Regional differences in organic composition of submicron and single particles during INTEX-B 2006

Abstract

Organic functional group and elemental concentrations were measured with Fourier transform infrared (FTIR) spectroscopy and X-ray fluorescence (XRF) from an aircraft platform as part of the Intercontinental Chemical Transport Experiment – Phase B (INTEX-B) conducted over the Eastern Pacific and Western North America. Single particle spectra were obtained using scanning transmission X-ray microscopy-near edge X-ray absorption fine structure spectrometry (STXM-NEXAFS). Organic mass (OM) concentrations ranged from 1 to 7 μg m−3 and averaged 2.4–4.1 μg m−3. Alkane functional groups were the largest fraction of OM, averaging 1.9–2.1 μg m−3 or 50–76% of OM. Alcohol functional groups comprised the second largest fraction of OM (0.35–0.39 μg m−3, 9–14%). Organic and elemental concentrations are compared within and among geographical air mass regions: "Pacific" free troposphere, "Continental" free troposphere, "Seattle" metropolitan region, and the California "Central Valley". OM concentrations were highest and most variable in the Central Valley (3.5±2 μg m−3). Alcohol functional group concentrations were highest in the Continental and Central Valley and lowest in the Pacific and Seattle air masses. Oxygen-to-carbon ratios were relatively constant in the Central Valley but variable for the Continental air masses. Most elemental concentrations did not show large variations among or within air mass categories. Overall, the OM concentrations showed greater variability within air mass categories as compared to averages among them, suggesting sampled air mass regions included a variety of sources, processing, and losses of organic aerosol. Single particle spectra obtained by STXM-NEXAFS were classified into metaclasses associated with different sources and atmospheric processing. Particles with spectra indicative of secondary organic aerosol production and combustion sources were found at several locations and a range of altitudes. At lower altitudes, particles with spectra resembling soil dust and biomass burning fingerprints were commonly observed. Single particle spectra provided evidence that condensation and surface-limited oxidation contributed to particle growth.

Full text

Page 1

ACPD
9, 6657–6690, 2009
Organic aerosol
composition during
INTEX-B
D. A. Day et al.
Title Page
Abstract Introduction
ConclusionsReferences
Tables Figures
??
??
BackClose
Full Screen / Esc
Printer-friendly Version
Interactive Discussion
Atmos. Chem. Phys. Discuss., 9, 6657–6690, 2009
www.atmos-chem-phys-discuss.net/9/6657/2009/
© Author(s) 2009. This work is distributed under
the Creative Commons Attribution 3.0 License.
Atmospheric
Chemistry
and Physics
Discussions
This discussion paper is/has been under review for the journal Atmospheric Chemistry
and Physics (ACP). Please refer to the corresponding final paper in ACP if available.
Regional differences in organic
composition of submicron and single
particles during INTEX-B 2006
D. A. Day1, S. Takahama1, S. G. Gilardoni1,*, and L. M. Russell1
1Scripps Inst. of Oceanography, University of California, San Diego, La Jolla, California, USA
*now at: European Commission, Institute for Environment and Sustainability, Ispra (VA), Italy
Received: 5 February 2009 – Accepted: 11 February 2009 – Published: 10 March 2009
Correspondence to: L. M. Russell (lmrussell@ucsd.edu)
Published by Copernicus Publications on behalf of the European Geosciences Union.
6657

Page 2

ACPD
9, 6657–6690, 2009
Organic aerosol
composition during
INTEX-B
D. A. Day et al.
Title Page
AbstractIntroduction
ConclusionsReferences
TablesFigures
??
??
BackClose
Full Screen / Esc
Printer-friendly Version
Interactive Discussion
Abstract
Organic functional group and elemental concentrations were measured with Fourier
transform infrared (FTIR) spectroscopy and X-ray fluorescence (XRF) from an aircraft
platform as part of the Intercontinental Chemical Transport Experiment – Phase B
(INTEX-B) conducted over the Eastern Pacific and Western North America. Single par-
ticle spectra were obtained using scanning transmission X-ray microscopy-near edge
X-ray absorption fine structure spectrometry (STXM-NEXAFS). Organic mass (OM)
concentrations ranged from 1 to 7µgm−3and averaged 2.4–4.1µgm−3. Alkane func-
tional groups were the largest fraction of OM, averaging 1.9–2.1µgm−3or 50–76%
of OM. Alcohol functional groups comprised the second largest fraction of OM (0.35–
0.39µgm−3, 9–14%). Organic and elemental concentrations are compared within and
among geographical air mass regions: “Pacific” free troposphere, “Continental” free
troposphere, “Seattle” metropolitan region, and the California “Central Valley”. OM
concentrations were highest and most variable in the Central Valley (3.5±2µgm−3).
Alcohol functional group concentrations were highest in the Continental and Central
Valley and lowest in the Pacific and Seattle air masses. Oxygen-to-carbon ratios were
relatively constant in the Central Valley but variable for the Continental air masses.
Most elemental concentrations did not show large variations among or within air mass
categories. Overall, the OM concentrations showed greater variability within air mass
categories as compared to averages among them, suggesting sampled air mass re-
gions included a variety of sources, processing, and losses of organic aerosol. Single
particle spectra obtained by STXM-NEXAFS were classified into metaclasses associ-
ated with different sources and atmospheric processing. Particles with spectra indica-
tive of secondary organic aerosol production and combustion sources were found at
several locations and a range of altitudes. At lower altitudes, particles with spectra
resembling soil dust and biomass burning fingerprints were commonly observed. Sin-
gle particle spectra provided evidence that condensation and surface-limited oxidation
contributed to particle growth.
5
10
15
20
25
6658

Page 3

ACPD
9, 6657–6690, 2009
Organic aerosol
composition during
INTEX-B
D. A. Day et al.
Title Page
AbstractIntroduction
Conclusions References
TablesFigures
??
??
Back Close
Full Screen / Esc
Printer-friendly Version
Interactive Discussion
1 Introduction
The organic fraction of submicron particles has been shown to be significant in nearly
every region of the troposphere: urban (Zhang et al., 2005), rural (Alfarra et al., 2004),
marine (Middlebrook et al., 1998), and the free troposphere (Novakov et al., 1997).
Organic particles can enter the atmosphere directly though anthropogenic processes
such as fossil fuel combustion and biomass burning or natural pathways such as wind-
driven dust and soil suspension. Despite the recognition of the ubiquity of organic
aerosol in the atmosphere, the organic composition is largely uncharacterized, in part
due to the large number of chemicals involved (Lewis et al., 2000) and the analytical
challenges associated with this heterogeneity (Turpin et al., 2001).
The free troposphere is an under-sampled part of the atmosphere, primarily due
to the limitations of conducting aircraft measurements. The Intercontinental Chemical
Transport Experiment – Phase B (INTEX-B) campaign was designed to investigate
one of these regions, the free troposphere above the Eastern Pacific and Western
North America. The primary goal of INTEX-B was to gain understanding of transport
and evolution of Asian pollution and how it impacts air quality and climate in Western
North America (Singh et al., 2009). There are very few measurements of organic
aerosol composition in the free troposphere over the Pacific between the Asian and
North American continents, and an even larger gap in the understanding of the export
of organic aerosol from Asia and trans-Pacific transport and processing. For example,
models have been shown to under-predict organic carbon in the free troposphere over
the Northwest Pacific by 1–2 orders of magnitude (Chung and Seinfeld, 2002; Heald
et al., 2005). Understanding these discrepancies is vital to quantifying the impact that
Asian pollution may have on North American air quality.
In this manuscript, we present measurements of organic functional groups and el-
emental composition of submicron particles collected as part of the INTEX-B study.
Since many of the air masses sampled in the free troposphere included a variety of
sources in addition to Asian pollution, we focus our discussion on characterizing dif-
5
10
15
20
25
6659

Page 4

ACPD
9, 6657–6690, 2009
Organic aerosol
composition during
INTEX-B
D. A. Day et al.
Title Page
AbstractIntroduction
Conclusions References
TablesFigures
??
??
BackClose
Full Screen / Esc
Printer-friendly Version
Interactive Discussion
ferences in organic composition between and within geographically-defined air mass
regions, namely the Northeastern Pacific free troposphere, the Western North Amer-
ican free troposphere, the Seattle metropolitan region, and the California Central Val-
ley. The organic functional group quantification was achieved using Fourier transform
infrared (FTIR) spectroscopy. Single particle measurements were also obtained using
scanning transmission X-ray microscopy – near edge X-ray absorption fine structure
spectrometry (STXM-NEXAFS), allowing classification of particles into metaclasses in-
dicative of general sources and atmospheric processing.
5
2Experimental methods
2.1Sample collection
10
Aerosol measurements discussed in this manuscript were collected during the Inter-
continental Chemical Transport Experiment – Phase B (INTEX-B) from the NCAR
C130 aircraft during April and May 2006. The aircraft was based at Paine Field in
Everett, Washington, just north of Seattle. Twelve flights were conducted, all based at
Paine field except the transit fights bracketing the study between Colorado and Wash-
ington. Flight tracks are available at: http://www-air.larc.nasa.gov/missions/intex-b/
images/thumbnailmaps c130 IMPEX.pdf. The majority of the flight time was in the free
troposphere above the Eastern Pacific or Western Washington, Oregon, and California
with the primary goal to intercept pollution layers transported from Asia. In addition to
profiles of the free troposphere, some flights included legs in and just above the bound-
ary layer over the Pacific Ocean, Seattle metropolitan area, and the California Central
Valley.
From a common inlet, 16.7 ambient Lmin−1was sampled, with the mass flow rate
varying inversely with altitude. Sample air passed through a PM1cyclone to select for
submicron particles. The sample flow was split, and aerosol particles were collected
on two sets of duplicate 37mm diameter Teflon filters. Particles were collected contin-
15
20
25
6660

Page 5

ACPD
9, 6657–6690, 2009
Organic aerosol
composition during
INTEX-B
D. A. Day et al.
Title Page
AbstractIntroduction
ConclusionsReferences
TablesFigures
??
??
BackClose
Full Screen / Esc
Printer-friendly Version
Interactive Discussion
uously on one filter set for the duration of each flight (as a backup). The other “short”
samples were collected on several different filters that were sampled for periods rang-
ing from ∼5min to ∼1 h, with resulting total flow volumes of 0.02 to 0.3m3. Typically
these “short” samples were in place for the duration of a level flight leg. Teflon filters
were stored in polystyrene Petri dishes, sealed with Teflon tape, and kept at −4◦C for
later analysis. A total of 76 “short” samples were collected during the 2 transit and
10 research flights of this study.
In addition to the submicron filter samples collected for quantitative mass-based
analysis, particles for single particle analysis were collected on Si3N4windows (Sil-
son Ltd.) that were mounted on a rotating impactor (Streaker, PIXE International, Inc.).
One standard Lmin−1(SLM) of air was drawn through Al tubing to the impactor. Sam-
ples were frozen after collection until time of analysis.
5
10
2.2Organic and Elemental Analysis
FTIR spectroscopy was used to quantify organic functional group concentrations for
the particles collected on each Teflon filter. Detailed descriptions of the method can
be found in Maria et al. (2002, 2003) and Gilardoni et al. (2007). Revisions to those
method descriptions include use of an automated algorithm for baselining, peak fit-
ting, and integration and additional calibrations of primary amine and carboxylic acid
functional groups (Liu et al., 2009; Russell et al., 2009). Functional groups quantified
include: saturated aliphatic (hereafter alkane) groups, unsaturated aliphatic (hereafter
alkene) groups, aromatic groups, alcohol groups (including phenols), carboxylic acid
groups, non-acidic carbonyl groups, primary amine groups, and organosulfate groups.
Due to a combination of generally low organic aerosol concentrations and the limited
sampling intervals dictated by the short flight legs, only 17 “short” samples contained
organic functional groups above the detection limits required for total organic mass
(OM) quantification. Alkene, aromatic, amine, and organosulfate groups were below
detection limit for all samples, with alkene and aromatic functional groups account-
ing for 4% and 3% of OM, respectively. The spectra were analyzed for evidence of
6661
15
20
25