Source apportionment of fine particulate matter in Houston, Texas: insights to secondary organic aerosols
Authors: Al-Naiema Ibrahim M. , Hettiyadura Anusha P. S. , Wallace Henry W. , Sanchez Nancy P. , Madler Carter J. , Cevik Basak Karakurt , Bui Alexander A. T. , Kettler Josh , Griffin Robert J. , Stone Elizabeth A. 
Published: October 2018

Abstract. Online and offline measurements of ambient particulate matter (PM) near the
urban and industrial Houston Ship Channel in Houston, Texas, USA, during May
2015 were utilized to characterize its chemical composition and to evaluate
the relative contributions of primary, secondary, biogenic, and anthropogenic
sources. Aerosol mass spectrometry (AMS) on nonrefractory PM1 (PM ≤
1 µm) indicated major contributions from sulfate (averaging
50 % by mass), organic aerosol (OA, 40 %), and ammonium (14 %).
Positive matrix factorization (PMF) of AMS data categorized OA on average as
22 % hydrocarbon-like organic aerosol (HOA), 29 % cooking-influenced
less-oxidized oxygenated organic aerosol (CI-LO-OOA), and 48 %
more-oxidized oxygenated organic aerosol (MO-OOA), with the latter two
sources indicative of secondary organic aerosol (SOA). Chemical analysis of
PM2.5 (PM ≤ 2.5 µm) filter samples agreed that organic
matter (35 %) and sulfate (21 %) were the most abundant components.
Organic speciation of PM2.5 organic carbon (OC) focused on molecular
markers of primary sources and SOA tracers derived from biogenic and
anthropogenic volatile organic compounds (VOCs). The sources of PM2.5 OC
were estimated using molecular marker-based positive matric factorization
(MM-PMF) and chemical mass balance (CMB) models. MM-PMF resolved nine factors
that were identified as diesel engines (11.5 %), gasoline engines
(24.3 %), nontailpipe vehicle emissions (11.1 %), ship emissions
(2.2 %), cooking (1.0 %), biomass burning (BB, 10.6 %), isoprene
SOA (11.0 %), high-NOx anthropogenic SOA (6.6 %),
and low-NOx anthropogenic SOA (21.7 %). Using available
source profiles, CMB apportioned 41 % of OC to primary fossil sources
(gasoline engines, diesel engines, and ship emissions), 5 % to BB,
15 % to SOA (including 7.4 % biogenic and 7.6 % anthropogenic),
and 39 % to other sources that were not included in the model and are
expected to be secondary. This study presents the first application of in situ AMS-PMF, MM-PMF, and
CMB for OC source apportionment and the integration of these methods to
evaluate the relative roles of biogenic, anthropogenic, and BB-SOA. The three
source apportionment models agreed that ∼ 50 % of OC is associated
with primary emissions from fossil fuel use, particularly motor vehicles.
Differences among the models reflect their ability to resolve sources based
upon the input chemical measurements, with molecular marker-based methods
providing greater source specificity and resolution for minor sources. By
combining results from MM-PMF and CMB, BB was estimated to contribute
11 % of OC, with 5 % primary emissions and 6 % BB-SOA. SOA was
dominantly anthropogenic (28 %) rather than biogenic (11 %) or
BB-derived. The three-model approach
demonstrates significant contributions of anthropogenic SOA to fine PM. More
broadly, the findings and methodologies presented herein can be used to
advance local and regional understanding of anthropogenic contributions to
SOA.