The authors present a carefully done study looking prmarily at the impacts of an airport on nearby residential ultrafine particle concentrations at locations not underneath the flightpaths.  Most previous studies have focused on impacts under the flightpaths.  Therefore this is a novel study that deserves publication.  I had several comments:

line 63:  There are also potential ground level impacts from landing aircraft due to lateral dispersion enhanced by wingtip vortices.

Line 81: Yu et al was based on a 4km spatial resolution model that might not account for the finer spatial scale resolution at locations near the airport under the flight path.  This is mentioned late in the reference to the Zurich study and also demonstrated initiall futher downwind by Hudda et al.

line 248: Where is the airport parking?  This is also relevant to the conclusion on line 569.

line 299:  This paragraph could do a better job of simply describing the cumulative method used here, especially starting on line 306.  It took me a while to figure out what was going on.

line 330: need a comma rather than a period

line 458: "strong evidence" is a bit too strong.  When the wind is coming from the airport, the evidence is clear.  It is just that it is not always coming from the airport.  

line 550: at site s229 the effect is evident, although not at the other site

The manuscript explores a valuable aspect of UFP from airports and its publication is supported.  I have a few comments:

lines 174-185  - More detail about the site locations would be helpful.  The locations are somewhat compromised by the proximity of local sources.  For example, N322 appears to be screened by trees to the south and in the middle of a working gardeners yard. S229 appears to be similarly sheltered by trees and a factory building.  As the authors note later in the manuscript, both locations are also influenced by contributions not from the airport.

lines 375-385  - Was any attempt made to correlate departures/arrivals with measurements?  Did the airport provide basic aircraft movements and operation modes for the survey periods?  This would help to attribute changes in particle diameters to aircraft activity.

line 400  - Given the possible screening of the sites by vegetation, do the authors think that meteorology measurements at 5,3m will be sufficiently representative of the sampling inlets at ca.3,0m?  Will winter and summer leaf growth cause any differences?

line 480-484  - Is it possible to disaggregate the contributions of road traffic from the airport related wind direction for N322?  Diurnal traffic flow and composition information might help

Figures 6 - 8 suggest that arriving and departing aircraft, once they are airborne and beyond the airfield boundaries, have very little influence on measured ground level UFP.  Is it worth mentioning this?  It's an important observation that is contrary to many other studies.