Isoprene, formaldehyde and acetaldehyde are important reactive organic compounds which strongly impact atmospheric oxidation processes and formation of tropospheric ozone. Monsoon meteorology and the topography of Himalayan foothills cause surface emissions to get rapidly transported both horizontally and vertically, thereby influencing atmospheric processes in distant regions. Further in monsoon, Indo-Gangetic Plain is a major rice growing region of the world and daytime hourly ozone can frequently exceed phytotoxic dose of 40 ppb O₃. However, the sources and ambient variability of these compounds which are potent ozone precursors are unknown. Here, we investigate the sources and photochemical processes driving their emission/formation during monsoon season from a sub-urban site at the foothills of the Himalayas. The measurements were performed in July, August and September using a high sensitivity mass spectrometer. Average ambient mixing ratios (±1σ variability) of isoprene, formaldehyde, acetaldehyde, and the sum of methyl vinyl ketone and methacrolein (MVK+MACR), were 1.4 ± 0.3 ppb, 5.7 ± 0.9 ppb, 4.5 ± 2.0 ppb, 0.75 ± 0.3 ppb, respectively, and much higher than summertime values in May. For isoprene these values were comparable to mixing ratios observed over tropical forests. Surprisingly, despite occurrence of anthropogenic emissions, biogenic emissions were found to be the major source of isoprene with peak daytime isoprene driven by temperature (r ≥ 0.8) and solar radiation. Photo-oxidation of precursor hydrocarbons were the main sources of acetaldehyde, formaldehyde and MVK+MACR. Ambient mixing ratios of all the compounds correlated poorly with acetonitrile (r ≤ 0.2), a chemical tracer for biomass burning suggesting negligible influence of biomass burning during monsoon season. Our results suggest that during monsoon season when radiation and rain are no longer limiting factors and convective activity causes surface emissions to be transported to upper atmosphere, biogenic emissions can significantly impact the remote upper atmosphere, climate and ozone affecting rice yields.

异戊二烯，甲醛和乙醛是重要的反应性有机化合物，它们会严重影响大气氧化过程和对流层臭氧的形成。季风气象学和喜马拉雅山麓的地形导致地表排放物在水平和垂直方向上都得到快速传输，从而影响了遥远地区的大气过程。进一步在季风中，印度恒河平原是世界上一个主要的水稻种植区，白天每小时的臭氧含量经常超过40 ppb O _3的植物毒性剂量。但是，尚不清楚这些有效臭氧前体化合物的来源和环境变异性。在这里，我们研究了在季风季节从喜马拉雅山山脚下的一个郊区站点驱动其排放/形成的来源和光化学过程。使用高灵敏度质谱仪在7月，8月和9月进行测量。异戊二烯，甲醛，乙醛以及甲基乙烯基酮和甲基丙烯醛的总和（MVK + MACR）的平均环境混合比（±1σ变异性）为1.4±0.3 ppb，5.7±0.9 ppb，4.5±2.0 ppb，0.75±0.3 ppb，分别远高于5月份的夏季值。对于异戊二烯，这些值与热带森林中观察到的混合比相当。令人惊讶的是，尽管发生了人为排放，生物成因排放被发现是异戊二烯的主要来源，白天异戊二烯的高峰由温度（r≥0.8）和太阳辐射驱动。前体烃的光氧化是乙醛，甲醛和MVK + MACR的主要来源。所有化合物的环境混合比与乙腈（r≤0.2）之间的相关性很弱（r≤0.2），这是生物质燃烧的化学示踪剂，表明季风季节中生物质燃烧的影响可忽略不计。我们的结果表明，在季风季节，辐射和雨水不再是限制因素，对流活动导致地表排放物被输送到高层大气，生物源排放物会显着影响偏远的高层大气，气候和臭氧，影响水稻产量。8）和太阳辐射。前体烃的光氧化是乙醛，甲醛和MVK + MACR的主要来源。所有化合物的环境混合比与乙腈（r≤0.2）之间的相关性很弱（r≤0.2），这是生物质燃烧的化学示踪剂，表明季风季节中生物质燃烧的影响可忽略不计。我们的结果表明，在季风季节，辐射和雨水不再是限制因素，对流活动导致地表排放物被输送到高层大气，生物源排放物会显着影响偏远的高层大气，气候和臭氧，影响水稻产量。8）和太阳辐射。前体烃的光氧化是乙醛，甲醛和MVK + MACR的主要来源。所有化合物的环境混合比与乙腈（r≤0.2）之间的相关性很弱（r≤0.2），这是生物质燃烧的化学示踪剂，表明季风季节中生物质燃烧的影响可忽略不计。我们的结果表明，在季风季节，辐射和雨水不再是限制因素，对流活动导致地表排放物被输送到高层大气，生物源排放物会显着影响偏远的高层大气，气候和臭氧，从而影响水稻产量。一种生物量燃烧的化学示踪剂，表明在季风季节生物量燃烧的影响可以忽略。我们的结果表明，在季风季节，辐射和雨水不再是限制因素，对流活动导致地表排放物被输送到高层大气，生物源排放物会显着影响偏远的高层大气，气候和臭氧，影响水稻产量。一种生物量燃烧的化学示踪剂，表明在季风季节生物量燃烧的影响可以忽略。我们的结果表明，在季风季节，辐射和雨水不再是限制因素，对流活动导致地表排放物被输送到高层大气，生物源排放物会显着影响偏远的高层大气，气候和臭氧，影响水稻产量。