

Fig. 5 shows the composite diurnal nitric acid profile

for both intensive days and non-intensive days. The

measured concentrations for nitric acid peak around

15:30 each day. This is in agreement with the FTIR

profile found during the 1995 Claremont HNO3

campaign (Fitz et al., 2003). In that study TDLAS data

were also taken but showed a time delay of 2 h compared

with the FTIR. The reason for the time lag was

attributed to the type of inlet filters used. For both

studies the pressure of the sampled ambient air was

0

5

10

15

20

25

0 3 6 9 12 15 18 21 0

Time (PDT)

Mix

ing

Rat

io (

ppb)

NO2

Fig. 3. Median diurnal NO2 profiles for intensive measurement

days ( ) and non-intensive days (- - - - - -). All points are 15-

min averages.

0

10

20

30

40

50

0 10 20 30

TDLAS-NO2 Mixing Ratio (ppb)

Lum

inol

GC

-NO

2 M

ixin

g R

atio

(pp

b)

Fig. 4. Plot of collocated luminol GC-NO2 data versus TDL-

NO2 data. All points are 15-min averages. Linear regression:

y ¼ ð1:0770:04Þx þ ð6:770:4Þ; R2 ¼ 0:66; errors stated are one

standard deviation.

C.G. Sauer et al. / Atmospheric Environment 37 (2003) 1583–1591 1587

reduced to 20 Torr immediately behind the Teflon

sample inlet filter. At this low pressure the adsorption

and desorption to/from the sample line wall was found

to be negligible during calibrations. After having

sampled about 60 ppb of nitric acid via the sample line

into the White cell and switching back to sampling

ambient air, the nitric acid concentration in the cell

returned to ambient level within o10 min. Maximum

concentrations during intensive days were only slightly

higher than during non-intensive days; however, night-

time profiles showed a marked difference. During

intensives the nighttime concentrations did not decrease

as low as during non-intensive periods, when nitric acid

mixing ratios were below the detection limit of 1 ppb.

For the later part of the study in August/September

2000 a dual converter/chemiluminescence instrument

collected data collocated with the TDLAS, the sample

inlets being about 5m apart. This analyzer measured

NOy on one channel; the second sample inlet was

equipped with a sodium chloride coated diffusion

denuder to specifically remove nitric acid and thus

measure NOy minus HNO3. The instrument cycled

between these modes every 30 s. Nitric acid concentra-

tions were derived by subtracting the concentration of

the NOy channel from the NOy–HNO3. Fig. 6 compares

the nitric acid measured by the TDLAS with the dual

converter/chemiluminescent analyzer. Although there is

considerable scatter, the majority of the data points

show a linear correlation. The scatter is likely due to the

chemiluminescent instrument using the difference be-

tween two large values to determine nitric acid. The

nitric acid concentrations measured by TDLAS were

generally by a factor of 4 higher than the dual converter/

chemiluminescence values. The reason for this discre-

pancy is not known, but the TDLAS data collected in

Claremont, California, in 1995 also showed that the

TDLAS gave the highest concentrations compared with

other analyzers (Fitz et al., 2003).

The data for the last intensive period in September

show a markedly different correlation between the nitric

acid concentrations measured by TDLAS and those

measured by the dual converter instrument than all the

previous data. There is still a linear correlation, but in

addition there is a significant y-intercept as the TDLAS-

measured nighttime nitric acid concentrations were not

decreasing to the detection limit but building up from

night to night (see Fig. 2). To facilitate a clearer picture

of this trend the last intensive period was divided into

two sections, 30 h on September 19/20 and all but these

30 h. During this 30-h period the nighttime concentra-

tion of nitric acid was significantly higher than during

the rest of this intensive measurement period. The same

trend, elevated nighttime concentrations, was also

observed in the formaldehyde data measured by the

second TDLAS in a trailer about 50m away (see Fig. 2).

Both instruments showed normal behavior during span

and zero checks throughout this intensive period.

When comparing the nitric acid concentrations to

ambient ozone concentrations, as shown in Fig. 7, there

appears to be a strong correlation between the two.

However, the last intensive period in September again

differs from the rest of the data insofar that the buildup

of nighttime nitric acid results in a different correlation

pattern with a significant y-intercept. This change also

reflects in the analysis of the nitric acid/ozone ratios with

observation-based methods (Tonnesen and Qin, 2003).

According to this study peak ozone concentrations

0

2

4

6

8

10

12

14

16

0 3 6 9 12 15 18 21 0

Time (PDT)

Mix

ing

Rat

io (

ppb)

HNO3

Fig. 5. Median diurnal HNO3 profiles for intensive measure-

ment days ( ) and non-intensive days (- - - - - -). All points

are 15-min averages.

0

5

10

15

20

25

0 2 4 6

DRI-HNO3 Mixing Ratio (ppb)

TD

L-H

NO

3 M

ixin

g R

atio

(pp

b)

Fig. 6. Plot of TDL-HNO3 data versus collocated dual

converter/chemiluminescence-HNO3 data. All points are 15-

min averages. The data set is split into three groups: non-

intensive measurement days ( ), y ¼ ð3:470:1Þx �ð0:970:2Þ; R2 ¼ 0:79; the last intensive period except a 30-h

period on September 19/20 ( ), y ¼ ð1:370:1Þx �ð5:670:2Þ; R2 ¼ 0:56; and this latter 30-h period ( ), y ¼ð2:570:2Þx � ð9:970:3Þ; R2 ¼ 0:81: Errors stated are one

standard deviation of the linear regression.

C.G. Sauer et al. / Atmospheric Environment 37 (2003) 1583–15911588

during the last intensive period in September are

generally NOx sensitive. However, the ratio ([O3]�[O3]background)/[HNO3] indicates VOC sensitivity around

September 19/20, coinciding with the 30-h period of high

nighttime nitric acid concentrations.

3.3. Formaldehyde

Fig. 8 shows the composite diurnal formaldehyde

profile for intensive measurement days and non-inten-

sive days. The concentrations peak each day around

noon with the maximum concentrations during intensive

days being by a factor of 2–3 higher than during non-

intensive days. During intensives the nighttime concen-

trations also did not decrease as low as during non-

intensive periods, when formaldehyde concentrations

dropped below the detection limit of 1 ppb around

06:00.

The formaldehyde mixing ratios show a strong

positive correlation with the solar radiation measured

at the same site by San Joaquin Valley Air Pollution

Control District during non-intensive days. One day (9

August 2000) showed a sharp peak in formaldehyde

concentration of about 2-h duration in the late morning

(Fig. 2), which we attribute to a fire in the vicinity of the

sampling location. During intensive days the correlation

between formaldehyde concentrations and light intensity

is not as pronounced, especially during the night, which

indicates direct sources for formaldehyde being formed

by enhanced levels of precursor hydrocarbons and

transported over longer distances.

3.4. Hydrogen peroxide

Fig. 9 shows the composite diurnal hydrogen peroxide

profile for intensive measurement days and non-inten-

sive days. During non-intensive days the concentration

maximum is reached in the early afternoon; during

intensives a pronounced maximum occurs in the early

morning on most of the days. Since the morning

maximum is less pronounced in the average values

(not shown here) than in the median values, this suggests

a large deviation in the measured concentrations. In fact

one of the intensives (in August) was aborted after one

day since the forecasted ozone episode did not occur. If

this day is calculated as a non-intensive day, the

0

5

10

15

20

25

0 50 100 150

O3 Mixing Ratio (ppb)

HN

O3

Mix

ing

Rat

io (

ppb)

Fig. 7. Plot of TDL-HNO3 data versus collocated ozone data.

All points are 1-h averages due to the available ozone data. The

data are split into three groups: non-intensive measurement

days ( ), y ¼ ð0:1470:01Þx � ð2:370:3Þ; R2 ¼ 0:73; the last

intensive period except a 30-h period on September 19/20

( ), y ¼ ð0:0570:1Þx þ ð5:170:2Þ; R2 ¼ 0:65; and this latter

30-h period ( ), y ¼ ð0:1070:1Þx þ ð9:270:6Þ; R2 ¼ 0:83:Errors stated are one standard deviation of the linear

regression.

0

2

4

6

8

10

12

0 3 6 9 12 15 18 21 0

Time (PDT)

Mix

ing

Rat

io (

ppb)

HCHO

Fig. 8. Median diurnal HCHO profiles for intensive measure-

ment days ( ) and non-intensive days (- - - - - -). All points

are 15-min averages.

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0 3 6 9 12 15 18 21 0

Time (PDT)

Mix

ing

Rat

io (

ppb)

H2O2

Fig. 9. Median diurnal hydrogen peroxide profile for intensive

measurement days ( ) and non-intensive days (- - - - - -). All

points are 1-h averages.

C.G. Sauer et al. / Atmospheric Environment 37 (2003) 1583–1591 1589

difference between the average and median diurnal

hydrogen peroxide patterns becomes significantly smal-

ler. Detection limits were 0.1–0.25 ppb. This is close

to most of the actually measured concentrations. Thus,

it can be assumed that much of the scatter observed

in the data and responsible for the discrepancies

between average and median diurnal patterns is due

to instrument noise rather than changing ambient

concentrations.

The pattern on non-intensive days agrees with

findings on other sites (Watkins et al., 1995a; Watkins

et al., 1995b; Balasubramanian and Husain, 1997; Das

and Husain, 1999), while other groups have not yet

reported the pattern observed on intensive measurement

days. However, there have been some findings of

nighttime hydrogen peroxide maxima, which were

attributed to meteorological factors rather than gas

phase chemistry (Das and Husain, 1999; Clairborn and

Aneja, 1991). The early morning maximum also possibly

could indicate the buildup of hydrogen peroxide

precursors during the night that react upon sunrise to

give increased levels of H2O2. This is supported by the

nighttime buildup of formaldehyde and nitric acid

during the last intensive measurement period. Formal-

dehyde is known to photolyze in sunlight acting as a

radical source.

4. Conclusions

Two tunable diode laser systems were successfully

employed for the measurement of four trace gases as

part of the CCOS study. Results for non-intensive and

three of the four intensive measurement periods are in

accordance to what was previously recorded in the

literature for similar areas. The data from the last

intensive period in September 2000 are contrary to what

has been published concerning HNO3, H2O2 and HCHO

in similar studies. The buildup of nitric acid and

formaldehyde from night to night indicates that there

was a deviation from the normal source/sink equilibrium

during these nights. The early morning peak of

hydrogen peroxide concentrations suggests a photolytic

source of H2O2. Modeling studies are ongoing to explain

this unusual diurnal hydrogen peroxide profile.

Acknowledgements

This study was funded by the San Joaquin Valleywide

Air Pollution Study Agency through contract 00-

6CCOS. The authors would like to thank Chuck

Bufalino and Carl Camp for their help during the setup

and dismantling of the instruments. Jill Locke and Carl

Camp from the San Joaquin Valley Air Pollution

Control District in Fresno provided the ozone and

weather data; John Bowen from the Desert Research

Institute in Reno made the dual converter nitric acid

measurements available, and Kurt Bumiller and Mark

Chitjian from CE-CERT shared the luminol-GC data

for NO2. A special thank goes to Dave Karecki from

Unisearch-Associates, Toronto, Canada, for his help

and support with the TDL systems.

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