acp-16-7623-2016.pdf

Atmos. Chem. Phys., 16, 7623–7637, 2016

www.atmos-chem-phys.net/16/7623/2016/

doi:10.5194/acp-16-7623-2016

The lifetime of nitrogen oxides in an isoprene-dominated forest

Paul S. Romer

, Kaitlin C. Duffey

, Paul J. Wooldridge

, Hannah M. Allen

2,3

, Benjamin R. Ayres

, Steven S. Brown

William H. Brune

, John D. Crounse

, Joost de Gouw

4,7

, Danielle C. Draper

2,8

, Philip A. Feiner

, Juliane L. Fry

Allen H. Goldstein

9,10

, Abigail Koss

4,7

, Pawel K. Misztal

, Tran B. Nguyen

6,11

, Kevin Olson

, Alex P. Teng

Paul O. Wennberg

6,12

, Robert J. Wild

4,7

, Li Zhang

, and Ronald C. Cohen

1,13

Department of Chemistry, University of California at Berkeley, Berkeley, CA, USA

Department of Chemistry, Reed College, Portland, OR, USA

Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, CA, USA

Chemical Sciences Division, Earth System Research Laboratory, National Oceanic and Atmospheric Administration,

Boulder, CO, USA

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

Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA, USA

Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, CO, USA

Department of Chemistry, University of California, Irvine, CA, USA

Department of Civil and Environmental Engineering, University of California at Berkeley, Berkeley, CA, USA

Department of Environmental Science, Policy and Management, University of California at Berkeley, Berkeley, CA, USA

Department of Environmental Toxicology, University of California, Davis, CA, USA

Division of Engineering and Applied Science, California Institute of Technology, Pasadena, CA, USA

Department of Earth and Planetary Sciences, University of California at Berkeley, Berkeley, CA, USA

Correspondence to:

Ronald C. Cohen (rccohen@berkeley.edu)

Received: 12 January 2016 – Published in Atmos. Chem. Phys. Discuss.: 21 January 2016

Revised: 4 May 2016 – Accepted: 16 May 2016 – Published: 23 June 2016

Abstract.

The lifetime of nitrogen oxides (NO

) affects the

concentration and distribution of NO

and the spatial pat-

terns of nitrogen deposition. Despite its importance, the life-

time of NO

is poorly constrained in rural and remote con-

tinental regions. We use measurements from a site in central

Alabama during the Southern Oxidant and Aerosol Study

(SOAS) in summer 2013 to provide new insights into the

chemistry of NO

and NO

reservoirs. We find that the life-

time of NO

during the daytime is controlled primarily by

the production and loss of alkyl and multifunctional nitrates

(

ANs). During SOAS,

AN production was rapid, averag-

ing 90 ppt h

−

during the day, and occurred predominantly

during isoprene oxidation. Analysis of the

AN and HNO

budgets indicate that

ANs have an average lifetime of un-

der 2 h, and that approximately 45 % of the

ANs produced

at this site are rapidly hydrolyzed to produce nitric acid. We

find that

AN hydrolysis is the largest source of HNO

and

the primary pathway to permanent removal of NO

from the

boundary layer in this location. Using these new constraints

on the fate of

ANs, we find that the NO

lifetime is 11

5 h

under typical midday conditions. The lifetime is extended by

storage of NO

in temporary reservoirs, including acyl per-

oxy nitrates and

ANs.

1 Introduction

The concentration and chemistry of nitrogen oxides

(NO

≡

) in Earth’s troposphere has a significant

and non-linear effect on the oxidative capacity of the atmo-

sphere. This in turn affects the production, composition, and

aging of aerosols and the lifetime of greenhouse gases such

as methane. Concentrations of NO

control the production

of ozone, a respiratory health hazard, important oxidant, and

greenhouse gas. In addition, the deposition of reactive nitro-

gen is an important source of nutrients in some ecosystems

(e.g. Fowler et al., 2013).

Published by Copernicus Publications on behalf of the European Geosciences Union.

7624

P. S. Romer et al.: The lifetime of nitrogen oxides

is emitted by both anthropogenic and biogenic

sources, including motor vehicles, power plants, forest fires,

and soil bacteria (e.g. Dallmann and Harley, 2010; Mebust

and Cohen, 2014; Hudman et al., 2012), and is temporarily

or permanently removed from the atmosphere by chemical

conversion to higher oxides of nitrogen. Across much of the

globe, the balance of these sources and sinks is in a period

of dramatic change, with large reductions of NO

emissions

occurring in North America and Europe and significant in-

creases occurring in Asia (e.g. Russell et al., 2012; Curier

et al., 2014; Reuter et al., 2014). Understanding the effects

of changes in NO

emissions on the concentration and spa-

tial distribution of NO

requires detailed knowledge of the

chemistry and transport of NO

and NO

reservoirs. These

reservoirs are poorly understood and represent a significant

uncertainty in analyses of NO

emissions and ozone produc-

tion (e.g. Ito et al., 2007; Browne and Cohen, 2012; Mao

et al., 2013).

The net chemical loss of NO

is difficult to directly ob-

serve. Observational methods for determining the lifetime of

are easiest to apply in the outflow of isolated emissions,

where the declining concentration of NO

or the changing

ratio of NO

to total reactive nitrogen (NO

) provide clear

evidence for NO

loss (e.g. Ryerson et al., 1998; Dillon et al.,

2002; Alvarado et al., 2010; Valin et al., 2013). In rural and

remote regions, emissions and concentrations of NO

and

are typically slowly varying over large distances (e.g.

Browne et al., 2013), preventing the loss of NO

from be-

ing directly observable. Nor can the lifetimes found in plume

studies be easily translated into an appropriate lifetime in

the regional background. Short-lived NO

reservoirs such as

peroxy acyl nitrate (PAN) can efficiently remove NO

in a

plume, but act as a source of NO

in rural and remote regions

(Finlayson-Pitts and Pitts, 1999). In addition, the non-linear

interactions between NO

and OH make the lifetime of NO

in a fresh plume very different from its lifetime several hours

downwind (e.g. Martinez et al., 2003; Valin et al., 2013).

To constrain the lifetime of NO

in rural and remote re-

gions, observations of reactive nitrogen species must be com-

bined with an understanding of the chemical transformations

between NO

and its higher oxides. If the production, loss,

and fate of these higher oxides are accurately understood,

then the lifetime of NO

can be calculated by tracing the flow

of reactive nitrogen through the system. Here, we evaluate

the daytime lifetime of NO

in the rural southeastern United

States, using measurements taken from 1 June–15 July 2013

as part of the Southern Oxidant and Aerosol Study (SOAS).

In situ measurements of volatile organic compounds (VOCs),

atmospheric oxidants, and a wide range of reactive nitrogen

compounds are used to determine the production and loss

rates for nitric acid, alkyl and multifunctional nitrates, and

peroxy nitrates. These rates are used to assess the lifetime of

in this region.

2 The NO

family and the lifetime of NO

During the day, NO

is lost by associating with other radicals

to produce higher oxides of nitrogen, primarily nitric acid,

alkyl and multifunctional nitrates (

ANs

RONO

), and

peroxy nitrates (

PNs

(

)

OONO

) (e.g. Day et al.,

2003; Perring et al., 2010). The sum of these and other higher

oxides such as N

and HONO are collectively known as

(NO

≡

−

is oxidized to produce the major daytime classes of

through Reactions (R1), (R2b), and (R3).

→

HNO

(R1)

→

RONO

(

R2b

)

(

)

(

)

OONO

(R3)

can also be converted to NO

through reactions of the

radical. Although these reactions are most important at

night, previous studies have shown that NO

chemistry can

produce NO

during the day if concentrations of alkenes are

high (e.g. Fuentes et al., 2007; Mogensen et al., 2015; Ayres

et al., 2015).

The production and fate of different NO

species deter-

mine the lifetime of NO

. Some of these species are short-

lived and re-release NO

back to the atmosphere within

hours of being formed. If the lifetime for the conversion of

an NO

species back to NO

is shorter than typical NO

life-

times in the atmosphere, then NO

and these NO

species in-

teract, and their concentrations will approach a steady-state

ratio. As NO

is removed from the system, some of the short-

lived NO

species dissociate, buffering the concentration of

. In this way, the presence of NO

reservoirs directly

extends the lifetime of NO

One method to take this buffering into account when cal-

culating the lifetime of NO

is to consider the sum of NO

and all NO

species with lifetimes to re-release of NO

shorter than the atmospheric lifetime of NO

. We define this

sum as short-lived reactive nitrogen, or NO

. The remaining

forms of reactive nitrogen are defined as long-lived reactive

nitrogen (NO

). The division between NO

and NO

de-

pends on the lifetime of NO

. For the initial discussion in

this manuscript, we use a provisional lifetime of 7 h to divide

species between NO

and NO

. This cutoff is in the

middle of the range of NO

lifetimes found in plume studies

(e.g. Ryerson et al., 1998; Dillon et al., 2002; Alvarado et al.,

2010; Valin et al., 2013). The provisional cutoff chosen as a

starting point does not affect the final results.

In areas well removed from large NO

sources, NO

and

its short-lived reservoirs interconvert significantly faster than

the rate of change of NO

. Under these conditions, the life-

time of NO

(

) is equal to the lifetime of NO

. If the

conversion of NO

to NO

is negligible, then the lifetime

of NO

can be calculated by Eq. (1).

[

]

L

(

)

(1)

Atmos. Chem. Phys., 16, 7623–7637, 2016

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P. S. Romer et al.: The lifetime of nitrogen oxides

7625

Figure 1.

A schematic representation of the chemistry of NO

and

, showing the typical components of both classes.

Throughout this paper, we use

L

(X)

to indicate the gross loss

rate of the compound or class of compounds

The relationship and interactions between NO

and

, and their typical compositions in the planetary bound-

ary layer, are shown in Fig. 1. In the summertime at mid-

latitudes, peroxy nitrates typically release NO

within hours

of being formed (LaFranchi et al., 2009), making them a

component of NO

. Under these same conditions, nitric

acid typically converts back to NO

on timescales of 100 h

or greater (Finlayson-Pitts and Pitts, 1999) and is a com-

ponent of NO

. The fate and lifetime of

ANs, the third

major component of NO

, remain poorly understood, mak-

ing it uncertain whether

ANs act as a component of NO

or NO

(Perring et al., 2013, and references therein). This

is especially true for the multifunctional, biogenically de-

rived nitrates that are the predominant component of

ANs

in forested areas (e.g. Beaver et al., 2012).

Recent studies of multifunctional nitrates suggest that the

main daytime loss pathways of these species are deposition,

reaction with OH, photolysis, and heterogeneous hydrolysis

to produce nitric acid (e.g. Darer et al., 2011; Browne et al.,

2013; Lee et al., 2014; Müller et al., 2014; Nguyen et al.,

2015; Lee et al., 2016). These recent studies, combined with

the extensive measurements made during SOAS, allow us to

provide new constraints on the lifetime and fate of

ANs and

therefore to more accurately determine the lifetime of NO

3 Instrumentation and measurements

The primary ground site for SOAS was located in Bibb

County, Alabama (32.90289

◦

N, 87.24968

◦

W) at the Cen-

treville (CTR) long-term monitoring site in the SouthEast-

ern Aerosol Research and CHaracterization (SEARCH) Net-

work (Hansen et al., 2003). This location is 40 km southeast

of Tuscaloosa (population 95 000), and 90 km southwest of

Birmingham (population 210 000). Comparison with long-

term measurements indicate that the summer of 2013 was

cooler and cloudier than typical for previous summers (Hidy

et al., 2014). Gas-phase measurements used in this study

were located on a 20 m walk-up tower at the edge of the

forest. Nitrate ion and meteorological parameters were mea-

sured in a clearing approximately 50 m away from the tower.

A nearly complete suite of reactive nitrogen species, in-

cluding NO, NO

PNs,

ANs, HNO

, and NO

−

, was

measured during SOAS. NO was measured using the chemi-

luminescence instrument described in Min et al. (2014). The

reaction of ambient NO with added excess O

formed excited

∗

molecules. A fraction of these fluoresce, and the emit-

ted photons were collected on a red-sensitive photomultiplier

tube (Hamamatsu H7421-50). Calibrations were performed

every 2 h by diluting NO standard gas (5.08 ppm

5 % NO

in N

, Praxair) to 3–20 ppb in zero air and adding it to the in-

strument inlet. The mixing ratio was corrected for enhanced

quenching by water vapor (Thornton et al., 2000) using co-

located measurements of relative humidity and temperature.

PNs, and

ANs were measured via thermal dis-

sociation laser induced fluorescence (TD-LIF), as described

by Day et al. (2002). Ambient air was drawn into a multipass

White cell, where a 532 nm Nd-YAG laser excited the NO

molecules, and their fluorescence signal was collected on a

photomultiplier tube (Hamamatsu H7421-50). The same in-

strument was used to measure the sum of peroxy nitrates and

the sum of alkyl and multifunctional nitrates by first pass-

ing the air through a heated oven, where the organic nitrates

dissociated to form NO

. Organic nitrates present in the par-

ticle phase undergo evaporation and thermal dissociation in

the heated ovens to form NO

. The TD-LIF measurement

ANs therefore includes alkyl and multifunctional ni-

trates in both the gas and particle phases, but does not in-

clude HNO

or particle-phase inorganic nitrate (Day et al.,

2002; Rollins et al., 2010). All of the channels were cali-

brated by injecting NO

standard gas (5.03 ppm

5 % NO

in N

, Praxair) and corrected for enhanced quenching by wa-

ter vapor.

Nitric acid was measured in the gas phase by chemical

ionization mass spectrometry (CIMS), using CF

−

as the

reagent ion (Crounse et al., 2006). The ions were quantified

using a compact time-of-flight mass spectrometer, and the in-

strument was calibrated in the field using isotopically labeled

nitric acid. Particle-phase inorganic nitrate (NO

−

) was mea-

sured using a Monitor for AeRosols and GAses (MARGA)

(Allen et al., 2015). Ambient air was drawn through a rotat-

ing wet-walled denuder which collected water-soluble gas-

phase compounds. Particle-phase compounds were captured

by a steam-jet aerosol collector downstream of the denuder.

Water-soluble ions from both phases were then quantified

via ion chromatography. This measurement of NO

−

is de-

signed to be specific to inorganic nitrate, and is not affected

ANs in the particle phase (Allen et al., 2015).

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Atmos. Chem. Phys., 16, 7623–7637, 2016

7626

P. S. Romer et al.: The lifetime of nitrogen oxides

Figure 2.

Diurnal cycle of measured reactive nitrogen species dur-

ing SOAS. Reactive nitrogen species are classified as likely compo-

nents of NO

(a)

, likely components of NO

(b)

or unknown

(c)

The classification into NO

and NO

is based on typical sum-

mertime afternoon lifetimes. The measurement of HNO

represents

nitric acid in the gas phase, while the measurement of NO

−

rep-

resents inorganic nitrate in the particle phase. The measurement of

ANs includes alkyl and multifunctional nitrates in both the gas

and particle phase.

Measurements of reactive nitrogen species are summa-

rized in Fig. 2. Concentrations of NO

compounds (NO,

, and

PNs) are shown in Fig. 2a. Afternoon concentra-

tions of NO

and NO were typically around 220 and 50 ppt

respectively. After sunset, NO dropped to near zero, and NO

began to increase. At sunrise, NO concentrations rapidly rose

to over 200 ppt between 06:00 and 08:00 Central Standard

Time (CST) while NO

decreased sharply. By 11:00, when

the daytime boundary layer was well developed, the concen-

trations of NO and NO

returned to their typical afternoon

values. Concentrations of

PNs were 160 ppt at sunrise, in-

creased to a maximum concentration of 300 ppt at 09:00 and

declined slowly throughout the rest of the day.

Concentrations of HNO

and inorganic NO

−

, components

of NO

, are shown in Fig. 2b. Both species increased slowly

after sunrise and reached a maximum combined concentra-

tion of 300 ppt at 13:00, before declining to a combined

concentration of 175 ppt at night. The total concentration

ANs in both the gas and particle phase, whose par-

titioning into NO

and NO

is not known, is shown in

Fig. 2c.

ANs averaged 150 ppt during the night and in-

creased sharply after sunrise. After reaching a maximum of

250 ppt at 08:00,

ANs declined slowly to a minimum con-

centration of 125 ppt at sunset.

OH, HO

, and OH reactivity (OHR) were measured via

fluorescence assay by gas expansion (FAGE) of OH. A

308 nm dye laser excited the OH radicals and their fluores-

Figure 3.

Diurnal cycle of OH, HO

, O

and VOCs during SOAS.

The top plot shows the concentration of OH and HO

; the middle

plot shows the concentration of O

; the bottom plot shows the OH

reactivity.

cence was detected by an electronically gated microchannel

plate detector (Faloona et al., 2004). Calibration of the sys-

tem was performed by in situ generation of OH radicals via

photolysis of water vapor. Chemical zeroing was performed

by periodically adding C

to the sampling inlet in order to

quantify the interference from internally generated OH ob-

served in previous field campaigns (Mao et al., 2012). HO

was measured in a second channel by adding NO to chem-

ically convert HO

to OH. The amount of added NO was

regulated such that HO

but not RO

was converted to OH

(Fuchs et al., 2011). Total OH reactivity (OHR) was mea-

sured by drawing ambient air through a flow tube and mix-

ing it with a fixed concentration of OH. At the end of the flow

tube, the concentration of OH was measured. The OH reac-

tivity is determined by the slope of the OH signal vs. reaction

time (Mao et al., 2009).

Measured concentrations of OH peaked at 0.045 ppt and

concentrations of HO

at 30 ppt during SOAS (Fig. 3, top

panel). Both OH and HO

increased slowly throughout the

morning and reached their maximum in the early afternoon.

Concentrations then fell as the sun set, with OH usually

dropping below 0.01 ppt by 19:00 The measured OH reactiv-

ity was high, reaching an afternoon peak of close to 25 s

−

(Fig. 3, bottom panel). OHR decreased throughout the night,

reaching a minimum of 10 s

−

just before sunrise.

Measurements of ozone were made using a cavity ring

down spectrometer (Washenfelder et al., 2011). O

was

chemically converted to NO

by reaction with excess NO,

and the resulting NO

was measured by cavity ring-down

spectroscopy at 404 nm. The concentration of ozone in-

creased from a minimum of 15 ppb at sunrise to a maximum

of 38 ppb in the late afternoon (Fig. 3, middle panel).

Atmos. Chem. Phys., 16, 7623–7637, 2016

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P. S. Romer et al.: The lifetime of nitrogen oxides

7627

Volatile organic compounds were measured primarily by

gas chromatography–mass spectrometry (GC-MS). Samples

were collected in a liquid-nitrogen cooled trap for 5 min,

and then transferred by heating onto an analytical column,

and detected using an electron-impact quadrupole mass-

spectrometer (Goldan et al., 2004; Gilman et al., 2010). This

system was able to quantify a wide range of compounds in-

cluding alkanes, alkenes, aromatics, isoprene, and multiple

monoterpenes. The sum of methyl vinyl ketone (MVK) and

methacrolein (MACR) was measured using a proton transfer

reaction time-of-flight mass spectrometer (PTR-TOF-MS)

(Kaser et al., 2013). The interference in this measurement

from the decomposition of isoprene hydroperoxides on in-

strument inlets (Rivera-Rios et al., 2014) is not corrected for,

and increases the uncertainty in this measurement by approx-

imately 20 %.

VOC measurements at the site show that the OHR was

dominated by reaction with biogenic compounds. Figure 3

shows the OH reactivity of individually measured com-

pounds as a stacked area plot. In the daytime, isoprene ac-

counted for nearly half of the total reactivity, while VOCs

typically attributed to anthropogenic activities, including

alkanes, aromatics, and simple alkenes, were responsible

for less than 10 % of the measured OHR. Not included

in Fig. 3 is the reactivity of VOCs whose reaction with

OH does not lead to net loss of OH, and therefore does

not contribute to the measured OHR. These compounds,

primarily isoprene hydroperoxides (ISOPOOH) and C5-

hydroxyaldehydes (HPALD), have an average daytime re-

activity of 2 s

−

. The sum of individual reactivities shows

a similar diurnal pattern to the measured OHR, and accounts

for 70–85 % of the total. Unknown biogenic emissions, small

aldehydes and alcohols, and other 2nd and 3rd generation

VOC oxidation products are all possible contributors to the

missing reactivity (e.g. Di Carlo et al., 2004; Goldstein and

Galbally, 2007; Pusede et al., 2014; Kaiser et al., 2016). Me-

teorological parameters including temperature and solar radi-

ation were measured by Atmospheric Research and Analysis

as part of SEARCH.

4 The production and loss of individual NO

reservoirs

4.1 Nitric acid

In the boundary layer, the production of nitric acid is typ-

ically followed by deposition and thus leads to the perma-

nent removal of reactive nitrogen from the atmosphere. Ni-

tric acid can undergo photolysis or reaction with OH to pro-

duce NO

, but these processes are slow (Burkholder et al.,

1993; Atkinson et al., 2006), with an average calculated rate

during SOAS of less than 0.2 ppt h

−

. Gas-phase nitric acid

can also partition into aerosols. Nitric acid is long-lived in

the particle phase and is typically lost by re-evaporation into

the gas phase (e.g. Hennigan et al., 2008). The loss of ni-

tric acid through deposition of aerosols is typically negligi-

ble compared to its gas-phase deposition (e.g. Zalakeviciute

et al., 2012). Because nitric acid releases NO

so slowly, it is

a component of NO

The deposition velocity (

dep

) of HNO

in the gas phase

was measured during SOAS by Nguyen et al. (2015). Around

midday, when the boundary layer is well developed, the de-

position velocity can be combined with the boundary layer

height (BLH) to calculate a loss rate of using Eq. (2).

L

(

HNO

)

dep

BLH

·[

HNO

]

(2)

Using this method, we find the lifetime of nitric acid in the

gas and particle phase to be 6 h at noon. In the late afternoon,

changing boundary layer dynamics make this calculation of

the loss rate inaccurate (e.g. Papale et al., 2006; Millet et al.,

2015). The loss of nitric acid in the late afternoon was there-

fore calculated by fitting periods of consistent decay between

15:00 and 19:00 with an exponential curve. By fitting only

the periods of consistent decay, we aim to select for periods

where the production of nitric acid is at a minimum and the

observed net decay of nitric acid is similar to its gross loss

rate. Because nitric acid reversibly partitions between the gas

and particle phases, the lifetime was calculated based on the

concentration of nitric acid in both phases. The lifetime cal-

culated using this method is 5

−

h, similar to the lifetime of

nitric acid calculated using Eq. (2) at noon.

By combining the loss rate of nitric acid with the rate of

change of its concentration, we can calculate an inferred pro-

duction rate of nitric acid (Fig. 4). This inferred production

rate for each hour is defined as the difference between the rate

of change in the concentration of nitric acid and the loss rate.

The rate of change was determined as the slope of a best-fit

line of the concentration of nitric acid vs. time for each hour.

Since the calculation of the inferred production rate con-

siders only the hour-to-hour change in nitric acid and not its

gross concentration, the inferred production rate is not af-

fected by distant nitric acid sources. We find very little (less

than 15 %) variation in the concentration of NO

with wind

direction and no correlation of the inferred production rate

around noon with sulfate (a power plant tracer) or benzene

(an urban tracer). As the transport time from these sources

to the CTR site is significantly greater than 1 h, this result

is not surprising. The changing boundary layer height could

significantly impact the inferred production rate of nitric acid

during the early morning, but it is likely unimportant at mid-

day.

Also shown in Fig. 4 is the rate of nitric acid production

from the reaction of OH

(Reaction R1), using the rate

constant measured by Mollner et al. (2010). The vertical bars

for the inferred rate represent the combined effects of the

uncertainty in both the fit of concentration vs. time and in

the calculated nitric acid lifetime, as well as the day-to-day

variations in the observations. The vertical bars shown for

the production of nitric acid from the OH

reaction in-

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Atmos. Chem. Phys., 16, 7623–7637, 2016