previous studies that 
 directly measured the impact of aviation 
 activity on air quality have mostly conducted measurements in close 
 proximity of airports .
2006 analyzed 
 differences in pollutant concentrations by wind speed and direction 
 along with differences in aircraft and ground traffic activity at 
 heathrow airport in london .
they found airport contributions of up 
 to 15% of total oxides of nitrogen ( nox ) at a site 1.5 km downwind of the nearest runway . at hong
2004 used nonparametric 
 regression analysis on pollutant concentrations by wind speed and 
 direction .
they calculated that aircraft nearly doubled sulfur dioxide 
 concentrations 3 km away and also increased concentrations of carbon 
 monoxide and respirable suspended particles under similar wind speeds 
 and directions .
2007 measured 
 particle numbers concentrations in the 10100 nm range and 
 found significant increases above background at 1.9 , 2.7 , and 3.3 
 km downwind of the los angeles international airport ( lax ) blast fence . 

although measurements were stationary and not concurrent , they also 
 noted that takeoffs produced high concentrations and downwind gradients 
 within 600 m of the blast fence .
2009 found that aircraft activity at a regional airport in warwick , 
 ri contributed 2428% of the total black carbon ( bc ) measured 
 at five sites 0.163.7 km from the airport .
several other 
 airport and aviation emissions studies focused on 
 quantifying the air quality impacts from jet takeoffs and measured air pollutant concentrations very close to runways . 
 of particular relevance to this study , hsu et al .
their models suggested that aircraft produced a median pn concentration 
 of nearly 150  000 particles / cm at the end of the 
 departure runway .
pn concentrations decreased rapidly with distance 
 to 19  000 particles / cm at a location 250 m downwind 
 and to 17  000 particles / cm at a location 500 m 
 further downwind . the rapid drop - off in concentration , however , may 
 have reflected an increasing offset from the centerline of impacts 
 with greater downwind measurement distance .
similar magnitude pn concentrations 
 and correlations with departures were reported by westerdahl et al . 

our study 
 was motivated by mobile monitoring platform ( mmp ) based 
 observations of large but gradual increases in pn concentrations as 
 we approached locations under lax jet landing trajectories on multiple 
 transects up to 10 km downwind of lax .
we hypothesized that emissions 
 from lax activities were increasing pn concentrations over much larger 
 areas and longer downwind distances than previously observed in studies 
 that focused on near freeway and jet takeoff impacts to air quality . 

an extensive monitoring campaign confirmed that lax - related emissions 
 increased pn concentrations downwind at least 2-fold to 16 km .
this 
 large , previously undiscovered spatial extent of the air quality impacts 
 downwind of major airports may mean a significant fraction of urban 
 dwellers living near airports likely receive most of their outdoor 
 pn exposure from airports rather than roadway traffic .
lax is the sixth busiest airport in 
 the world and third busiest in the united states .
about 95% of flights 
 take off and land into the prevailing westerly / west - southwesterly 
 ( w / wsw ) onshore winds ( i.e. , 263 degrees , 
 the direction of runway alignment ) using 
 two sets of parallel runways separated by about 1.5 km . in the busiest 
 hours , 4060 jets per hour arrive during hours 07001900 
 and
2040 jets per 
 hour arrive during hours 0600 and 10000100 and depart during 
 hours 0700 and 22002300 . during other hours
the airport complex is about 4.5 km east to west ( e - w ) and 
 about 2.5 km north to south ( n  s ) and is surrounded by major 
 roadways and freeways , as highlighted in figure 1 ( figure s.1 in supporting information ( si ) shows a map of this area with street name labels ) .
the federal aviation 
 administration noise contours of the modeled annual 65 db a - weighted 
 equivalent ( laeq ) noise threshold are 
 shown extending eastward along the predominant 
 downwind direction and reflect the jet trajectories used for landing . 

they also extend west of the airport over the pacific ocean ( not shown ) .
los angeles 
 international airport and 65 db noise contours indicating 
 eastern jet trajectories .
monitoring consisted of transects 
 416 km in length , nearly perpendicular ( i.e. , n 
s ) 
 to the direction of the prevailing winds , at varying downwind distances . 
 different monitoring routes were required to fully capture the changes 
 in impact locations due to shifts in wind direction .
a general downwind 
 direction was chosen based on meteorological predictions but transect 
 lengths and locations were determined during the monitoring run based 
 on observations of the rate of change of pn concentrations . for each 
 transect , monitoring was extended several hundred meters beyond the 
 location where baseline pn concentrations appeared stable .
measurements 
 were conducted over 29 days with the university of southern california 
 ( usc ) mmp , a gasoline - powered hybrid vehicle .
a second mmp , the university 
 of washington ( uw ) mmp , a gasoline - powered minivan , joined the monitoring 
 on 3 days ( june 22 , 27 and july 1 , 2013 ) .
urban background 
 value concentrations 
 are reported to nearest 2500 particles / cm and are the 
 average baseline values in the unimpacted areas away from local traffic 
 sources concurrent mmp sampling 
 times : june 
 22:13201720 , june 27:13251510 , july 1:12401640 . monitoring route did not cover 
 the 
 full n  s extent of the impact on western av ( 10 km downwind ) 
 on these days , values have been reported for crenshaw blvd .
easterly flow 
 was recorded in morning 
 hours ( until 1000 ) and westerly later morning to afternoon 08/25/2013 was not counted as an 
 additional monitoring day because only 1 h of monitoring ( 00000100 ) 
 was conducted on this date most measurements were conducted during times of onshore westerly 
 winds , typically strongest during 11001600 , but we also conducted 
 measurements during early morning and late night hours when air traffic 
 was low and onshore winds were reduced ( august 13 , 16 , 23 , 24 and 
 25 , december 03 , 09 , 15 and 16 , 2013 ) . monitoring focused on the area 
 east of lax ( i.e. , the predominant downwind direction ) but included 
 several runs along the boundary of the airport in the upwind direction 
 and south of the airport complex during occasions of northerly winds 
 in winter months .
concentration measurements 
 included 
 pn , bc , no , no2 , nox , and particle 
 surface uv - photoionization potential ( measured using ecochem photoelectric 
 aerosol sensor [ pas ] that responds to elemental carbon and particle - bound 
 polycyclic aromatic hydrocarbons [ pb  pah ] ) . instrument details 
 are provided in si ( table s.1 and s.2 ) . 

instruments were powered by two deep - cycle marine batteries via dc - to - ac 
 inverter .
our power arrangement allowed for 5 h of run time if all 
 instruments were running . for sampling runs that were anticipated 
 to exceed 5 h ,
several instruments were shut down to extend battery 
 life and the condensation particle counter ( cpc ) was run on the vehicle s 
 12 v cell phone power outlet . if other instruments were turned on 
 later , the required warm - up time was 25 min .
instrument clock 
 times were regularly synchronized to be within 1 s of the global positioning 
 system device time , which also recorded speed and location .
measurements 
 from instruments with a delayed response time were advanced to match 
 the instantaneous instruments and the gps time and location recorded 
 at 1 s intervals . for pollutant measurements recorded at 10 s intervals , 
 all locations within the recording interval
minute and hourly wind speed and 
 wind direction data were obtained from the automated surface observing 
 systems monitor at lax airport ( latitude 33.943 and longitude 118.407 ) . 
 due to the 16 km distance between eastern edge of the study area and 
 the meteorological station located at lax
, we could not assume that 
 wind speed and direction were identical to those measured at lax , 
 but wind direction in this region of los angeles tends to be similar 
 over large areas during daytime .
daytime southwesterly sea breezes typically 
 occur 16 h per day in the summer ( 09000100 for june  august ) , 
 decreasing to 6 h in the winter ( 12001800 in december ) . only 
 during the winter months ( november  february , 00000900 ) 
 are light easterly off - shore winds common .
wind roses based on 1 min data 
 are shown in figure s.2 and s.3 of the si .
mmp measurements included a localized 
 traffic emissions signal representing microscale and middle scale 
 variations ( 10100 m and 100500 m , respectively ) and 
 an underlying  baseline  pollutant concentration that 
 varied gradually over the neighborhood scale ( 500 m4 km ) .
1997 derived these categories by considering the spatial scales of impact 
 of various types of air pollution sources .
we adopted a smoothing 
 methodology to estimate baseline pn concentrations that excluded the 
 microscale and middle scale impacts due to local sources , usually 
 specific vehicles .
baseline pn concentrations were derived from 
 our mobile measurements by taking a rolling 30-s fifth percentile 
 value of the 1-s concentration time series , and assigning that value 
 to the measured location .
this removed the microscale and middle scale 
 impacts from traffic sources such as specific vehicle plumes .
baseline 
 concentrations for a run were relatively spatially uniform outside 
 of the lax impact areas , with coefficients of variation ( cv ) of less 
 than 5% . in 
 comparison ,
the raw pn concentrations on roadways outside the lax 
 impact areas had cvs on the order of 40% . on rare occasions ,
the mmp 
 was behind a high emitter for longer than 30 s. such events , only 
 if verifiable by video and field notes , were censored .
however , less 
 than 0.5% of data were censored in this manner , generated from about 
 a dozen instances of prolonged influence from high emitting vehicles . 

an illustration of both raw and smoothed concentration time series 
 is presented in the si ( figures s.4s.7 ) . 

downwind of lax we observed gradual but large 
 increases in baseline 
 pn concentrations occurring over transect distances of multiple kilometers . 

pn concentrations were elevated 4-fold or more above nearby unimpacted 
 baseline concentrations up to 10 km in the downwind direction from 
 lax .
figure 2 shows an example of the spatial 
 pattern of the elevated pn concentrations . spatial pattern of pn 
 concentration ( colored by deciles ) for the 
 afternoon and evening hours of august 23 , 2013 .
the size of the impacted areas with high pn concentration 
 increases 
 was remarkable . at 16 km
downwind , a 2-fold increase in pn concentration 
 over baseline concentrations was measured across 6.5 km .
assuming 
 a trapezoidal shaped plume with parallel edges of length 1.5 and 6.5 
 km , pn concentrations were at least doubled over an area of 60 km .
eight km downwind , a 5-fold increase in pn concentrations 
 over baseline concentrations extended across 3 km and covered a total 
 area of 24 km .
( concentrations in this large area exceeded 
 71  000 particles / cm , the average concentration 
 on los angeles freeways . ) within 3 km 
 of the airport boundary , concentrations were elevated nearly 10-fold , 
 exceeding 100  000 particles / cm , with concentrations 
 of 150  000 particles / cm occurring over a several 
 km area .
this pattern of elevated pn concentrations 
 over large areas east 
 of lax was consistently observed during periods when there were both 
 westerly winds and high air traffic volumes , typically all daylight 
 hours and well into the night .
figure 3 shows 
 the consistency of the patterns over eight monitoring runs at various 
 times of day , displayed in each row by similarity of spatial scale . in directions other than the downwind direction ,
figures 3(c)(e ) include concentrations measured upwind of the 
 lax boundary ( these are indicated by faint yellow lines within the 
 noise contour ) ; the concentrations recorded were typical of the coastal 
 baseline concentrations , less than 10  000 particles per cm ( also see figure s.8 in si ) .
of 
 possible other pn sources , a large refinery is located south of the 
 airport but we did not observe elevated pn or other pollutant concentrations 
 directly downwind of this source .
in general , industrial point sources 
 of pollution in the los angeles air basin are very tightly regulated 
 by the south coast air quality management district .
we did not observe distinct day versus night differences , 
 as might 
 be expected based on the large change in meteorologically driven dilution 
 between day and night for ground level sources .
it appeared that the 
 distant impacts we observed downwind of lax required sufficient wind 
 speeds for the jet climbing and landing emissions to reach the ground , 
 as observed in yu et al .
, 2004 at lax 
 and hong kong international airports and carslaw et al . 2006 at heathrow airport . at lax ,
this probably corresponded 
 to the development of the on - shore sea breezes that typically started 
 46 h after sunrise and lasted until 36 h after sunset .
we also did not see the impacts of individual 
 jets at the distances 
 monitored , but the merging of individual jet impacts is not unexpected 
 at distances of multiple km .
considering the frequency of landings 
 and takeoffs ( > 90 per hour from 09002100 ) , at an average wind speed of 4 m / s , for example , an incoming 
 parcel of air will travel only about 160 m before another jet landing 
 or takeoff occurs . under normal daytime air turbulence and the enhanced 
 turbulence produced by jets , significant
the 
 generally smooth increases and decreases observed across the length 
 of transects at such distances are additional evidence that mixing 
 of plumes occurs .
examples of these smooth concentration increases 
 for individual transects are shown in figures s.6 and s.7 in the si .
the consistent and distinctive spatial 
 pattern of elevated concentrations 
 was aligned to prevailing westerly winds and landing jet trajectories , 
 and roughly followed the shape of the contours of noise from landing 
 jets , indicating that landing jets probably are an important contributor 
 to the large downwind spatial extent of elevated pn concentrations . 
 as defined by the international civil aviation organization ,
typical 
 engine thrust during landing is 30% , as compared to 100% for takeoff 
 and 85% for the climbing phase .
2011 calculated 18% of total nox emissions from landings , with 12% from taxiing 
 and holding , 18% from takeoff , and 52% from the climb and climb out 
 phases , respectively .
when the extra upwind distance of the climb 
 and climb out phases are taken into account , the landing approach 
 emissions likely produce a significant fraction of the increased pn 
 concentrations observed downwind .
the 
 downwind location of the impact changed with shifts in the prevailing 
 wind direction , although significant shifts in wind direction during 
 the daytime are not typical of this area of los angeles .
figure 4(a ) and ( b ) illustrate 
 one such change in impacted locations due to a shift in wind direction 
 on a gusty day with frontal weather that also resulted in cleaner 
 upwind baseline pn concentrations of less than 5000 particles / cm .
the impacted locations were aligned along the ne direction 
 during 20002220 h when winds were from w to wsw ( 250280 ) . 

the impact then moved southwards between 22200000 h as winds 
 turned more w to wnw ( 280330 ) . during this shift , the 
 impact centerline moved by 5.5 km on transects 810 km east 
 of lax .
monitoring was also conducted during n to ne prevailing 
 winds that tend to occur late at night in november and december ( 21002300 ) .
this n to ne wind direction resulted in impacts 
 that were centered south of the airport ( figure 4(c ) ) .
the pn concentrations in this southerly impact were roughly 
 twice as high as on other days , in part because the baseline pn concentrations 
 reflected urban air from northerly winds instead of marine air from 
 westerly winds .
onshore westerly winds 
 are common during midday 
 hours , even in winter . as a result
, areas of elevated pn concentrations 
 downwind and east of lax likely occur in all seasons .
monitoring in 
 different seasons demonstrated the consistent year round presence 
 of this impact .
examples of similarly extensive impacts in non - summer 
 months are shown in the si ( figures s.8 
 and s.9 ) . over large areas downwind of
figure 5(a)(c ) show nearly indistinguishable spatial 
 patterns for pn , bc , and no2 concentration measured simultaneously 
 at distances of 9.512 km from lax .
this suggests a common 
 source for these pollutants , although the bc concentration increases 
 were not large when compared to pn and nox , about 0.51 g / m at 810 km downwind . 
 while jet aircraft are not known to produce large amounts of bc , two 
 studies found elevated bc from plane takeoffs at lax .
2011 measured an increase of about 1 g / m of bc due to plane activity 140 m downwind of the runway . 

2008 measured increases 
 in bc concentration of several g / m during takeoff 
 events near the eastern lax boundary , but also observed elevated bc 
 concentrations at all times . at a smaller airport , dodson et al .
2009 found median contributions of about 0.1 g / m , about one - quarter of total bc measured at five sites ranging 
 in downwind distance from 0.33.7 km , and also observed departures 
 producing about twice the impact as arrivals .
therefore , it appears 
 some jets at lax are capable of producing measurable increases in 
 bc , particularly at takeoffs
. spatial pattern of simultaneously measured pollutants 
 during 14001530 
 on june 27 , 2013 .
panels ( a)(c ) show data measured by the uw mmp and ( d)(f ) 
 show data measured by the usc mmp .
spatial patterns of simultaneously measured pn and pas response 
 ( pb  pah and ec ) were also similar on transects 4.57.5 
 km from lax ( figure 5(d)(e ) ) .
this was likely due to smaller lax related contributions 
 compared to baseline concentrations , thus reducing the signal - to - noise 
 ratio .
overall , the top quartile concentrations ( highly impacted ) 
 of all 
 pollutants were about three times higher than the lowest quartile 
 within 7.5 km from lax and two times higher at 12 km distance . in 
 addition , concurrent sampling with the two mobile platforms demonstrated 
 high temporal ( si figure s.10 ) and spatial 
 consistency ( si figure s.11 ) for pn measurements .
pn concentration 
 increases from ground level line sources such as freeways , under conditions 
 of daytime crosswind dilution , decrease exponentially with increasing 
 downwind distance and return to baseline concentrations within 200300 
 m. the two n  s freeways ( i-405 
 and i-110 that run perpendicular to the prevailing winds ) did not 
 contribute appreciably to elevated pn concentrations in areas where 
 we observed large impacts from lax on pn concentrations .
this is illustrated 
 in figure 6 , which contains two enlargements 
 to show the increase in pn number concentrations over approximately 
 250 m distance downwind of i-405 , a distance and an increase in pn 
 concentration that is not discernible at the scale of figures 2 and 3 .
the panel in figure 6(c ) at 1:10  000 scale shows the pn concentration 
 increase of about 24  000/cm . the maximum pn concentration 
 was not immediately downwind of the freeway
because at this location 
 there is an elevated overpass and some distance is needed for emissions 
 to reach the ground .
comparison of the spatial scale of freeway impacts compared 
 to 
 airport impacts for monitoring during nighttime on august 2324 , 
 2013 . for the days we captured the fullest downwind extent 
 of the impact 
 under typical daytime wind conditions ( august 15 , 23 , and 24 )
, we 
 calculated an integrated pn impact above baseline pn concentrations 
 of 2.3 , 1.6 , and 1.1  10 ( particles / cm )  km , respectively .
impacted areas 
 were calculated using arcgis spatial analysis tools and were conservatively 
 defined as areas where increased pn concentration were at least double 
 the baseline concentrations measured north and south of the impact 
 zone .
the resulting impact areas were 3065 km . 
 for comparison , a less conservative criterion for defining the impact 
 area such as a 50% or 33% increase over baseline pn concentrations 
 increased the impacted area by 40% and 80% , respectively .
to 
 calculate pn impacts downwind of freeways , we combined the exponential 
 regression fit of near - freeway measurements made downwind of i-405 
 by zhu et al .
2002a with updated average 
 daytime on - freeway pn concentrations taken from li et al .
pn concentrations 
 were at least double the baseline pn concentrations of 15  00020  000 
 particles / cm for 90130 m downwind .
this resulted in a concentration - weighted impact area of 
 29303930 ( particles / cm )  km per 
 km of freeway length . based on these concentration - weighted 
 impact areas , 280790 
 km of freeway
are needed to produce the equivalent pn - concentration - weighted 
 impact area of lax .
( the less conservative criteria resulted in ranges 
 of freeway length of 3401000 km and 4301100 km for 
 thresholds of 50% and 33% , respectively . )
there are only about 1500 
 km of freeways and highways in los angeles county .
therefore , lax should be considered one of the most important 
 sources of pn in los angeles .
for comparison , within the 60 km area of elevated pn concentrations downwind and east of lax , 
 the 1525 km of freeways contributed less than 5% of the pn 
 concentration increase .
this provided 
 the several hours necessary for a single mobile platform to monitor 
 a sufficient number of transects to cover the large area impacted 
 by lax emissions . at airport locations
where the prevailing wind direction 
 frequently shifts during the day , multiple platforms would be necessary 
 to quickly capture the full spatial extent of emissions impacts to 
 surrounding air quality .
the emissions from lax are likely not 
 unique on a per - activity basis .
the large area of impact from lax 
 suggests that air pollution studies involving pn , localized roadway 
 impacts , or other sources whose impacts are in the influence zone 
 of a large airport should carefully consider wind conditions and whether 
 measurements are influenced by airport emissions .
source apportionment 
 of specific airport sources or activities 
 was beyond the scope of our study but would be necessary to evaluate 
 the effectiveness of possible mitigation options .
differing no2 to nox ratios at different levels 
 of engine thrust might be used to distinguish 
 the contributions of jet landing , idling or takeoff activities .
takeoff 
 and idling emission also differ in surface properties ( i.e. , the ratio 
 of active surface area to surface bound photoionizable species ) and particle size distributions differ between 
 aircraft and ground support equipment emissions .
 we 
 measured the spatial pattern of particle number ( pn ) concentrations 
 downwind from the los angeles international airport ( lax ) with an 
 instrumented vehicle that enabled us to cover larger areas than allowed 
 by traditional stationary measurements . 
 lax emissions adversely impacted 
 air quality much farther than reported in previous airport studies . 
 
 we measured at least a 2-fold increase in pn concentrations over unimpacted 
 baseline pn concentrations during most hours of the day in an area 
 of about 60 km2 that extended to 16 km ( 10 miles ) downwind 
 and a 4- to 5-fold increase to 810 km ( 56 miles ) downwind . 
 
 locations of maximum pn concentrations were aligned to eastern , downwind 
 jet trajectories during prevailing westerly winds and to 8 km downwind 
 concentrations exceeded 75  000 particles / cm3 , more 
 than the average freeway pn concentration in los angeles . during infrequent 
 northerly winds , the impact area remained large but shifted to south 
 of the airport . the freeway length that would cause an impact equivalent 
 to that measured in this study ( i.e. , pn concentration increases weighted 
 by the area impacted ) was estimated to be 280790 km . 
 the total 
 freeway length in los angeles is 1500 km . 
 these results suggest that 
 airport emissions are a major source of pn in los angeles that are 
 of the same general magnitude as the entire urban freeway network . 
 
 they also indicate that the air quality impact areas of major airports 
 may have been seriously underestimated . 
Introduction
Materials and
Methods
Results
and Discussion