Atmospheric low volatility vapors play an essential role in aerosol particle formation, growth, and cloud condensation nuclei production, thereby influencing climate. While intensive measurements have been carried out in different locations, little is known about their seasonal variability due to a lack of long-term measurements. To address this gap, we present nearly 4 years of continuous observations of low volatility vapors in a boreal forest measured using a NO₃- atmospheric pressure interface time-of flight (MION-Api-TOF) mass spectrometer. Our results reveal the seasonal variation in the concentration, molecular composition, formation mechanism and volatility distribution of highly oxygenated organic molecules (HOMs). We show that while temperature-dependent terpene emissions regulate the overall seasonal abundance of HOMs, distinct formation pathways govern their diurnal profiles: monoterpene-derived monomers peak during the day due to the availability of precursors and oxidants, whereas their dimers peak at night driven by low concentrations of terminating species (NO and HO₂) and extended RO₂ radical lifetimes. We observed differing seasonal changes across different HOMs species and, although C₁₀ HOMs remains the dominant HOM species during the whole year, sesquiterpene-derived C₁₅ HOMs shows the steepest increase during summer. Using binned Nonnegative Matrix Factorization (bin-NMF), we found that HOM composition is strongly modulated by seasonality. While monoterpene-derived HOMs containing nitrogen atoms (“CHON”) dominate during colder months, summertime chemistry is characterized by a shift toward “CHO” species and a substantial contribution from heavy terpenes (sesquiterpenes and diterpenes), which can account for 40 %–80 % of the signal in the high-mass range (m/z 450–700, including NO₃-). We also analysed the seasonal volatility distribution of HOMs and identified the primary contributors to each volatility class. Crucially, challenging the long-held assumption that monoterpene derived C_17-20 HOM dimers are the most important biogenic precursors of new particle formation in the boreal forest, we found sesquiterpene-derived C_13-15 HOMs and C_17-20 HOMs have comparable contribution to Ultra-Low Volatility Organic Compounds (ULVOCs) during daytime throughout much of the year. Moreover, in spring – the season with the highest NPF frequency at our measurement site, sesquiterpene-derived C_13-15 HOM account for approximately 40 % of ULVOC during daytime, compared to 28 % for C_17-20 HOMs. Our findings provide critical new insights into the seasonal dynamics of HOM composition and their broader atmospheric implications.