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This work demonstrates mass measurement of individual submicron air-borne particles using resonant micromechanical nano-balances. Thermally actuated high-frequency single crystalline silicon resonators fabricated using a single mask process have been used as mass sensors. Mass sensitivity of the resonators have been characterized using artificially generated airborne particles of known size and composition. Mass sensitivities as high as 1.6 kHz/pg have been demonstrated for devices with resonant frequencies in the tens of MHz range. The measured mass sensitivities are in good agreement with the calculated values based on the resonator physical dimensions. Due to the high mass sensitivities, the shift in the resonator frequencies caused by individual particles as small as ~200 nm in diameter is distinguishable. Counting and individual mass measurement of single arbitrary particles in air samples from a cleanroom have also been demonstrated. The results in this work present the possibility of implementation of low-cost and small-size instruments for airborne particle mass and size distribution analysis in highly controlled environments (e.g., for cleanroom classification) or for environmental applications.