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. Submicron aerosol was measured to the southwest of Houston, Texas, duringwinter and summer 2014 to investigate its seasonal variability. Data from ahigh-resolution time-of-flight aerosol mass spectrometer (HR-ToF-AMS)indicated that organic aerosol (OA) was the largest component ofnonrefractory submicron particulate matter (NR-PM 1 ) (on average, 38 %  ±  13 % and 47 %  ±  18 % of the NR-PM 1 mass loading inwinter and summer, respectively). Positive matrix factorization (PMF)analysis of the OA mass spectra demonstrated that two classes of oxygenatedOA (less- and more-oxidized OOA, LO and MO) together dominated OA mass insummer (77 %) and accounted for 39 % of OA mass in winter. The fractionof LO-OOA (out of total OOA) is higher in summer (70 %) than in winter(44 %). Secondary aerosols (sulfate  +  nitrate  +  ammonium  +  OOA) accountedfor ∼76  % and 88 % of NR-PM 1 mass in winter andsummer, respectively, indicating NR-PM 1 mass was driven mostly bysecondary aerosol formation regardless of the season. The mass loadings anddiurnal patterns of these secondary aerosols show a clear winter–summercontrast. Organic nitrate (ON) concentrations were estimated using the NO x + ratio method, with contributions of 31 %–66 % and 9 %–17 % toOA during winter and summer, respectively. The estimated ON in summerstrongly correlated with LO-OOA ( r=0.73 ) and was enhanced at nighttime. The relative importance of aqueous-phase chemistry and photochemistry inprocessing OOA was investigated by examining the relationship of aerosolliquid water content (LWC) and the sum of ozone ( O3 ) and nitrogendioxide ( NO2 ) ( Ox = O3+NO2 ) with LO-OOA and MO-OOA.The processing mechanism of LO-OOA apparently was related to relativehumidity (RH). In periods of RH   80 %, aqueous-phase chemistrylikely played an important role in the formation of wintertime LO-OOA,whereas photochemistry promoted the formation of summertime LO-OOA. Forperiods of high RH  >  80 %, these effects were opposite those oflow-RH periods. Both photochemistry and aqueous-phase processing appear tofacilitate increases in MO-OOA concentration except during periods of highLWC, 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 and0.01  µ g MO-OOA per µ g of LWC, respectively. The increase in LO-OOAwas larger than that for MO-OOA, with increase rates of 0.033 and 0.055  µ g LO-OOA per µ g of LWC at night during winter and summer, respectively. On average, the mass concentration of LO-OOA in summer waselevated by nearly 1.2  µ g m −3 for a ∼20 µ gchange in LWC, which was accompanied by a 40 ppb change in Ox .

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