The objective of this study was to improve our understanding on secondary organic aerosol (SOA) formation with specific aims on the influence of NOx, H2O2, aerosol water chemistry, and acidity of seed particles. It will improve our understanding on SOA formation and its relevant environmental phenomena. It was examined for two abundant biogenic reactive organic gases (ROGs) of α-pinene and d-limonene. An indoor smog chamber was newly developed and characterized for photochemical SOA formations. For experiments of NOx effects in dry condition with no seed particles, d-limonene with 2 double bonds showed much higher SOA yield of 50.0%-91.6% relative to 9.7%-37.7% for α-pinene with a double bond. It agrees to the fact that ROGs with more double bonds tend to form lower volatility products with multifunctional groups, leading to higher SOA yield. The SOA yield peaked at ROG/NOx ratio approximately 1 for both α-pinene and d-limonene. It decreased gradually at higher ROG/NOx ratio and abruptly with reducing ROG/NOx ratio. d-Limonene showed SOA formation significantly suppressed by decreasing NO2/NOx ratio unlike α-pinene with consistent SOA formation yields at low and high NO2/NOx ratios. The results might be associated with the formation of high volatility products via NO reaction channel of alkyl peroxy radical, a key intermediate in atmospheric photochemistry. The influence of OH radical on SOA formations was elucidated by indirectly varying the concentration ratio of ROG to H2O2, OH radical precursor, in dry air without seed particles. α-Pinene and d-limonene showed SOA formation complicated by varying ROG/H2O2 ratio. Further researches are necessary to explain the complex phenomena. The terpene compounds showed SOA formations obviously enhanced in wet condition (RH>80%) compared to dry condition (RH<5%) in NOx-free air with (NH4)2SO4 seed particles and oxidant of H2O2. It suggests the possibility of SOA formation through aerosol water chemistry. The influence of acidity was examined using two sets of seed particle with different acidities of (NH4)2SO4, (NH4)2SO4 + NH4HSO4, and NH4HSO4 and NH4NO3 and NH4NO3 + HNO3. More acidic seed particles increased SOA formation in the photochemical reaction of α-pinene and d-limonene in dry and NOx-free air with an oxidant of H2O2. The magnitude was dependent on seed particle in smog chamber experiments.