Atmospheric Environment 37 (2003) 3627–3637

HOx concentrations and OH reactivity observationsin New York City during PMTACS-NY2001

Xinrong Ren*, Hartwig Harder1, Monica Martinez1, Robert L. Lesher,Angelique Oliger, Terry Shirley, Jennifer Adams, James B. Simpas,

William H. Brune

Department of Meteorology, Pennsylvania State University, University Park, PA 16802, USA

Received 6 March 2003; accepted 27 May 2003

Abstract

Hydroxyl (OH) and hydroperoxy (HO2) radicals (collectively called HOx) were measured by a laser-induced

fluorescence instrument during the PMTACS-NY (PM2.5 Technology Assessment and Characterization Study–New

York) intensive campaign in New York City in summer 2001. Measurement results for OH and HO2 are presented for

the month-long study. The detection limits were about 3.0� 105 cm�3 for OH and 2.5� 106 cm�3 (B0.1 ppt) for HO2

with a 1-min integration time and a 2s confidence level. The daytime maximum concentrations were 5–20� 106 cm�3

for OH and 0.4–6� 108 cm�3 (2–24 pptv) for HO2, usually appearing later than the peak of ozone photolysis frequency,

J(O1D). Relative high OH and HO2 persisted into early evening and were frequently observed during nighttime. The

ratios of HO2 to OH were typically between 5 and 40, which are smaller than those obtained in relatively clean

environments. The OH reactivity, measured by an instrument named total OH loss rate measurement was on average

1973 s�1 in this urban environment. It was the highest in the morning and the lowest in the afternoon. The comparison

of measured OH and HO2 with model calculations is given in a companion paper (OH and HO2 chemistry in the urban

atmosphere of New York City, Atmospheric Environment (2003a) this issue).

r 2003 Elsevier Ltd. All rights reserved.

Keywords: Hydroxyl radical; Hydroperoxy radical; Measurement; Urban environment; Nighttime hydroxyl

1. Introduction

The hydroxyl radical (OH) and hydroperoxy radical

(HO2), collectively called HOx, play key roles in many

environmental issues such as ozone production, acid

rain and fine particle formation. In polluted urban

environments, high levels of anthropogenic NOx and

volatile organic compounds (VOCs) cause rapid HOx

cycling and termination. Understanding the sources,

propagation, and sinks of HOx is as important to

creating effective pollution control as understanding the

gaseous and particulate emissions and the meteorology

that determines the transport and mixing of these

emissions and their oxidation products.

Under the polluted conditions that typically occur in

urban areas, the HOx production from photolysis of

some oxygen-containing species, such as formaldehyde

(HCHO), nitrous acid (HONO) and hydrogen peroxide

(H2O2), can be as important as ozone photolysis

following by reaction of O(1D) with water vapor (Eisele

et al., 1997). However, these are not the only HOx

sources. Recent studies show that O3 can react with

alkenes to produce significant amounts of OH and HO2

(Paulson and Orlando, 1996; Donahue et al., 1998; Kroll

et al., 2001). Because these sources require no photolysis,

they would produce OH and HO2, both day and night.

Nighttime HOx can be also produced directly from the

ARTICLE IN PRESS

AE International – North America

*Corresponding author. Fax: 1-814-865-3663.

E-mail address: ren@essc.psu.edu (X. Ren).1Now at Max-Planck-Institut f .ur Chemie, D-55116 Mainz,

Germany.

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

doi:10.1016/S1352-2310(03)00460-6

NO3 reaction with HCHO or indirectly from the

conversion to HO2 of organic peroxy radicals (RO2)

that are produced by NO3 reactions with VOCs (Bey

et al., 2001). The chemically diverse urban environment

provides an interesting and important context for

studying HOx production and loss.

OH and HO2 have been successfully measured in

relatively clean atmosphere on the surface ground

(Hofzumahaus et al., 1996; Mount et al., 1997; Mather

et al., 1997; Holland et al., 1999; Creasey et al., 2001,

2002; Kanaya et al., 2001; Faloona et al., 2001) and on

the aircraft (Brune et al., 1998; Mauldin et al., 1999; Tan

et al., 2001a; Hanisco et al., 2002) in recent intensive

field campaigns. The measurements of OH and HO2 in

polluted urban environments are more limited. The

urban areas where OH and/or HO2 have been measured

include Portland, Oregon (Hard et al., 1984, 1986, 1992),

London, UK (Abram et al., 2000), Los Angeles, CA

(George et al., 1999), Nashville, TN (Martinez et al.,

2003), Houston, TX (Martinez et al., 2002), and Berlin,

Germany (Holland et al., 2003).

For most studies, simultaneous measurements of

chemicals that influence HOx were used in models to

calculate HOx, which could be compared to the HOx

measurements. While measured and expected HO2 often

agreed to within a factor of 2, measured HO2 tended to

be higher than expected HO2 when NO was high or

when the solar zenith angle (SZA) was large, including at

night. Measured and expected OH levels were sometimes

in agreement to within combined uncertainties, roughly

a factor of 1.5, but in some environments, measured and

modeled OH differed by a factor of two or even more. If

these differences are real, then the current understanding

of oxidation chemistry may need modification. More

intensive field campaigns that include HOx measure-

ments are needed to test these observed discrepancies.

This paper presents the measurements of OH, HO2

and OH reactivity during the PM2.5 Technology

Assessment and Characterization Study—New York

(PMTACS-NY) intensive in summer 2001. One purpose

of this study was to determine the concentrations of OH

and HO2 and to substantiate our understanding of the

chemistry controlling these chemicals in this highly

polluted environment. Measurements of HOx concen-

trations and OH reactivity both day and night are

presented. The comparison of measured OH and HO2

with model calculations and a discussion of HOx budget

are given in a companion paper (Ren et al., 2003a).

2. Experimental description

2.1. Site description

The PMTACS-NY 2001 intensive campaign took

place at a US EPA host site on the campus of Queens

College (40�4401500N, 73�4901800W) in the Borough of

Queens, New York City. The campaign occurred from

the end of June to the beginning of August. The site was

in a parking lot that overlooks a running track and

soccer field to the west. The instruments were located on

a scaffolding tower near a string of air-conditioned

trailer laboratories. West winds usually brought the

polluted air from Manhattan Island to the site, while

east winds brought relatively clean air from the Atlantic

Ocean. The air at the site was characterized by relatively

high levels of NOx and anthropogenic VOCs.

2.2. HOx measurements

2.2.1. HOx instrument description

Our technique for measuring OH and HO2 is laser-

induced fluorescence (LIF) at low pressure (often called

FAGE) (Hard et al., 1984; Stevens et al., 1994; Mather

et al., 1997). In this technique, the air sample is drawn

into a low-pressure chamber with a vacuum pump. As

the air passes through a laser beam, OH is excited by a

laser that was tuned to the wavelength of an A2Sþðv0 ¼0Þ-X 2Pðv00 ¼ 0Þ OH transition near 308 nm and is

detected by the resulting fluorescence near 308 nm.

Collisional quenching of the excited state is slow enough

at the chamber pressure that the weak OH fluorescence

extends beyond the prompt scattering (Rayleigh, Mie

and wall scattering) and is detected with time-gated

microchannel plate (MCP) detectors. HO2 is detected

when NO, added to the low-pressure air stream, reacts

with HO2 to form OH, which is then detected by LIF.

During this campaign, OH and HO2 measurements

were made with the Penn State ground-based tropo-

spheric hydrogen oxides sensor (GTHOS). GTHOS had

three main subsystems: the detection module, the laser

system, and the electronics. The detection module

consisted of two detection axes in series that were

mounted on the top of a scaffolding tower (Fig. 1). The

sampled air was pulled by a vacuum pump through a

1.0mm-diameter inlet, down a 20-cm long, 5-cm-

diameter tube, into the low-pressure (4.7 hPa) detection

axes. The first detection axis was for OH; the second

one, 20 cm downstream of the first, was for HO2. In each

detection axis, the air stream, laser beam, and detector

field-of-view intersected. The laser beam was passed

through the air stream 32–36 times at each detection axis

with a White cell. An injector loop positioned between

the axes was used to add Ascarite-filtered NO for

converting HO2 to OH. Because of the two-axis

arrangement, the detection of OH and HO2 was

simultaneous.

The laser was a spectra-physics T40-X30S-532Q

diode-pumped frequency-doubled Nd:YAG laser

(532 nm, 3 kHz repetition rate, 25 ns pulse width, 3W

average power) that pumped a Harvard-modified

Chromatix frequency-doubled dye laser (Wennberg

ARTICLE IN PRESSX. Ren et al. / Atmospheric Environment 37 (2003) 3627–36373628

et al., 1994). The dye used was Pyrromethene 597

(Lambda Physik) in 2-propanol. This dye laser had an

intracavity etalon that both narrowed the laser line

width to 3.5GHz, which was about the OH line width,

and allowed for tuning the laser on and off the OH

resonance. The OH transition used was the Q1(2) line

near 308 nm. The total UV laser power was 15–20mW.

A reference cell containing OH, which was made on a

hot filament, indicated times when the laser wavelength

coincided with the OH transition, called on-line, and

when it did not, called off-line (Stevens et al., 1994).

Each hour, a spectral scan was taken of the OH

fluorescence line and the surrounding background to

ensure the identity of the excitation transition and to test

for spectral interferences.

A schematic view of the GTHOS instrument during

the PMTACS-NY2001 campaign (Fig. 1) shows the

arrangement of its components. The laser system,

controlling units, and the data acquisition system were

in a rack that was housed inside the trailer. The

detection module of GTHOS was mounted on top of

the construction scaffolding that was erected beside the

trailer. The GTHOS inlet was 6.4m above the ground.

The vacuum pump was situated under the trailer near

the tower and was connected to the detection cell with a

7.5 cm-diameter vacuum hose. Electrical cables con-

nected the equipment in the trailer to the detection

module on the scaffolding. The UV laser beam was

delivered from the trailer to OH and HO2 axes by two

8m-long, 0.20mm-diameter optical fibers (ThorLabs,

Newton, NJ). The UV power was about 5.0mW in the

OH axis and about 2.0mW in the HO2 axis. The

detection module was housed in a thin sheet-metal

enclosure that was purged with dry nitrogen, thus

minimizing water condensation on the optics when

temperature dropped at night.

The OH fluorescence and background signals were

determined when the laser was tuned on and off

resonance with the OH transition every 10 s. Collection

optics gathered about 8% of the fluorescence and sent it

to detectors behind narrow-band 308 nm interference

filters. The MCP-PMT detectors (Hamamatsu) were

gated on for approximately 300 ns to detect the OH

fluorescence about 60 ns after each laser pulse had

cleared the detection cells. Two counter gates were used

for each detector: one counted the OH fluorescence

signal; a second counted the Rayleigh and chamber

scattering. With the detector gain gate off, and thus the

detector gain several orders of magnitude smaller, the

second counter gate was opened for 50 ns during

the laser pulse to measure the Rayleigh and chamber

scattering. All signals were recorded 5 times a second.

A time series shows the actual detector signals

collected during this campaign (Fig. 2). The high and

low reference cell signals in Fig. 2(c) correspond to

periods when the laser wavelength coincided with the

Q1(2) OH absorption at 307.9951 nm and when it did

not. The off-line position alternated between wave-

lengths that were longer or shorter than the wavelength

of the on-line position. An on-line dithering routine

continually tested for the largest reference cell signal and

adjusted the on-line position. The measured OH and

HO2 signals, shown in Fig. 2(a) and (b), respectively,

track the signal in the OH reference cell. The scattering

signals in the detection cells (Fig. 2(a) and (b)), which

were proportional to the OH sensitivity, indicated the

stability of GTHOS detection sensitivity.

2.2.2. HOx instrument calibration

GTHOS was calibrated before, during, and after the

field measurements. To determine the instrument’s

sensitivity, C, known amounts of OH and HO2 were

generated through the photolysis of water vapor at

184.9 nm with the subsequent reaction with O2

H2Oþ hn ðl ¼ 184:9 nmÞ þO2-OHþHO2: ðR1Þ

The calibration system was carried to the top of the

scaffolding tower to perform the calibration. Ultra zero

air was run at a flow rate of 50 SLPM from the base of

the tower up to the calibration system. A small portion

was passed through a bubbler that contained HPLC-

grade water. Adjusting the fraction of air through the

bubbler produced a range of different water mixing

ARTICLE IN PRESS

Fig. 1. Schematic diagram of the GTHOS instrument for OH

and HO2 measurements and the TOHLM instrument for OH

reactivity measurements during the PMTACS-NY 2001 cam-

paign. The laser system and data acquisition system were in the

trailer and the detection module was mounted on top of a

scaffolding tower, with sample inlet height of about 6.4m above

ground level. The detection module was connected with the

vacuum pump that was located under the trailer.

X. Ren et al. / Atmospheric Environment 37 (2003) 3627–3637 3629

ratios from 0% to 2%. The water concentration was

detected by a LICOR LI-6262 H2O/CO2 instrument

(LICOR Instruments, Lincoln, NE), which was cali-

brated against a chilled mirror hygrometer. A low-

pressure Hg lamp generated the 184.9-nm actinic flux in

a 1.3� 3.8 cm2 irradiation region near the outlet of the

calibration system. The UV flux at 184.9 nm was

measured with a solar-blind Cs–I photomultiplier tube

calibrated against a NIST-calibrated photodiode at the

University of Colorado. The resulting OH and HO2

concentrations were then calculated with knowledge of

the following parameters: the air flow rate, from which

the exposure time, t, could be calculated; the mixing

ratio of water vapor in the air, wH2O; the absolute crosssection of water vapor, sH2O (7.2� 10�20 cm2, Cantrell

et al., 1997); the quantum yield of OH, FOH (1.0,

DeMore et al., 1997); and the integrated actinic flux,

Fl¼184:9 nm: Thus,

wOH ¼ wHO2¼ wH2OsH2OFOHFl¼184:9 nmt: ð1Þ

The upper-limit of the absolute calibration uncertainty

was estimated at 732% (at the 95% confidence level).

The sensitivity of instrument (C) was then obtained from

C ¼DS

wOHP; ð2Þ

where DS is the difference between on-line and off-line

signals (counts s�1), wOH is the calculated volume mixing

ratio from Eq. (1) (pptv), and P is the laser power

monitored at the output of the White cell (mW). The

ambient measurements of wOH were then calculated from

simply inverting Eq. (2). The minimum detectable wOHcan be defined from twice the standard deviation of the

background signal

wOHmin¼

ðS=NÞC

ffiffiffiffiffiffiffi2sb

t

r: ð3Þ

The detection limits (with 2s confidence and 1-min

integration time) were estimated to be in the order of

0.012 pptv (B3� 105 cm�3) for OH and 0.1 pptv

(B2.5� 106 cm�3) for HO2.

2.2.3. Interference tests

The interference of photolytic production of OH

within the detection cell from ozone and water (Smith

and Crosley, 1990) has been overcome in more recent

techniques by using low-pressure detection, low laser

energy per pulse, and 308 nm excitation. The inter-

ference has been found to be negligible for this technique

(Stevens et al., 1994; Zeng et al., 1998), confirmed by

laboratory experiments in which no significant OH

signal was observed when ozone and water vapor were

admitted to the instrument. However, other chemical

interferences might bias the measurement results.

We carried out the interference studies during the

campaign and in the laboratory after the campaign.

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0 20 40 60 80 100 120 1400

500

1000

1500

307.95307.96307.97307.98307.99308.00308.01

-40

0

40

80

120

0

50

100-20

0

20

40

60

0

50

100

150

200

250

Ref

. cel

l sig

nal (

cps)

Elapsed time (s)

Wav

elen

gth

(nm

)

HO

2 sig

nal

(cps

)

Sca

tterin

g si

gnal

at H

O2 c

ell (

cps)

OH

sig

nal

(cps

)

(a)

(b)

(c)

Sca

tterin

g si

gnal

at O

H c

ell (

cps)

Fig. 2. Typical data acquisition cycles recorded at the starting time of 15:36 eastern daylight time (EDT) on 9 July 2001 during the

PMTACS-NY2001 campaign: (a) OH fluorescence signal (solid line, left axis) in counts s�1 (cps, where 1 cps corresponds to

B2.8� 105 cm�3) and Rayleigh scattering (dotted line, right axis) from OH cell; (b) HO2 fluorescence signal (solid line, left axis) in

counts s�1 (cps, where 1 cps corresponds to B1.0� 106 cm�3 or 0.04 pptv) and Rayleigh scattering (dotted line, right axis) from HO2

cell. (c) Signal from reference cell (solid line, left axis) and excitation laser wavelength (dotted line, right axis).

X. Ren et al. / Atmospheric Environment 37 (2003) 3627–36373630

During the field measurements, an automatic procedure

was run every hour on the half-hour to check instrument

performance. Spectral scans showed that there was little

spectral interference from other atmospheric species and

that the laser was alternating wavelengths properly

between the on-line and off-line positions. For HO2, the

reagent NO flows were varied to ensure that the peak

OH signal was obtained from the NO addition.

Occasionally turning off the NO and measuring OH

for two days prior to turning on the NO to measure HO2

showed that NO addition did not affect the observed

OH signals in the first axis. We also occasionally tested

the background signal by internal addition of perfluor-

opropylene (C3F6), by sampling zero air, and by

sampling through a glass-wool filter to remove OH

radicals. Results from this diagnosis also indicated the

lack of interferences. In addition, the OH signal

decreased proportionally with the UV laser power,

whereas a quadratic decrease is expected if OH is both

generated and excited by the laser. All of these tests

indicated that the observed OH signal was not generated

internally and was not produced by the laser beam.

Possible interfering chemical species tested in the

laboratory include formaldehyde (HCHO), hydrogen

peroxide (H2O2), nitrous acid (HONO), nitrite acid

(HNO3) and acetone. Ozone and the alkenes—ethene,

propene and isoprene—were added together just outside

of the inlet or internally to test for possible dark

reactions that produce HOx. All these tests provide

evidence that GTHOS had no significant interferences

(Ren et al., 2003b).

When NO is added to the HO2 axis, ambient RO2

might react with NO to form RO, which then reacts with

O2 to produce HO2, as in the following mechanism:

RO2 þNO-ROþNO2; ðR2Þ

ROþO2-R0CHOþHO2; ðR3Þ

HO2 þNO-OHþNO2: ðR4Þ

Calculations indicate that at the levels of NO used in the

instrument, about 1000 ppmv, the reaction RO+

NO+M-RONO+M is competitive with reaction

(R3) due to the low O2 number density inside the

detection cell. Calculations show that RO2 contributes

less than a 2% interference (Tan et al., 2001b). In

addition, the RO2 interference was examined in the

laboratory, where several different RO2 species at the

levels of B20–100 pptv were produced by photolysis

of air/Cl2/hydrocarbon mixtures. RO2 was produced

from methane (CH4), ethane (C2H6), propane (C3H8)

and n-butane (n-C4H10). The signals from the

generated RO2 were not measurable; the upper limit to

the RO2 interference in the HO2 signal was less than

0.1 pptv.

2.3. OH reactivity measurements

The OH reactivity, kOH, was measured with a new

instrument named the total OH loss-rate measurement

(TOHLM), which is described in detail in Kovacs and

Brune (2001) and Kovacs et al. (2003). Basically,

ambient air was pulled by a blower down a tube and

past a HOx detection axis. HOx was detected in the same

way that ambient HOx was detected with GTHOS. OH

was mixed into the ambient flow through a movable

injector and was detected by the HOx detection axis. As

the injector was pulled back, the decay in the OH signal

was measured. The injector was pulled back in steps,

allowing the OH signal to be monitored as it decayed.

The decay rate was determined by the equation

ktotal ¼ �lnSOH=SOH initial

distance=velocity

� �þ kwall; ð4Þ

where distance/velocity = time of reaction and the wall

loss, kwall, was determined by looking at the OH decay

in zero air (kwallB1.571.0 s�1). The HOx detection

sensitivity does not need to be absolutely calibrated, but

we calibrated it to ensure that the OH mixing ratio

was not too large. Typical OH mixing ratios were

10–30 pptv, large enough to provide strong signals and

avoid possible interferences from ambient oxidants, but

small enough that OH reactants are not depleted.

To calibrate TOHLM, known amounts of CO and

hydrocarbons were added either to the ambient flow or

to ultra zero air. Calibrated OH decays agreed on

average to within 10% of OH decays determined from

knowledge of the ambient flow velocity, step distance,

and kwall. Ambient decays maintained linearity to within

10% for ambient NO of o1 ppbv, but decays could be

corrected for much greater values of NO, although the

uncertainty increases as NO increases. Ambient NO

recycles HO2 into OH, causing curvature in the OH

decays. The absolute accuracy of the TOHLM measure-

ment was 715%, with 2s confidence and a71.0 s�1

uncertainty in the zero air offset.

The instrument was placed on the same top of the

scaffolding tower, about 2m away from the HOx LIF

instrument (Fig. 1).

3. Results of HOX and OH reactivity measurements

During the campaign the weather in New York City

was typical for summer—mostly clear and sunny or

sunny with some clouds. There were occasional rain

showers on 8, 17, and 26 July, and thick clouds on

26 July. The temperature was as high as 35�C during

daytime, with an average of 24�C throughout the

campaign. The average of the relative humility was 55%.

HOx mixing ratios and OH reactivity were continu-

ously measured for about 1 month during the field

ARTICLE IN PRESSX. Ren et al. / Atmospheric Environment 37 (2003) 3627–3637 3631

campaign; more than 600 h of 1-min averaged OH and

HO2 data were obtained.

3.1. OH measurements

OH was measured on 34 days, from 30 June to 2

August. The time series of OH measurements is

presented as 1-min measurements in Fig. 3. Gaps in

the data are due to the instrument calibrations and

maintenance. As can be seen from Fig. 3, the distinct

diurnal variation of OH had daytime peak values

between 2� 106 and 20� 106 cm�3. The observed OH

concentrations are comparable to those measured by

George et al. (1999) and Martinez et al. (2000) in the

similar urban environments.

The composite diurnal variation of OH is shown in

Fig. 4(a). For this figure, data are averaged for 10min.

For reference, also shown in Fig. 4(a) are the median

diurnal variation of OH and the average diurnal

variation of O3 photolysis frequency, J(O1D), which

was calculated from the photolysis frequency expression

by Jenkin et al. (1997) and was scaled to the measured

UV radiation (see the companion paper). On average the

maximum OH concentration was B7� 106 cm�3 and

occurred in the afternoon. Both the OH peak and the

whole shape are shifted from those of J(O1D) by about

2 h. This shift was also found by Kanaya et al. (2001)

and Creasey et al. (2002) in the marine boundary layer.

In the late afternoon and early evening, J(O1D)

decreased to zero, but OH concentration remained at

significant level, about 2� 106 cm�3 on an average.

The correlation coefficient, R2, between OH and

J(O1D) was only 0.35 during this campaign. This poor

correlation contrasts with the good relationship that has

been observed in clean marine and polluted rural

atmosphere (Eisele et al., 1997; Holland et al., 1999;

Ehhalt and Rohrer, 2000).

Two dominant factors controlling the OH and HO2

amounts are NOx and HOx production rate, P(HOx).

According to the expected behavior, as NOx is

increased, OH first increases and then, when NOx is

about 1–2 ppbv, OH decreases due to reactions such as

OH+NO2+M-HNO3+M. For greater P(HOx), OH

is higher and the peak OH moves to greater NOx

(McKeen et al., 1997; Carslaw et al., 2001). These

observed OH dependences in PMTACS-NY agree

qualitatively with the expected behavior, as shown in

Fig. 5(a).

3.2. HO2 measurements

HO2 was measured on 32 days, from 2 July to 2

August (Fig. 6). During daytime, typical peak HO2

concentrations observed were B0.5� 108–2� 108 cm�3

(2–8 pptv) with levels reaching as high as 6� 108 cm�3

(24 pptv) on 25 July. Fig. 4(b) shows the diurnal median

variation of HO2 over the entire campaign, together

with the corresponding average J(O1D) values. The

average maximum HO2 concentration during daytime

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Jun 30 Jul 1 Jul 2 Jul 3 Jul 4 Jul 5 Jul 6 Jul 7 Jul 8 Jul 9 Jul 10 Jul 11 0

10

20x 10

6

Jul 12 Jul 13 Jul 14 Jul 15 Jul 16 Jul 17 Jul 18 Jul 19 Jul 20 Jul 21 Jul 22 0

10

20x 10

6

Jul 23 Jul 24 Jul 25 Jul 26 Jul 27 Jul 28 Jul 29 Jul 30 Jul 31 Aug 1 g 2 Au 0

10

20x 10

6

Date of 2001 (EDT)

OH

(cm

-3)

OH

(cm

-3)

OH

(cm

-3)

Fig. 3. Time series of all the 1-min average OH data (small dots) during the PMTACS-NY2001 field campaign. Local daylight time

(EDT) was used.

X. Ren et al. / Atmospheric Environment 37 (2003) 3627–36373632

was about 1.0� 108 cm�3 (4 pptv), which is much

lower than that obtained in the marine boundary

layer (Holland et al, 1999; Kanaya et al., 2001;

Creasey et al., 2002) and the forest sites (Tan et al.,

2001b; Creasey et al., 2001). This low value is due to

high NO concentrations in this urban area, which

can quickly convert HO2 to OH. The average NO

concentration over the campaign was 10.2 ppbv

while its level in the morning rush hour was as high as

200 ppbv.

Similar to the OH peak, the HO2 peak is shifted

behind the peak of J(O1D) by about 2 h (Fig. 4(b)). The

HO2 profile shows that the HO2 concentration increased

slowly in the morning while it remained as high as

2� 107–4� 107 cm�3 (0.8–1.6 pptv) in the early evening

and was about 1� 107 cm�3 (0.4 pptv) at night. This

significant amount of HO2 acts as the dominant source

of OH (Geyer et al., 2003).

For any value of P(HOx), HO2 is expected to remain

fairly constant as NOx increases to 1–2 ppbv, but then

decreases more rapidly than OH as NOx increases to

much higher amounts. Just as for OH, HO2 is expected

to be higher for great P(HOx) at any given value of NOx

(Fig. 5(b)). These observed HO2 dependences on NO

and P(HOx) agree qualitatively with this expected

behavior.

3.3. Nighttime observations

Nighttime OH and HO2 measurements were also

carried out over the campaign. Unlike our previous field

studies during which NO approached 0 ppbv at night,

nighttime NO in New York City was typically between a

few ppbv and 20–30 ppbv (Fig. 7). As seen from Figs. 3

and 6, OH and HO2 reached near-zero values for several

hours on more than 20 nights. However, relatively high

nighttime OH and HO2 were also frequently observed

during this campaign. In the late afternoon and early

evening, OH was B1.3� 106 cm�3 (Fig. 4(a)) and HO2

was B2.5� 107 cm�3 (1 pptv) (Fig. 4(b)) on average.

Small but significant average levels of OH and HO2,

B7� 105 and B8� 106 cm�3 (0.32 pptv), respectively,

remained through midnight to 4:00 in the morning. At

around 5:00–6:00 in the morning, both OH and HO2

reached the average minima of 5� 105 and

2.5� 106 cm�3 (0.1 pptv), respectively.

ARTICLE IN PRESS

10-1

100

101

102

105

106

107

NO (ppbv)

10-1

100

101

102

105

106

107

108

109

NO (ppbv)

HO

2 (c

m)

OH

(cm

-3)

(a)

(b)

Fig. 5. Dependence of the measured OH (a) and HO2 (b) upon

the observed NO and calculated HOx production, P(HOx).

Gray dots are all 10-min measurements and are binned by same

log(NO) intervals to get average points linked by four lines

according to different P(HOx). Ranges for P(HOx) are indi-

cated by different symbols: diamonds (>3.0� 107 cm�3 s�1);

asterisks (1.5� 107–3� 107 cm�3 s�1); circles (0.5� 107–

1.5� 107 cm�3 s�1); squares (o0.5� 107 cm�3 s�1).

0:00 6:00 12:00 18:00 0:000

5

10

15x 10

6

0

1

2

3x 10

-5

0:00 6:00 12:00 18:00 0:000

1

2

3x 10

8

0

1

2

x 10-5

OH

(cm

-3)

OH

(cm

-3)

J(O

1 D)(

s-1)

J(O

1 D)(

s-1)

Time of day (EDT)

(a)

(b)

Fig. 4. Composite diurnal variations of measured OH (a) and

HO2 (b) during the PMTACS-NY2001 campaign. The small

gray dots show all 10-min average data and the solid thick line

indicates the median diurnal variation. Average diurnal profile

of ozone photolysis frequencies, J(O1D), is shown as the dashed

thick line. The time of day is the local daylight time (EDT),

which was 1:05 later than the local solar time.

X. Ren et al. / Atmospheric Environment 37 (2003) 3627–3637 3633

OH and HO2 increased after midnight on several

days, especially on 17 and 18 July. O3 also increased,

perhaps as a result of transport. The correlation between

O3 and HOx makes sense; O3 can react with alkenes and

directly produce OH and HO2. Meanwhile NO3 can be

produced by the O3 reaction with NO2. Relatively high

NO2 existed throughout the whole campaign, normally

around 20–30 ppbv. NO3 can react with HCHO and

alkenes to produce HO2 and b-nitrooxy-peroxy radicals.The nitrooxy-peroxy radicals can react with NO

producing alkoxy radicals, which can produce HO2

through the reaction with O2. Details about nighttime

HOx observations after the midnights of 16/17 and 17/18

July are described in a separate paper (Ren et al., 2003c).

Results indicate that nighttime OH and HO2 were

primarily produced via the reactions of ozone with

alkenes, while the nighttime NO3 reactions were not

important due to its low concentrations. The relatively

high observed OH and HO2 suggest that HOx chemistry

can play an important role in the nighttime oxidation

processes in the polluted atmosphere.

3.4. Observed HO2/OH ratios

The HO2/OH ratio reflects the interconversion be-

tween OH and HO2, thus it is a very useful parameter to

test the fast cycling between OH and HO2 (Stevens et al.,

1997). For this campaign, most HO2/OH ratios were

between 5 and 40 and the average ratio was B15

(Fig. 7). These measured ratios are much lower than

those obtained in relatively clean atmosphere (Stevens

et al., 1997; Carslaw et al., 2001; Tan et al., 2001b;

Kanaya et al., 2001). One reason for these lower HO2/

OH ratios is the high NO concentrations in this urban

atmosphere which can accelerate the exchange rate from

HO2 to OH. The diurnal profile indicates that the lower

ARTICLE IN PRESS

Jul 2 Jul 3 Jul 4 Jul 5 Jul 6 Jul 7 Jul 8 Jul 9 Jul 10 Jul 11 Jul 120

1

2

x 108

Jul 13 Jul 14 Jul 15 Jul 16 Jul 17 Jul 18 Jul 19 Jul 20 Jul 21 Jul 22 Jul 230

1

2

x 108

Jul 24 Jul 25 Jul 26 Jul 27 Jul 28 Jul 29 Jul 30 Jul 31 Aug 1 Aug 2 0

2

4

6x 10

8

Date of 2001 (EDT)

HO

2 (c

m-3)

HO

2 (c

m-3)

HO

2 (c

m-3)

Fig. 6. Time series of 1-min average HO2 measurements (small dots) during the PMTACS-NY2001 campaign. Time is in local daylight

time (EDT), which is 1:05 later than the local solar time.

0:00 6:00 12:00 18:00 0:000

10

20

30

40

Time of day (EDT)

0

10

20

30

40

HO

2/O

H r

atio

NO

(pp

bv)

Fig. 7. Composite diurnal variation of measured HO2/OH

ratios during the PMTACS-NY2001 campaign. The small dots

show all 10-min average data and the solid circles indicate the

median diurnal variation of HO2/OH ratios in 1 h intervals.

Average diurnal profile of measured NO is shown as the thick

line.

X. Ren et al. / Atmospheric Environment 37 (2003) 3627–36373634

HO2/OH ratios appeared in the early morning at around

4:00–10:00 when the NO concentrations were usually

high. In the afternoon, NO was lower than in the

morning, leading to the higher HO2/OH ratios.

3.5. OH reactivity measurements

As discussed by Kovacs et al. (2003), an important

consideration for the OH reactivity measurements in

polluted environments is the recycling of the HO2 by

reaction (R4), which slows the OH decay. The kinetic

rate equation for OH thus becomes

d½OHdt

¼ �ktotal½OH þ kNOþHO2½NO½HO2: ð5Þ

Inverting this equation to get the corrected OH

reactivity, k0OH; in the equation

k0OH ¼ kOH þ kNOþHO2

½NOSHO2

SOH; ð6Þ

where SHO2=SOH ¼ ½HO2=½OH; since the detector

calibrations for OH and HO2 are the same. HO2 signal

decays were measured by adding pure NO downstream

of the inlet in the lower pressure sampling tube upstream

of the detection axis. The average of SHO2=SOH was

about 4.971.5. Using the measured NO concentrations,

the observed kOH values were corrected. For the kOH of

10 s�1 and NO o1.0 ppbv, k0OH was less than 1.10 times

kOH; when NO was 5 ppbv, k0OH was 1.48 times kOH; and

when NO was 10 ppbv, k0OH was 1.97 times kOH. Recent

laboratory studies show that, within the NO range of

0–25 ppbv, which includes 90% of the measurements in

New York City, the agreement between the calculated

and corrected measured OH reactivity is better than

B10% when SHO2=SOH is measured, while in the higher

NO range (25–100 ppbv), the discrepancy increased to

about 15–50%.

During this campaign, the OH reactivity was typically

20 s�1, with a maximum of B25 s�1 at morning rush

hour and a minimum in the mid-afternoon of B15 s�1

(Fig. 8). These values are about twice those found in

Nashville in 1999 (Kovacs et al., 2003). As in Nashville,

the scatter of OH reactivity measurements is greater at

night than during the day and is smallest in the mid-

afternoon. The OH reactivity correlates well with

measured NOx, with the highest values in the morning

when NOx was higher and the lowest in the afternoon

while NOx was lower. OH lifetime in this environment is

very short, about 0.05 s due to the high concentrations

of pollutants. As shown in the companion paper (Ren

et al., 2003a), the measured OH reactivity agrees to

within 10% with the OH reactivity that is calculated

from the sum of the individual OH reactants and their

rate coefficients.

4. Summary

OH and HO2 radicals were measured during the

PMTACS-NY2001 intensive field campaign by a laser-

induced fluorescence instrument. Average diurnal varia-

tions show that the maximum concentrations of OH and

HO2 were about 7� 106 and 1� 108 cm�3 (4 pptv),

respectively, which usually appeared in the afternoon.

The relationships between OH and NO, HO2 and NO,

and HO2/OH ratio and NO were qualitatively consistent

with our understanding of atmospheric HOx chemistry.

HO2/OH ratios found in this polluted environment are

lower than those obtained in relatively clean atmosphere

due to the relatively high NO concentrations in New

York City. Small but significant OH and HO2 were

frequently observed at night, indicating that OH and

HO2 may play important roles in the nighttime

chemistry of the polluted urban atmosphere. OH

reactivity measurements show that due to high concen-

trations of pollutants, the OH lifetime in this urban

environment was only about 0.05 s. A modeling paper

will describe the comparison of these results with theory

and models.

Acknowledgements

The authors would like to thank Kenneth Demerjian

for asking us to participate in PMTACS-NY2001. We

also thank all the other researchers for their kind

cooperation during this field campaign. Two anonymous

reviewers are acknowledged for providing insightful

comments and suggestions. This work was supported by

NSF (ATM-9974335), the New York State Energy

ARTICLE IN PRESS

0:00 6:00 12:00 18:00 0:000

10

20

30

40

50

Time of day (EDT)

-20

0

20

40

60

80

NO

x (

ppbv

)

OH

rea

ctiv

ity (

s-1 )

Fig. 8. Diurnal variation of OH reactivity measured by

TOHLM. The gray points show all measured values and the

solid circles are the median values in 1-h intervals. Error bars

on circles indicate the absolute uncertainties (1s) of the hourlyaverages. Also shown in the figure as a thick line is the average

diurnal variation of NOx during the campaign.

X. Ren et al. / Atmospheric Environment 37 (2003) 3627–3637 3635

Research and Development Authority (NYSERDA)

(contract x 4918ERTERES99), the US Environmental

Protection Agency (EPA) (cooperative agreement x

R828060010), and New York State Department of

Environmental Conservation (NYS DEC) (contract x

C004210). Although the research described in this article

has been funded in part by the US Environmental

Protection Agency, it has not been subjected to the

Agency’s required peer and policy review and, therefore,

does not necessary reflect the views of the Agency and

no official endorsement should be inferred.

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