Dear Authors,

Please adress the two final comments by Referee #1:
Page 10, lines 4-13: In their discussion of the differing O/I mass concentration ratios for the three PM size fractions, the authors attribute the trend of decreasing O/I ratio with increasing particle size to transport dynamics caused by the "piston effect". While this may be one factor contributing to different observed ratios, the authors should also mention that these differences could also be related to different emission rates within the tunnel for PM in different size fractions.

Page 15, paragraph beginning line 5: I find this paragraph on results of principal component analysis to be confusing and suggest revising to more clearly identify why specific sources were associated with specific factors. For example, I find the attribution of PC1 and PC2 in the coarse fraction to diesel and gasoline exhaust to be somewhat speculative. Given the primarily submicron particle mass distribution generally shown for motor vehicle emissions, how do the authors support attributing coarse fraction factors to vehicle exhaust?

Best regards,
Harald Saathoff