Since biogenic volatile organic compounds (BVOCs) are important precursors of ozone, the monitoring of the BVOC concentration distributions is needed. In general, forest BVOC concentrations increase in summer as well as in other seasons. This study aims to detect temporally sporadic increases in BVOC concentrations in the non-summer months and to analyze the occurring climatic conditions. Using a uniform sampling system and shared gas chromatography–mass spectrometry, the concentrations of isoprene and monoterpenes in six Japanese forests were observed approximately once a month for 3 years. Using the observed data, the relations between the BVOC concentration increases and meteorological factors were evaluated. Twenty instances of temporal increases in BVOC concentrations were observed. These mainly occurred in spring for isoprene and in autumn for monoterpenes. Most of the increases in the non-summer months were observed after a rainfall event, when the daily temperature range was large, suggesting that wind, rain, and a rapid diurnal temperature rise could be factors in the non-summer months. Thus, the network monitoring of BVOC concentrations might be effective for understanding the effects of factors other than temperature, and the mechanisms and frequency of the temporal increases, on the BVOC concentrations in various forests.
