Airborne measurements of organosulfates over the continental U.S.

Airborne measurements of organosulfates

over the continental U.S.

Jin Liao

1,2

, Karl D. Froyd

1,2

, Daniel M. Murphy

, Frank N. Keutsch

3,4

,GeYu

, Paul O. Wennberg

5,6

Jason M. St. Clair

, John D. Crounse

, Armin Wisthaler

7,8

, Tomas Mikoviny

7,8

, Jose L. Jimenez

2,9

Pedro Campuzano-Jost

2,9

, Douglas A. Day

2,9

, Weiwei Hu

2,9

, Thomas B. Ryerson

, Ilana B. Pollack

1,2

Jeff Peischl

1,2

, Bruce E. Anderson

, Luke D. Ziemba

, Donald R. Blake

, Simone Meinardi

and Glenn Diskin

Chemical Sciences Division, Earth System Research Laboratory, NOAA, Boulder, Colorado, USA,

Cooperative Institute for

Research in Environmental Sciences, University of Colorado Boulder, Boulder, Colorado, USA,

Department of Chemistry,

University of Wisconsin-Madison, Madison, Wisconsin, USA,

Now at Department of Chemistry and Chemical Biology,

Harvard University, Cambridge, Massachusetts, USA,

Division of Geology & Planetary Sciences, Pasadena, California, USA,

Division of Engineering and Applied Science, Pasadena, California, USA,

Institut für Ionenphysik und Angewandte Physik,

Leopold-Franzens Universität Innsbruck, Innsbruck, Austria,

Now at Department of Chemistry, University of Olso, Oslo,

Norway,

Department of Chemistry and Biochemistry, University of Colorado Boulder, Boulder, Colorado, USA,

NASA

Langley Research Center, Hampton, Virginia, USA,

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

Abstract

Organosulfates are important secondary organic aerosol (SOA) components and good tracers for

aerosol heterogeneous reactions. However, the knowledge of their spatial distribution, formation conditions,

and environmental impact is limited. In this study, we report two organosulfates, an isoprene-derived isoprene

epoxydiols (IEPOX) (2,3-epoxy-2-methyl-1,4-butanediol) sulfate and a glycolic acid (GA) sulfate, measured

using the NOAA Particle Analysis Laser Mass Spectrometer (PALMS) on board the NASA DC8 aircraft over the

continental U.S. during the Deep Convective Clouds and Chemistry Experiment (DC3) and the Studies of

Emissions and Atmospheric Composition, Clouds, and Climate Coupling by Regional Surveys (SEAC4RS). During

these campaigns, IEPOX sulfate was estimated to account for 1.4% of submicron aerosol mass (or 2.2% of

organic aerosol mass) on average near the ground in the southeast U.S., with lower concentrations in the

western U.S. (0.2

–

0.4%) and at high altitudes (

<

0.2%). Compared to IEPOX sulfate, GA sulfate was more

uniformly distributed, accounting for about 0.5% aerosol mass on average, and may be more abundant globally.

A number of other organosulfates were detected; none were as abundant as these two. Ambient measurements

con

fi

rmed that IEPOX sulfate is formed from isoprene oxidation and is a tracer for isoprene SOA formation.

The organic precursors of GA sulfate may include glycolic acid and likely have both biogenic and anthropogenic

sources. Higher aerosol acidity as measured by PALMS and relative humidity tend to promote IEPOX sulfate

formation, and aerosol acidity largely drives in situ GA sulfate formation at high altitudes. This study suggests

that the formation of aerosol organosulfates depends not only on the appropriate organic precursors but also on

emissions of anthropogenic sulfur dioxide (SO

), which contributes to aerosol acidity.

1. Introduction

Atmospheric aerosols affect climate forcing by directly absorbing and scattering sunlight and acting as

cloud condensation nuclei (CCN) to initiate cloud formation [

Charlson et al

., 1992;

Scott et al

., 2014]. Aerosols

are also atmospheric pollutants because they are harmful to human health, especially the respiratory and

cardiovascular systems [

Pope et al

., 2002], and they decrease atmosphere visibility. Secondary organic aerosol

(SOA) accounts for a signi

fi

cant fraction of organic aerosol (OA) mass [

Murphy et al

., 2006;

Zhang et al

., 2007a]. Due

to our limited knowledge of SOA forma

tion, the modeled SOA mass often has discrepancies with observations

[

Heald et al

., 2005;

Volkamer et al

., 2006]. The formation mechanism, properties, and fates of SOA must be

characterized to evaluate their environmental impact [

Hallquist et al

., 2009].

Organosulfates are important secondary aerosol components. They are formed by reactions between

organic material and sulfate, which are the main chemical components of atmospheric aerosols [

Murphy

et al

., 2006;

Zhang et al

., 2007a]. Organosulfates have been detected in ambient aerosols [

Iinuma et al

., 2007;

Surratt et al

., 2007, 2008;

Gomez-Gonzalez et al

., 2008;

Froyd et al

., 2010;

Chan et al

., 2010;

Hatch et al

., 2011;

LIAO ET AL.

2990

PUBLICATION

Journal of Geophysical Research: Atmospheres

RESEARCH ARTICLE

10.1002/2014JD022378

Special Section:

Studies of Emissions and

Atmospheric Composition,

Clouds and Climate Coupling

by Regional Surveys, 2013

(SEAC4RS)

Key Points:

•

IEPOX sulfate is an isoprene SOA

tracer at acidic and low NO conditions

•

Glycolic acid sulfate may be more

abundant than IEPOX sulfate globally

•

impacts IEPOX sulfate by increasing

aerosol acidity and water uptake

Supporting Information:

•

Table S1

Correspondence to:

J. Liao,

jin.liao@noaa.gov

Citation:

Liao, J., et al. (2015), Airborne measure-

ments of organosulfates over the conti-

nental U.S.,

J. Geophys. Res. Atmos.

120

2990

–

3005, doi:10.1002/2014JD022378.

Received 1 AUG 2014

Accepted 26 FEB 2015

Accepted article online 28 FEB 2015

Published online 3 APR 2015

This is an open access article under the

terms of the Creative Commons

Attribution-NonCommercial-NoDerivs

License, which permits use and distri-

bution in any medium, provided the

original work is properly cited, the use is

non-commercial and no modi

fi

cations

or adaptations are made.

Olson et al

., 2011;

Zhang et al

., 2012;

Worton et al

., 2013;

Shalamzari et al

., 2013] and cloud water [

Pratt et al

2013] and are estimated to comprise up to 5

–

10% of OA mass over the continental U.S. [

Tolocka and Turpin

2012]. Organosulfates are thought to be good tracers for heterogeneous aerosol phase chemistry and SOA

formation since the known formation mechanisms involve reactive uptake of gas phase organic species onto

aerosol [

Surratt et al

., 2010;

McNeill et al

., 2012;

Zhang et al

., 2012]. Organosulfates are polar, hydrophilic, and

low-volatility SOA compounds, which may help nanoparticle growth [

Smith et al

., 2008;

Yli-Juuti et al

., 2013] and

increase their potential to become CCN. Therefore, investigation of organosulfate abundance, distributions,

sources, formation mechanisms, and fates is an important step to improve our knowledge of SOA.

One of the most well studied and abundant aerosol organosulfates is isoprene epoxydiols (IEPOX) (2,3-epoxy-

2-methyl-1,4-butanediol) sulfate (C

) (chemical structure shown in Figure 1a). IEPOX sulfate is one of

the most abundant individual organic molecules in aerosols (e.g., 1

–

2% of carbon mass in the southeast U.S.)

[

Chan et al

., 2010;

Lin et al

., 2013]. IEPOX sulfate was discovered to be a key intermediate in SOA formation

from isoprene, the largest nonmethane carbon source [

Surratt et al

., 2010;

Paulot et al

., 2009b]. Uptake of IEPOX

by acid-catalyzed ring opening of epoxydiol, followed by addition of inorganic sulfate, is known to form

IEPOX sulfate [

Darer et al

., 2011;

Eddingsaas et al

., 2010;

Surratt et al

., 2010;

Paulot et al

., 2009b]. Organosulfates

have also been proposed to form by reactive uptake of unsaturated compounds into the particle phase

and reaction with the sulfate radical [

Rudzinski et al

., 2009;

Nozière et al

., 2010;

Schindelka et al

., 2013]. The

vertical pro

fi

les of IEPOX sulfate were measured by the Particle Analysis by Laser Mass Spectrometry (PALMS)

instrument during previous airborne campaigns [

Froyd et al

., 2010]. In those studies IEPOX sulfate accounted

for about 2

–

3% of aerosol mass in the southeast U.S. and higher fractions in the tropical-free troposphere.

IEPOX sulfate temporal pro

fi

les measured by aerosol time-of-

fl

ight mass spectrometry (ATOFMS) in Atlanta

[

Hatch et al

., 2011] have also been reported. The IEPOX sulfate mass loading over the southeast U.S. has been

estimated to be 10

–

60ng/m

by a model study [

Pye et al

., 2013].

A few other organosulfates have been quanti

fi

ed. For example, an organosulfate derived from 2-methyl-3

buten-2-ol, an important biogenic volatile organic emitted from pine trees, was measured to account for

0.25% of OA mass in the Manitou Forest Observatory in Colorado [

Zhang et al

., 2012]. The aromatic

organosulfate benzyl sulfate, thought to form from anthropogenic emissions, was measured in Lahore,

Pakistan, and found to account for a very small mass fraction of OA (2 ppm) but might be a useful tracer

[

Kundu et al

., 2013].

Figure 1.

(a) Mass spectra of an ambient particle containing GA sulfate signal, (b) a laboratory particle generated from GA

sulfate standard, and (c) a particle generated from a mixed solution of glycolic acid, NH

HSO

, and H

Journal of Geophysical Research: Atmospheres

10.1002/2014JD022378

LIAO ET AL.

2991

Glycolic acid (GA) sulfate (C

) (chemical structure shown in Figure 1a) is another potentially important

organosulfate. Like IEPOX sulfate, the proposed formation mechanism of GA sulfate is the reaction of a

gas phase organic precursor with acidic aerosol sulfate, although the formation mechanism remains to be

proven. GA sulfate has been detected in ambient aerosols [

Olson et al

., 2011;

Surratt et al

., 2008] and in SOA

generated by isoprene oxidation in chamber studies [

Surratt et al

., 2008]. GA sulfate can also form from the

particle phase reaction of methyl vinyl ketone, a

fi

rst generation oxidation product of isoprene, with the

sulfate radical through a pathway similar to that proposed for hydroxyacetone sulfate formation [

Schindelka

et al

., 2013], although it is unclear how important this pathway is under ambient conditions. GA sulfate

formed from isoprene oxidation and other sources might account for signi

fi

cant aerosol mass. Ambient GA

sulfate was measured to be 1.9

–

11.3 ng/m

from

fi

lters collected on the ground in California, Ohio, and Mexico

[

Olson et al

., 2011]. C

was also observed as a gas phase ion and may be the only gas-phase observation

of an organosulfate [

Ehn et al

., 2010]. However, the spatial distribution of GA sulfate measurements is limited.

The sources, formation mechanisms, and atmosph

eric importance of these species are unclear.

This study reports measurements of IEPOX sulfate C

and the less studied GA sulfate C

Measurements were made with the NOAA PALMS instrument on board the NASA DC-8 airplane over the

continental U.S. during the Deep Convective Clouds and Chemistry Experiment (DC3) in May and June 2012

and the Studies of Emissions and Atmospheric Composition, Clouds, and Climate Coupling by Regional

Surveys (SEAC4RS) in August and September 2013. The potential sources and formation conditions of the two

organosulfates in the atmosphere are also discussed.

2. Methods

The NOAA PALMS instrument measures the chemical composition of individual particles using a laser

evaporation and ionization technique [

Murphy and Thomson

, 1995]. Particles with a diameter larger than

about 200 nm can be sized by two continuous laser beams (405 nm) and ionized by a pulsed excimer laser

(193 nm). The ions are extracted and detected by a time of

fl

ight mass spectrometer [

Murphy

, 2007]. The

PALMS instrument detects

>

500 with unit mass resolution. However, fragmentation can limit the

detection of large molecules. The PALMS instrument is able to detect inorganic (e.g., potassium, sulfate, and

metals) and organic compounds in single particles. Most of the organic compounds are fragmented due

to excimer laser ionization. However,

Froyd et al

. [2010] found that IEPOX sulfate is not fully fragmented and

can be detected as C

by PALMS at

215. PALMS measured the aerosol chemical composition on

board the NASA DC8 airplane during DC3 and SEAC4RS over the continental U.S. from about 400 m to 12 km

in altitude. The organosulfate aerosols were sampled by a forward facing solid diffuser inlet based on the

University of Hawaii design [

McNaughton et al

., 2007] for 90% and 84% of the data in DC3 and SEAC4RS and a

High Cross-

fl

ow Aerosol Sampler designed by Dr. Suresh Dhaniyala at Clarkson University as an experimental

inlet for the remaining data. The solid diffuser inlet sampled up to 2.8

μ

matunitef

fi

ciency and with a

0.5 s inlet residence time. The experimental inlet sampled particles up to at least 1

μ

m at unit ef

fi

ciency and

with a 1.4 s inlet residence time. Because almost all organosulfate signals (

>

95%) were detected in submicron

particles, both inlets have the same unit sampling ef

fi

ciency for organosulfates. The sampling tubing

temperature was about 20°C. Additional measurements are reported for

fl

ight campaigns over the Alaskan

Arctic (ARCPAC, Aerosol, Radiation, and Cloud Processes affecting Arctic Climate) in 2008 and over Central

America and nearby oceans: Pre-AVE (Pre-Aura Validation Experiment ) in 2004, CR-AVE (Costa Rica Aura

Validation Experiment) in 2006, and TC4 (Tropical Composition, Cloud, and Climate Coupling) in 2007.

A peak at

155 was the most intense organosulfate signal on average detected by PALMS during DC3

and SEAC4RS. The ratio of the isotopic signals at

157 to

155 of 0.04 is consistent with the presence

of sulfur in the molecule (Figure 1a). An accurate mass analysis [

Froyd et al

., 2010] of

155 for the DC3

and SEAC4RS data indicates that the empirical formula is C

. The structure of ambient C

was investigated by

Galloway et al

. [2009] who suggested that ambient C

is likely GA sulfate

since the mass and elution time of ambient C

is the same as that of a glycolic acid sulfate

standard C

. Since PALMS cannot distinguish betwe

en isomeric compounds, we adopt the

structural identi

fi

cation by

Galloway et al

. [2009] for this work. No signal at

155 was observed for

many spectra that contain IEPOX sulfate at

215 in the

fi

eld nor when the larger organosulfate BEPOX

Journal of Geophysical Research: Atmospheres

10.1002/2014JD022378

LIAO ET AL.

2992

(2,3-epoxy-1,4-butanediol, C

)

sulfate was sampled in the lab [

Froyd

et al

., 2010], suggesting that C

is not formed from fragmentation of a

larger organosulfate.

IEPOX sulfate was quanti

fi

ed using

the calibration described by

Froyd

et al

. [2010]. A synthesized GA sulfate

standard [

Olson et al

., 2011] was used

to calibrate PALMS signals at

155.

The mass spectrum of the GA sulfate

standard is shown in Figure 1b. Due to

the high variability of laser vaporization

and ionization, quantitative PALMS

measurements are challenging. The

ion signal intensity depends on the

excimer laser power and the ablation

position, both of which vary from

particle to particle. Therefore, the

average of the relative signal intensity

155, which is the signal intensity at

155 normalized by the total ion intensity, was used to

quantify the compounds instead of the individual absolute signal intensity. Aerosols generated from 13

solutions containing different known mass fractions of GA sulfate in the range from 0.3% to 2% were

delivered to the PALMS and the corresponding average relative signals of

155 at medium (1.7 mJ

or 1.7 × 10

W/cm

) and low (0.84 mJ or 8.4 × 10

W/cm

) excimer laser power were recorded. The chemical

compounds besides GA sulfate in the solutions were sulfate, ammonium, and an oxidized organic

(succinic acid). They were used to mimic the general composition of the atmospheric particles in which

organosulfates were detected. The mass ratios of organic to inorganic constituents were about 0.1, and the

detailed chemical compositions of the solutions are provided in Table S1 in the supporting information. The

ionization ef

fi

ciencies of PALMS do not depend strongly on the relative concentrations of the organic to

inorganic constituents as long as metal cations are absent. The calibration was also extrapolated to high

excimer laser power (2.8 mJ or 2.8 × 10

W/cm

). For the simulated aerosol the uncertainty in organosulfate

ionization ef

fi

ciency due to the aerosol matrix was smaller than the variation of laser power. Figure 2 shows

the calibration plot. The relative signal intensity at

155 as a function of aerosol GA sulfate mass fraction

is used to determine the sensitivity to GA sulfate and convert the PALMS

155 signal to GA sulfate

aerosol mass fraction. Due to the variation of laser power pulses and the inhomogeneous chemical

composition of individual aerosols, the calibration has signi

fi

cant uncertainty denoted as the error bars.

PALMS is about 3 times more sensitive to GA sulfate than IEPOX sulfate. Increased laser power reduces the

organosulfate parent peaks because of increased fragmentation. The effect is less pronounced for GA

sulfate than for IEPOX sulfate. This may be due to less fragmentation of the smaller GA sulfate compound.

Because different laser powers are optimal for different aerosol species, PALMS was operated alternately at

medium and low laser power during DC3 and at high and low laser power during SEAC4RS.

Other organosulfate signals observed during DC3 and SEAC4RS were much smaller. For example, an

organosulfate signal at

169 may be lactic acid sulfate [

Olson et al

., 2011]. A signal at

169 was

fi

rst observed by electrospray ionization mass spectrometry (ESI-MS) in isoprene oxidation studies with

methylglyoxal as the proposed precursor [

Surratt et al

., 2007]. The signal at

169 was 1.5% of the signal

155 on average by PALMS. The vertical distribution of the signals at

169 was similar to that

of GA sulfate. Other potential organosulfate species at

139, 141, 153, 183, and 199 had signal intensities

of about 9%, 7%, 0.3%, 0.1%, and 0.1% of GA sulfate signals, respectively. Except for

141, the above

organosulfate ions were derived mostly from isoprene oxidation chemistry [

Surratt et al

., 2007]. We estimated

they constitute a very small aerosol mass assuming that their sensitivities are between IEPOX sulfate and GA

sulfate. Such small peaks were often below the detection limit so we make no further attempt to quantify them

in this study. However, these organosulfates may account for a signi

fi

cant mass fraction if their sensitivities are

Figure 2.

Average PALMS relative signals at

155 (dots) for different GA

sulfate mass fractions at low and medium PALMS laser power. The

fi

ts at

low power (black line) and medium power (red line) and the extrapolated

fi

t at high power (blue line) are used to convert PALMS relative signals at

155 in ambient air to GA sulfate mass fractions.

Journal of Geophysical Research: Atmospheres

10.1002/2014JD022378

LIAO ET AL.

2993

orders of magnitude lower. Of

fl

ine analysis of organosulfates by liquid chromatography and ESI-MS and

availability of additional organosulfate standards are required to address this important question.

Aerosol acidity or neutralization can be estimated from the PALMS signal at

195, which corresponds to a

sulfuric acid cluster HSO

·H

. Acidic particles form this peak as well as HSO

, whereas neutralized

particles favor formation of only HSO

ions. Particles in the stratosphere and in volcanic plumes are

known to be more acidic and consequently have higher HSO

·H

signals compared to tropospheric

particles [

Murphy et al

., 2007;

Carn et al

., 2011]. The [HSO

·H

]/([HSO

] + [HSO

·H

]) signals

measured in the

fi

eld generally increased as particle-into-liquid sampler [NH

] to [SO

] ratios decreased

[

Froyd et al

., 2009, 2010]. Lab experiments also found that the [HSO

·H

]/[HSO

] signals from a

similar Aerosol Time-of-Flight Mass spectrometer (ATOFMS) increased with decreased aerosol neutralization

[

Yao et al

., 2011]. This acidity indicator signal also varies with laser power. Due to the different laser powers

used during these two campaigns, the acidity response ion HSO

·H

was biased high (more acidic)

during DC3 and biased low (less acidic) during SEAC4RS. The aerosol pH values were also estimated from the

thermodynamic model extended aerosol inorganic model (E-AIM) [

Clegg et al

., 1998;

Wexler and Clegg

, 2002;

Zhang et al

., 2007b] for DC3 and SEAC4RS.

Hennigan et al

. [2014] and

Guo et al

. [2014] evaluated the aerosol

acidity estimated from thermodynamic models ISORROPIA and E-AIM with and without gas phase inputs.

Submicron aerosol sulfate (SO

), ammonium (NH

), and nitrate (NO

) from a High-Resolution Aerosol

Mass Spectrometer (HR-AMS), gas phase nitric acid (HNO

) from the California Institute of Technology

(Caltech) chemical ionization mass spectrometer (CIMS), ambient relative humidity, temperature, and

pressure were used as inputs. The impact of organic compounds on pH values was not considered.

The predicted gas phase HNO

levels agreed well with the measurements. The predicted pH values

generally anticorrelated with the PALMS acidity signal, as expected (Figures 3c, 3d, 3g, and 3h). Since the

pH predictions from the thermodynamic model probably become less accurate as sulfate approaches

complete neutralization, and since gas phase NH

measurements were not available to constrain the model,

the PALMS acidity signal was used as the main aerosol acidity indicator.

To investigate the potential formation mechanisms and precursors of GA sulfate, laboratory experiments

were carried out to synthesize the organosulfate at

155 from both glyoxal and glycolic acid. Aerosols

were nebulized from a (0.4M: 0.2M: 0.2M) mixture of glyoxal, ammonium bisulfate (NH

HSO

), and sulfuric

acid (H

) or a (0.2M: 0.2M: 0

–

0.2M) mixture of glycolic acid, NH

HSO

, and H

. Some solutions were

exposed to UV light (254 nm) from a standard mercury pen lamp to initiate photochemistry for a range of

time periods (Table 1). A differential mobility analyzer was used to select the sizes representative of ambient

particles containing GA sulfate. The size-selected aerosols were sampled by PALMS. Table 1 shows the lab

experiment cases, number fraction of acidic aerosols containing

155, relative

155 signals in acidic

aerosols, and size distribution of the aerosols detected by PALMS. Figure 1c shows the mass spectrum of a

particle generated from a solution of glycolic acid, NH

HSO

, and H

Other trace gas and aerosol concentrations were measured by a suite of instruments on board the NASA

DC8 airplane during DC3 and SEAC4RS. Isoprene was measured by both proton transfer reaction mass

spectrometry (PTR-MS, University of Innsbruck) [

Hansel et al

., 1999] and by whole air sampling (WAS;

University of California Irvine) followed by laboratory analysis using gas chromatography [

Colman et al

2001]. PTR-MS measurements have a higher spatial and temporal resolution compared to WAS data.

Isoprene measurements by PTR-MS do, however, suffer from furan interference in biomass burning (BB)

plumes [

Christian et al

., 2004]. In BB-impacted air masses (identi

fi

ed by elevated acetonitrile levels), WAS

data were used for the analysis. Two gas phase isoprene oxidation products, hydroxy hydroperoxides

(ISOPOOH) and dihydroxyepoxides (IEPOX), were measured by the Caltech CIMS [

Paulot et al

., 2009a;

Clair et al

., 2010]. NO and O

were measured by a NOAA chemiluminescence instrument [

Ryerson et al

2001]. The aerosol mass of organic material, sulfate, and ammonium was measured by a University of

Colorado Aerodyne HR-AMS [

DeCarlo et al

., 2006] and is reported under standard temperature and pressure

conditions (1 atm and 273 K). Submicron total aerosol mass was derived from the total submicron aerosol

volume measured by the Ultra-High Sensitivity Aerosol Spectrometer and the laser aerosol spectrometer

aerosol sizing instruments operated by the NASA LARGE group [

Ziemba et al

., 2013] with the assumption of

an average aerosol density of 1.4 g/cm

. The submicron total aerosol mass was used to multiply the PALMS

organosulfate mass fraction products to get to the absolute organosulfate mass loading.

Journal of Geophysical Research: Atmospheres

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LIAO ET AL.

2994

3. Results and Discussion

3.1. Vertical Distribution of IEPOX Sulfate and GA Sulfate Over the Continental U.S.

The average vertical aerosol mass fractions of IEPOX sulfate and GA sulfate in the eastern U.S. and western

U.S. during DC3 and SEAC4RS are shown in Figures 3a and 3e, respectively. Flights during the DC3 campaign

targeted continental convective in

fl

ow and high altitude out

fl

ow. Most of the SEAC4RS

fl

ights targeted a

wide range of tropospheric environments. SEAC4RS campaign data were excluded for marine

fl

ights and

for the

fl

ight on 2 September that sampled convective out

fl

ow, where the vertical pro

fi

le was similar to DC3

with enhanced IEPOX sulfate at high altitudes. IEPOX sulfate is estimated to account for 1.3% and 0.2% of

submicron aerosol mass on average near the ground in the eastern and western U.S., respectively, during

DC3, and 1.4% and 0.2% during SEAC4RS. Near the ground is de

fi

ned as from the lowest GPS altitudes

sampled by DC-8 (about 400 m) to 1500 m since the typical daytime boundary layer height was about 1

–

2 km.

The IEPOX sulfate mass fraction decreased in the upper troposphere and lower stratosphere (to

<

0.2% of

submicron aerosol mass). GA sulfate accounted for 0.6% and 0.3% of submicron aerosol mass near the

ground and increased to 0.7% and 0.6% of submicron aerosol mass in the upper troposphere in the eastern

and western U.S., respectively, during DC3, compared to 0.4% and 0.6% near the ground and 0.6% and 0.4%

Figure 3.

Average vertical pro

fi

les of measured (a) IEPOX sulfate (red) and GA sulfate (black) in aerosol mass (%), (b) isoprene (green), gas phase IEPOX (blue),

(c) and PALMS acidity signal (orange), and (d) estimated pH (purple) in the eastern (solid) and western (dashed) U.S. during DC3 and (e

–

h) SEAC4RS. Longitude 96°W

is used to separate eastern U.S. data from western U.S. data. The error bars in the estimated pH panels represent ±1 standard deviation of the 5 min avera

ge data. The data

points in each bin for eastern and western U.S. are on the right and left of the error bars, respectively.

Journal of Geophysical Research: Atmospheres

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LIAO ET AL.

2995

in the upper troposphere in the eastern and

western U.S. during SEAC4RS. Total IEPOX sulfate

mass accounted for 2.2% and 0.7% of total organic

mass measured by AMS near the ground in the

eastern and western U.S. and for 5.0% and 1.9% of

total sulfate mass measured by AMS on average

in these two campaigns. Total GA sulfate mass

accounted for 0.9% and 0.7% of total organic mass

measured by AMS near the ground in the eastern

and western U.S. and for 2.0% and 2.5% of total

sulfate mass measured by AMS on average in these

two campaigns. Most of the eastern U.S.

fl

ights

were over the southeast U.S. There were several

low-altitude

fl

ight segments over the

“

isoprene

volcano

”

region in the Ozark Mountains of Arkansas

and Missouri during SEAC4RS. Although isoprene

observed in the

“

isoprene volcano

”

region was

signi

fi

cantly enhanced, there was no enhancement

of IEPOX sulfate observed compared to average

levels in the eastern U.S.

The IEPOX sulfate mass fraction was highest near

the ground in the eastern U.S. where the isoprene

emissions were most intense. Compared to IEPOX

sulfate, the vertical pro

fi

les of GA sulfate mass

fraction were nearly constant with altitude and

peaked in the upper troposphere, suggesting

that GA precursors are more widely distributed

and probably longer lived than the IEPOX-derived

sulfate. Accordingly, GA sulfate may account for

signi

fi

cant aerosol mass throughout the global

troposphere. The average IEPOX sulfate mass

fraction of 1.4% of total submicron aerosol mass

or of 2.2% of total OA mass in the southeast U.S.

from 400 m to 1500 m was close to the aircraft

measurements in 2004 by

Froyd et al

. [2010] (3%

of submicron aerosol mass at low altitude) and

the ground measurements by

Lin et al

. [2013]

–

2% of OA mass). The estimated mass loading

of GA sulfate of 20 ng/m

at low altitudes was

near the upper limit of GA sulfate detected from

fi

lter measurements collected from several

ground sites [

Olson et al

., 2011]. To our knowledge,

these are the

fi

rst-online and the

fi

rst-airborne

measurements of GA sulfate reported.

3.2. IEPOX Sulfate and Its Precursors

The IEPOX sulfate vertical pro

fi

les had a similar

pattern to those of isoprene and its gas phase

oxidation product IEPOX (Figures 3b and 3f).

Average isoprene mixing ratios were similar

during DC3 and SEAC4RS in the eastern U.S. The

gas phase IEPOX (and ISOPOOH) was signi

fi

cantly

lower during SEAC4RS, which was probably due

to higher NO concentration observed in the

Table 1.

Laboratory Experiments on GA Sulfate Formation and Precursors

Experiment Cases

Add H

Reaction Time

Before Making

Aerosols

Radiation

Acidic

Aerosols

Number Fraction

of Generated

Aerosols is Acidic

Number Fraction of Acid

Aerosols Containing

155

Relative Peak Area

155 in

Acidic Particles

Acid Particle Mode

Diameters

(

μ

A. Glyoxal + NH

HSO

<

1 h

yes

0.17

0.5

B. Glyoxal + NH

HSO

4 days

yes

0.47

0.01

2 × 10

0.6 (82%) and

1.8 (18%)

C. Glyoxal + NH

HSO

<

6 h

5 h

0.54

0.03

3 × 10

0.6 (80%) and

2.0 (20%)

D. Glyoxal + NH

HSO

6 days

5 h

yes

0.68

0.11

3.5 × 10

1 (23%) and

2.3 (78%)

E. Glycolic acid + NH

HSO

1 day

yes

0.90

0.28

1.5 × 10

0.6 (44%) and

2.0 (56%)

F. Glycolic acid + NH

HSO

yes

New (

<

2 h)

75 min

yes

0.69

0.53

6 × 10

0.6 (10%) and

2.0 (90%)

G. Glycolic acid + (NH

)

yes

New (

<

2 h)

40 min Less acidic pH = 3

0.1

0.02

8 × 10

distributed between

0.5 and 2.0

H. Glycolic acid + NH

HSO

yes

1 day

75 min

yes

0.91

0.41

3 × 10

1.6

The solutions were exposed to lab light in the daytime after they were made.

Journal of Geophysical Research: Atmospheres

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2996

southeast U.S. during SEAC4RS than DC3. Considering lower levels of IEPOX (523 pptv in DC3 versus 285pptv

in SEAC4RS on average) during SEAC4RS, a similar IEPOX sulfate mass fraction in both campaigns, and twice

higher submicron aerosol mass loading during SEAC4RS near the ground in the southeast U.S., production of

IEPOX sulfate from gas phase IEPOX seemed to be much more ef

fi

cient during SEAC4RS. The lower pH near

the ground estimated during SEAC4RS may contribute to the more ef

fi

cient IEPOX sulfate formation. Another

possibility is that reactive uptake of IEPOX formed other species such as 2-methyltetrols and C

alkene triols

[

Surratt et al

., 2010] in particle phase under the DC3 conditions. Further study is needed to explore the reasons

for the higher IEPOX sulfate production ef

fi

ciency during SEAC4RS.

The vertical pro

fi

les of IEPOX sulfate and its precursors may provide clues about the lifetime of IEPOX sulfate,

which has not been investigated. Generally, isoprene, gas phase IEPOX, and aerosol phase IEPOX sulfate

dropped to insigni

fi

cant levels above 3.5km, 4 km, and 8 km, respectively. The chemical lifetime of isoprene is

about 1

–

3h depending on hydroxyl radicals (OH) concentrations [

Paulot et al

., 2009a]. The lifetime of gas phase

IEPOX due to OH oxidation is about 3

–

28 h [

Jacobs et al

., 2013;

Bates et al

., 2014], and loss to aerosol uptake

varies from about 1 h to days depending on the aerosol acidity [

Surratt et al

., 2010;

Gaston et al

., 2014]. The

IEPOX sulfate vertical pro

fi

le suggests that the lifetime of IEPOX sulfate is longer than those of isoprene and gas

phase IEPOX. Higher IEPOX sulfate mass fractions in the upper troposphere observed during DC3 compared

to SEAC4RS may be due to shortened vertical transport time of either IEPOX sulfate or isoprene from convection

because DC3

fl

ights were designed to target convection, and SEAC4RS mostly sampled nonconvectively

active air masses. The observed vertical pro

fi

les of isoprene, IEPOX, and IEPOX sulfate and the relevant HO

and NO levels may help modelers to estimate the IEPOX reactive uptake rates under ambient conditions that

can be compared with rates measured in the laboratory at different conditions (e.g., aerosol acidity).

Two case studies of IEPOX sulfate and isoprene measurements are shown in Figure 4. The case study on the

11 June 2012

fl

ight to the southeast U.S. during DC3 (Figures 4a and 4b) further con

fi

rms that IEPOX sulfate

is formed from isoprene. IEPOX sulfate mass fraction generally tracks the concentrations of isoprene and

gas phase IEPOX. The consistent low levels of isoprene (

<

200pptv), IEPOX (

<

200pptv), and IEPOX sulfate

(

<

0.3% mass fraction) at low altitudes near 10:25 P.M. UTC are due to

fl

ying over the low vegetation and low

isoprene emission area between Arkansas and Mississippi (Figure 4a). This indicates that isoprene is required

in IEPOX sulfate formation and is consistent with the IEPOX sulfate formation from oxidation of isoprene

as proposed by

Surratt et al

. [2010] and

Paulot et al

. [2009a]. This also indicates that IEPOX sulfate formation can

be isoprene-limited in low-leaf areas in the southeast U.S. Meanwhile, the levels of IEPOX sulfate generally

followed the OA mass concentrations. This may indicate that SOA mass formed from biogenic VOCs oxidation

contributed substantially to OA mass in the southeast U.S. and that IEPOX sulfate can be used as a tracer for SOA

formed from isoprene oxidation. Figures 4c and 4d demonstrate a case study near Northern California and

southern Oregon on 6 August 2013 where there were signi

fi

cant isoprene and gas phase IEPOX concentrations,

but no IEPOX sulfate was detected. In the enhanced isoprene periods, the aerosols were mostly neutralized, and

the airplane was sampling biomass burning plumes. A similar case (not shown) with high isoprene and no

detectable IEPOX sulfate was found on the 26 August 2013

fl

ight. In general, little IEPOX sulfate was observed

in biomass burning plumes because the aerosols were neutralized. This indicates that either IEPOX does not

effectively react with neutralized aerosols or the reactive uptake forms other SOA products (e.g., 2-methyltetrols

and C

alkene triols) not IEPOX sulfate [

Surratt et al

., 2010].

In addition to the appropriate organic and sulfate precursors, the formation of IEPOX sulfate from isoprene

is modulated by sulfate aerosol acidity and gas phase NO levels. Figures 5a and 5b show IEPOX sulfate mass

fraction versus isoprene mixing ratios when the aerosols were acidic (PALMS acidity signal

>

0.002) (red)

and near neutralized (PALMS acidity signal

<

0.002) (black). Isoprene concentrations in Figure 5 are from

PTR-MS with a detection limit of 6 pptv (2

σ

for 5 min data) and were

fi

ltered for acetonitrile less than

200 pptv to exclude biomass burning plumes. More IEPOX sulfate was formed on average at the same

isoprene levels when aerosols were acidic compared to near neutralized. The same data are plotted for low

(

<

100 pptv) and high NO (

>

200 pptv) levels in Figures 5c and 5d. Generally, higher IEPOX sulfate mass

fractions were observed at low NO conditions for both

fi

eld campaigns, which is consistent with ef

fi

cient

formation of gas phase IEPOX from isoprene oxidation under low NO conditions. The signi

fi

cant IEPOX

sulfate present at high NO conditions (even

>

500 pptv) during SEAC4RS may be due to transport or

formation of IEPOX from isoprene hydroxynitrate oxidation [

Jacobs et al

., 2014], which probably plays a

small role. Aerosol SO

concentrations are known to be important for IEPOX sulfate formation, and NH

Journal of Geophysical Research: Atmospheres

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2997

concentrations may also be important [

Nguyen et al

., 2014]. The average IEPOX sulfate mass fraction

increased with aerosol SO

or NH

at low levels (SO

<

2 or NH4 +

<

μ

g/m

) but not at higher mass

loadings. This indicates that the aerosol SO

and NH

levels are important but may not be limiting

factors of IEPOX sulfate formation in most ambient conditions. It is worth noting that almost all IEPOX

sulfate and GA sulfate signals were observed in particles classi

fi

ed as sulfate-organic mixtures.

3.3. Aerosol Acidity

The PALMS acidity signal has undergone previous validation as a qualitative indicator of sulfate acidity

[

Murphy et al

., 2007;

Froyd et al

., 2009, 2010;

Carn et al

., 2011;

Yao et al

., 2011]. Our data show that under

some conditions aerosol acidity as measured by PALMS is important in both ambient IEPOX sulfate and GA

sulfate formation. At low altitudes (

<

1000 m) in the eastern U.S. where isoprene is more abundant, the

IEPOX sulfate and GA sulfate signals were well correlated with PALMS aerosol acidity signal [HSO

·H

([HSO

] + [HSO

·H

]) during DC3 and SEAC4RS when IEPOX sulfate and GA sulfate signals were

above their respective detection limits (see Figure 6). This correlation indicates that higher acidity tends to

promoteformationofIEPOXsulfateandGAsulfatenear thegroundwhentheorganicprecursorsareabundant.

This provides

fi

eld evidence for an important role of aerosol acidity in ambient IEPOX sulfate formation. The

importance of acidity agrees with both the acid-catalyzed epoxydiol ring opening formation mechanism

[

Surratt et al

., 2010] and the sulfate radical initiated organosulfate formation because ef

fi

cient formation of

sulfate radicals also requires acidity [

Schindelka et al

., 2013]. At comparable isoprene concentrations, higher

levels of IEPOX sulfate were generally observed in particles with higher acidity (see Figures 5a and 5b),

Figure 4.

(a) A low-altitude

fl

ight leg on 11 June 2012 over the southeast U.S. during DC3 color coded and sized with airborne PTR-MS isoprene measurements plotted

over an isoprene emission inventory BEIS3.13 map [

Pierce et al

., 1998]. (b) The corresponding time series of

fl

ight altitude (black), isoprene (light green), IEPOX (blue),

IEPOX sulfate (red), organic aerosol mass (dark green), and PALMS acidity signal (orange). (c) A low-altitude

fl

ight leg on 6 August 2013 over North California and south

Oregon during SEAC4RS color coded and sized with isoprene measurements by airborne whole air sampler, and (d) the corresponding time series plot.

Journal of Geophysical Research: Atmospheres

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2998