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

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-PM 1 ) (on average, 46 ± 13 % and 55 ± 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 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-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 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 (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 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 u003e 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 0.01 μg 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 0.055 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 is accompanied by a 40 ppb change in O x .