# Emission of Terpenoid Compounds from Rice Plants
## 1 Introduction  
Many species of volatile organic compounds (VOCs) are emitted from biogenic sources, including crops, trees, grasses, and weeds. Terpenoids, including isoprene and monoterpenes, are major biogenic VOCs (BVOCs), and their total annual emission is estimated to account for 600-800 TgC [1], which is higher than the annual emission estimates of anthropogenic VOCs (98-158 TgC) [2]. Because of their high reactivity in the atmosphere [3], terpenoids contribute to the production of photochemical oxidants, including ozone, and secondary organic aerosols (SOA). Photochemical oxidants degrade air quality and have negative effects on humans, other animals, and plants [4]. SOA may directly and indirectly affect the global radiation balance by forming cloud condensation nuclei, which reflects sunlight, but the degree of its contribution is still largely unknown [5]. Rice (_Oryza sativa_) is the most important cereal crop in Asia and other countries. In Japan, the cultivation area is approximately \(1.4\times 10^{6}\) ha, occupying 36% of the country's entire farmland [6]. Its global cultivation area is approximately \(167\times 10^{6}\) ha [7], which corresponds to 4% of the world's forest area. In Asia, the area of rice paddies accounts for 87% of the global cultivation area of rice [8]. Trace gas exchange between rice paddies and the atmosphere has been of great concern, including the emission sources of N\({}_{2}\)O [9] andmethane [8]. If terpenoids are abundantly emitted from rice paddies, they might be a strong source of BVOC in rural areas and affect regional atmospheric chemistry and air quality. To date, several studies have been conducted to qualify and quantify terpenoid emissions from rice paddy fields. In a rice paddy in California, no emission of isoprene or monoterenes was observed in a flow-through chamber enclosing the rice cultivar \"M202\" grown in a rice paddy [10]. [PERSON] et al. [11] employed a static chamber method for the rice cultivar \"Cocodrie\" grown in a rice paddy and detected no emissions of monoterenes, but did detect a trace amount of isoprene. The reported isoprene emission rate on a land-area basis was very small (3.7 ug m\({}^{-2}\) h\({}^{-1}\)\(\approx\) 0.015 nmol m\({}^{-2}\) s\({}^{-1}\)). Our previous study showed that a trace amount of monoterenes species linalool was emitted from the rice cultivar \"Mutsubomare,\" but not from \"Nipponbare,\" \"TCM916\", \"Tangin-bodzu\", or \"Waito C.\" The emission rate was not quantified because of the low concentration below the limit of quantification in our previous gas chromatography-mass spectrometry (GC-MS) system [12]. On the other hand, [PERSON] et al. [13] reported that monoterene species \(\alpha\)-pinene, limonene, myrene, p-cymene, and \(\beta\)-pinene were emitted from Japonica rice (cultivar information was not given), and the total emission rate of the monoterenes on a land-area basis was 600 ug m\({}^{-2}\) h\({}^{-1}\). By extrapolating the data, they estimated the monoterene emission from rice in the Kinki region of Japan to be 5% of that from the whole plant species in the region. As most of the monoterenes detected by these researchers were reactive in the atmosphere [3], [PERSON] et al. [14] estimated ozone formation using the Community Multiscale Air Quality (CMAQ) model and concluded that paddy fields in the Kinki region contributed to the increase in ozone concentration in the urban area of Osaka City and its surrounding region. Except for [PERSON]'s report [13], no other studies have reported substantial terpenoid emissions from rice. If large amounts of terpenoids are emitted from rice, the emissions might have a great impact on the production of regional photochemical oxidants and SOAs in Asia and other countries where rice is widely grown in rice paddy fields. As there is a discrepancy in terpenoid emission strength from rice plants between published reports, reliable emission data collected based on reliable measurement methods need to be provided.