Highly time-resolved measurements of non-refractory submicron particulate matter (NR-PM1) were performed using a high-resolution time-of-flight aerosol mass spectrometer (HR-ToF-AMS) over a big urban city (Ahmedabad, 23.0 degrees N, 72.6 degrees E, 49 m amsl) of western India during the post-monsoon season. All the components of NR-PM1 [i.e., organic aerosol (OA) SO42-, NO3-, NH4+ and Cl-] showed a strong diurnal variation with the overall dominance of OA (58%) in NR-PM1, and the OA composition was found to be a mixture of fresh and aged species. O/C varied from 0.04 to 0.91 whereas, H/C varied from 1.34 to 2.26. A strong diurnal variation in O/C caused a large variability in OM/OC ratio (ranging from 1.2 to 2.3) during the study period, which suggests that a constant conversion factor for OM estimation (from OC) can be a source of large uncertainty in their load assessment over the study region. Furthermore, the observed slope (-0.78 +/- 0.01) of Van Krevelen (VK) diagram suggests that plausible major functional groups of ambient oxidized OA could be the mixture of alcohols and carboxylic groups. Further, a drastic change in NR-PM1 concentrations and composition was observed during Diwali, a festival when huge amount of firecrackers are burnt across India. Three distinct sources of OA [i.e., primary OA (POA), semi-volatile oxygenated OA (SV-OOA), and low volatility OOA (LV-OOA)] were identified via positive matrix factorization (PMF) analysis. The OA mass was dominated by POA (42%), followed by LV-OOA (33%) and SV-OOA (25%). Diurnal variations in PMF factors suggest that OOA (SV-OOA + LV-OOA) were high during early morning and afternoon hours; whereas, POA was most abundant during traffic rush hours. Observations also revealed that the high OA loading events were dominated by POA during the study period. This study provides new insights on the atmospheric aging of OA, for the first time, over western India, which would be helpful in understanding the formation mechanism of secondary OA over this region.