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 Karacurt , Bui Alexander A. T. , Kettler Josh , Griffin Robert J. , Stone Elizabeth A. 
Published: April 2018

Abstract. Online and offline measurements of fine 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 non-refractory PM1 (PM ≤ 1 µm) indicated major contributions from sulfate (averaging 50 %), 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 semi-volatile oxygenated organic aerosol (CI-SV-OOA), and 48 % low-volatility oxygenated organic aerosol (LV-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 (VOC). 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 9 factors that were identified as diesel engines (11.5 %), gasoline engines (24.3 %), non-tailpipe 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, anthopogenic, 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 % of primary emissions and 6 % BB-SOA. SOA was dominantly anthropogenic (28 %) compared to 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.