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Published July 31, 2019 | Published + Supplemental Material
Journal Article Open

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


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 nonrefractory submicron particulate matter (NR-PM_1) (on average, 38 % ± 13 % and 47 % ± 18 % of the NR-PM_1 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 39 % of OA mass in winter. The fraction of LO-OOA (out of total OOA) is higher in summer (70 %) than in winter (44 %). Secondary aerosols (sulfate + nitrate + ammonium + OOA) accounted for ∼76 % and 88 % of NR-PM_1 mass in winter and summer, respectively, indicating NR-PM_1 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 NO^+_x ratio method, with contributions of 31 %–66 % and 9 %–17 % to OA during winter and summer, 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 (O_3) and nitrogen dioxide (NO_2) (O_x = O_3+NO_2) with LO-OOA and MO-OOA. The processing mechanism of LO-OOA apparently was related to 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 those of low-RH periods. Both photochemistry and aqueous-phase processing appear to facilitate increases in MO-OOA concentration except during periods of high LWC, which is likely a result of wet removal during periods of light rain or a negative impact on its formation rate. The nighttime increases in MO-OOA during winter and summer were 0.013 and 0.01 µg MO-OOA per µg of LWC, respectively. The increase in LO-OOA was larger than that for MO-OOA, with increase rates of 0.033 and 0.055 µg 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 1.2 µg m^(−3) for a ∼20 µg change in LWC, which was accompanied by a 40 ppb change in O_x.

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© 2019 Author(s). This work is distributed under the Creative Commons Attribution 4.0 License. Received: 6 December 2018 – Discussion started: 12 December 2018; Revised: 7 June 2019 – Accepted: 18 June 2019 – Published: 31 July 2019. Data availability: Datasets are available by contacting the corresponding author. Supplement: The supplement related to this article is available online at: https://doi.org/10.5194/acp-19-9641-2019-supplement. Author contributions: QD performed the data analysis and wrote the manuscript. RJG and YF assisted heavily with manuscript development and editing. HWW, AATB, JHF and BLL contributed to data collection during the field campaigns. BCS, HWW, AATB and NPS contributed with data analysis. XB, BCS, AATB, FG, NPS and JHF provided helpful comments and edits. The authors declare that they have no conflict of interest. The authors would like to acknowledge Yele Sun (Institute of Atmospheric Physics, Chinese Academy of Sciences) for providing the aq-OOA mass spectra and Qiao Zhu (Peking University Shenzhen Graduate School) for assistance in the calculation of organic nitrates and PMF analysis. The scholarships provided by China Scholarship Council to Qili Dai and Xiaohui Bi are gratefully acknowledged. Support of the Houston Endowment in the development and deployment of the MAQL also is gratefully acknowledged. This research has been supported by the National Key R&D Program of China (grant no. 2016YFC0208505), the Tianjin Science and Technology Plan Program (grant no. 18ZXSZSF00160) and the Houston Endowment (grant no. 2014-177-0163-03). Review statement: This paper was edited by Eleanor Browne and reviewed by two anonymous referees.

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