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Discussion papers
https://doi.org/10.5194/acp-2018-1269
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/acp-2018-1269
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 12 Dec 2018

Research article | 12 Dec 2018

Review status
This discussion paper is a preprint. It is a manuscript under review for the journal Atmospheric Chemistry and Physics (ACP).

Seasonal differences in formation processes of oxidized organic aerosol near Houston, TX

Qili Dai1,2, Benjamin C. Schulze2,a, Xiaohui Bi1,2, Alexander A. T. Bui2, Fangzhou Guo2, Henry W. Wallance2,b, Nancy P. Sanchez2, James H. Flynn3, Barry L. Lefer3,c, Yinchang Feng1, and Robert J. Griffin2,4 Qili Dai et al.
  • 1State Environmental Protection Key Laboratory of Urban Ambient Air Particulate Matter Pollution Prevention and Control, College of Environmental Science and Engineering, Nankai University, Tianjin 300350, China
  • 2Department of Civil and Environmental Engineering, Rice University, Houston, TX, 77005
  • 3Department of Earth and Atmospheric Sciences, University of Houston, Houston, TX, 77004
  • 4Department of Chemical and Biomolecular Engineering, Rice University, Houston, TX, 77005
  • anow at: Department of Environmental Science and Engineering, California Institute of Technology, Pasadena, CA, 91125
  • bnow at: Washington State Department of Ecology, Lacey WA, 98503
  • cnow at: Division of Tropospheric Composition, NASA, Washington, DC 20024

Abstract. Submicron aerosol was measured to the southwest of Houston, Texas during winter and summer 2014 to investigate its seasonal variability. Data from a high-resolution time-of-flight aerosol mass spectrometer (HR-ToF-AMS) indicated that organic aerosol (OA) was the largest component of non-refractory submicron particulate matter (NR-PM1) (on average, 46±13% and 55±18% of the NR-PM1 mass loading in winter and summer, respectively). Positive matrix factorization (PMF) analysis of the OA mass spectra demonstrated that two classes of oxygenated OA (less and more-oxidized OOA, LO and MO) together dominated OA mass in summer (77%) and accounted for 42% of OA mass in winter. The fraction of LO-OOA (out of total OOA) is higher in summer (69%) than in winter (44%). Secondary aerosols (sulfate+nitrate+ammonium+OOA) accounted for ~76% and 89% of NR-PM1 mass in winter and summer, respectively, indicating NR-PM1 mass was driven mostly by secondary aerosol formation regardless of the season. The mass loadings and diurnal patterns of these secondary aerosols show a clear winter/summer contrast. Organic nitrate (ON) concentrations were estimated using the NOx+ ratio method, with an average contribution of ~15% and 37% to OA during winter and summer campaign, respectively. The estimated ON in summer strongly correlated with LO-OOA (r=0.73) and was enhanced at nighttime.

The relative importance of aqueous-phase chemistry and photochemistry in processing OOA was investigated by examining the relationship of aerosol liquid water content (LWC) and the sum of ozone (O3) and nitrogen dioxide (NO2) (Ox=O3+NO2) with LO-OOA and MO-OOA. The processing mechanism of LO-OOA apparently depended on relative humidity (RH). In periods of RH<80%, aqueous-phase chemistry likely played an important role in the formation of wintertime LO-OOA, whereas photochemistry promoted the formation of summertime LO-OOA. For periods of high RH>80%, these effects were opposite that of low RH periods. Both photochemistry and aqueous-phase processing appear to facilitate MO-OOA formation except during periods of high LWC, which is likely a result of wet removal during periods of light rain.

The nighttime increases of MO-OOA during winter and summer were 0.013 and 0.01μg MO-OOA per μg of LWC, respectively. The increase of LO-OOA was larger than that for MO-OOA, with increase rates of 0.033 and 0.055μg LO-OOA per μg of LWC at night during winter and summer, respectively. On average, the mass concentration of LO-OOA in summer was elevated by nearly 1.2μgm−3 for a ~20μg change in LWC, which is accompanied by a 40ppb change in Ox.

Qili Dai et al.
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Short summary
The formation processes of secondary organic aerosol remain to be fully understood. We reported the measurement data from two field campaigns within Houston, TX to investigate the effects of aqueous-phase chemistry and photochemistry in processing oxygenated organic aerosol (OOA) in winter and summer. Both photochemistry and aqueous-phase processing appear to facilitate more oxidized OOA formation. The processing mechanism of less oxidized OOA apparently depended on relative humidity.
The formation processes of secondary organic aerosol remain to be fully understood. We reported...
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