# 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-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 $M11inlinescrollmathmlchem{\mathrm{normal NO}}_{x}^{+}$ 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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