Referee report LOAC: a small aerosol optical counter/sizer for ground- based and balloon measurements of the size distribution and nature of atmospheric particles: 1. Principle of measurements and instrument evaluation

 L. 4-5 p.9995: To be precise, in [Deshler et al., 2003, cited in the paper], the authors mention the value of 150 nm as smallest particle size measured by optical particle counters (OPC) at Laramie. However, these authors also measure condensation nuclei (radius ≤ 0.01 μm) by growing particle with sizes down to 0.01 μm in the sample stream.  L. 5-6 p.9995: “sensitive to nature of the particles”: Do the authors refer to the value of the refractive index? This could be mention for the sake of clarity.  L. 23-24 p.9995: The authors should remove the sentence “All these tests indicate that no bias is present in the LOAC measurements and in the corresponding data processing.” See the specific discussions further in the text.