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

Dai, Qili; Schulze, Benjamin C.; Bi, Xiaohui; Bui, Alexander A. T.; Guo, Fangzhou; Wallace, Henry W.; Sanchez, Nancy P.; Flynn, James H.; Lefer, Barry L.; Feng, Yinchang; Griffin, Robert J.

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-PMinline-formula1) (on average, 38 % inline-formula± 13 % and 47 % inline-formula± 18 % of the NR-PMinline-formula1 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 inline-formula+ nitrate inline-formula+ ammonium inline-formula+ OOA) accounted for inline-formula∼76 % and 88 % of NR-PMinline-formula1 mass in winter and summer, respectively, indicating NR-PMinline-formula1 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 inline-formula M11inlinescrollmathml chem normal NO x + 24pt14ptsvg-formulamathimge3a11eb649141fb79f3ce1f1127704e1 acp-19-9641-2019-ie00001.svg24pt14ptacp-19-9641-2019-ie00001.png 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 (inline-formular=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 (inline-formulaO3) and nitrogen dioxide (inline-formulaNO2) (inline-formulaOxinline-formula=inline-formulaO3+NO2) with LO-OOA and MO-OOA. The processing mechanism of LO-OOA apparently was related to relative humidity (RH). In periods of RH inline-formula< 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 inline-formula> 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 inline-formulaµg MO-OOA per inline-formulaµg of LWC, respectively. The increase in LO-OOA was larger than that for MO-OOA, with increase rates of 0.033 and 0.055 inline-formulaµg LO-OOA per inline-formulaµg 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 inline-formulaµg minline-formula−3 for a inline-formula∼20page9642inline-formulaµg change in LWC, which was accompanied by a 40 ppb change in inline-formulaOx.

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Dai, Qili / Schulze, Benjamin C. / Bi, Xiaohui / et al: Seasonal differences in formation processes of oxidized organic aerosol near Houston, TX. 2019. Copernicus Publications.

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