[Objective] Carbonyl compounds are important precursors of ozone and significantly affect air quality and human health. Diesel vehicles have been identified as a dominant contributor to carbonyl compound emissions. In China, agricultural machines have become an important emission source of off-road mobile pollution because of its increasing number and the backward control of its emission. With worsening atmospheric pollution and depleting petroleum resources, finding alternatives to petroleum is urgent. Biodiesel, mainly referred to as fatty acid methyl ester formed by esterification reactions of vegetable, animal, and waste oils with methanol or ethanol, is regarded as an important clean energy source. In this context, the impact of biodiesel on reducing carbonyl emissions compared with fossil diesel remains unclear, leading to a very large uncertainty in achieving China's goal of "reducing pollution and carbon" for diesel machinery. [Methods] Therefore, five representative agricultural machines (three tractors and two combine harvesters) fueled with different biodiesel and diesel blends--pure fossil diesel (B0), B5 biodiesel, and B20 biodiesel--were selected in this study to characterize carbonyl compound emissions. Real-world measurements were conducted using a dilution system in a rice field in Gongdao Town, Yangzhou City, Jiangsu Province. A total of 17 carbonyl compounds (formaldehyde, methanol, acetaldehyde, formic acid, acrolein, propionaldehyde, acetic acid, butenone, butyraldehyde, valeraldehyde, ethyl acetate, cyclohexanone, hexaldehyde, benzaldehyde, heptaldehyde, methylbenzaldehyde, and caprylaldehyde) were detected using a proton-transfer reaction time-of-flight mass spectrometer (Ionicon Analytik, Innsbruck, Austria). [Results] Overall, biodiesel-fueled engines exhibited significantly higher carbonyl emission factors than diesel-fueled engines, demonstrating the trend of B5 > B20 > B0. Average emission factors for B5, B20, and B0 fueled engines were (1.64 ± 1.04), (1.41 ± 0.84), and (1.18 ± 0.76) g/kg fuel, respectively. These results were consistent with emissions from light-duty diesel trucks (LDDTs) but were higher than those from three- and four-wheel rural vehicles and LDDTs. Propionaldehyde, acetic acid, acrolein, and butyraldehyde were the dominant carbonyl species for the tested agricultural machines, accounting for 71.4%, 73.2%, and 80.1% of the total carbonyls for B0, B5, and B20, respectively. The ozone formation potential (OFP) of carbonyl was higher for B5- and B20-fueled agricultural machines than those fueled by B0, owing to the biodiesel's higher oxygen content than diesel. Furthermore, the OFPs for agricultural machines were higher than those for diesel trucks. In addition, cancer risks from carbonyls were the highest for B5, followed by B20 and B0. [Conclusions] In conclusion, different biodiesel blend ratios, engine types, and operation modes affect carbonyl emissions. Although biodiesel is promoted as an alternative fuel in some cities, its impact on carbonyl emission, air quality, and human health should be carefully addressed owing to its higher oxygen content than diesel. [ABSTRACT FROM AUTHOR]