Four neighborhoods
in the metropolitan Boston area near Interstate 93 (I-93; 1.5 ×
105 vehicles per day in all seasons40 ) were studied: Somerville, Dorchester/South Boston (referenced
as “Dorchester”), Chinatown, and Malden (Figure1 ). The areas included both mixed residential-commercial
areas (Somerville, Dorchester, and Malden) and a highly urban area
with tall buildings, street canyons and multiple highways (Chinatown).
Somerville and Dorchester contained both near-highway (<400 m)
and urban background (>1000 m) areas; Chinatown (near-highway)
and
Malden (urban background) were paired because they have demographically
similar populations and Chinatown was too small to contain a background
area. We did not identify significant nonroad UFP sources (e.g., industry,
energy generation, shipping) in any of the study areas. Diesel vehicles
contributed ∼3.8% of highway traffic and <5% of local traffic
in all of the study areas.41 ,42 More detailed descriptions
of the study areas are available elsewhere.5 (link),8 (link),43 ,44 Mobile monitoring with
the Tufts Air Pollution Monitoring Laboratory
(TAPL) was conducted in Somerville between September 2009 and September
2010, in Dorchester between September 2010 and July 2011, and in Chinatown
and Malden between August 2011 and July 2012 (Supporting Information , SI, Table S1).43 ,44 The impacts of nonsimultaneous monitoring in the study areas (i.e.,
interannual variation in TRAPs measured at an EPA monitoring station)
were small compared to seasonal and diurnal differences in PNC, and
were therefore assumed to not play an important role in model differences.43 Monitoring was conducted under a wide range
of conditions at different times between the hours of 04:00 and 22:00
on 34–46 days per neighborhood distributed across all seasons
(∼21–70 h per season) and days of the week.43 This was more monitoring than suggested by Van
Poppel et al.,45 (link) who concluded that 3–16
h of mobile monitoring per season sufficiently characterized spatial
PNC variation in their neighborhoods. On each monitoring day the TAPL
was driven over a fixed route in one neighborhood for 2–6 h
at <20 m/s (72 km/h; mean and median = 5 m/s = 18 km/h). PNC was
monitored at 1-s intervals using a butanol condensation particle counter
(Dp,50 = 4 nm; CPC 3775, TSI, Shoreview,
MN) and matched to locations with a Garmin V GPS unit. In addition,
continuous monitoring for model performance evaluation was conducted
with a second CPC (identical to the one in the TAPL) at the Boston
Globe site ∼20 m east of I-93 in Dorchester between March and
May 2011 (Figure1 ).
The CPC used for
mobile monitoring was manufacturer-calibrated
at the start of the study in September 2009 and again in July 2011,
and the CPC used at the Globe site was received from TSI in March
2011. Side-by-side measurements by these CPCs differed by <3%.44 PNC measurements were censored for flow rate
errors (2% of observations) and for potential self-sampling of TAPL
exhaust when the TAPL speed dropped below 1.4 m/s (5 km/h; ∼14%
of observations, mainly during complete stops at intersections).44 Using the Particle Loss Calculator, we estimated
combined inlet and tubing particle losses of <10%.46 GPS coordinates >20 m from the centerline of the nearest
road (due to poor GPS reception in street canyons) were moved to the
monitoring route centerline using ArcGIS 10.1 (ESRI, Redmond, CA;
6% of data in Chinatown only).43 One-second PNC measurements were assigned spatial variables using
ArcGIS. GIS variables (e.g., road type, road features including width
and curb type, and distance and direction from I-93 or other major
roads) were obtained from MassGIS.47 ,48 Distances
from major intersections with estimated average vehicle delays ≥20
s were also calculated for Chinatown.49 Because higher-resolution covariate data were not available,
each
one-second PNC measurement was assigned hourly meteorological and
traffic values using SAS version 9.3 (SAS Institute, Inc., Cary, NC).
Hourly wind speed and direction (7.9 m above ground level) and temperature
(2 m above ground level) measurements for development of all models
were obtained from Logan International Airport.50 Hourly traffic volume and average speed on interstate highways
were provided by the Massachusetts Department of Transportation (stakeholder.traffic.com).
Neighborhood-specific real-time traffic and wind data were not available.
in the metropolitan Boston area near Interstate 93 (I-93; 1.5 ×
105 vehicles per day in all seasons40 ) were studied: Somerville, Dorchester/South Boston (referenced
as “Dorchester”), Chinatown, and Malden (Figure
areas (Somerville, Dorchester, and Malden) and a highly urban area
with tall buildings, street canyons and multiple highways (Chinatown).
Somerville and Dorchester contained both near-highway (<400 m)
and urban background (>1000 m) areas; Chinatown (near-highway)
and
Malden (urban background) were paired because they have demographically
similar populations and Chinatown was too small to contain a background
area. We did not identify significant nonroad UFP sources (e.g., industry,
energy generation, shipping) in any of the study areas. Diesel vehicles
contributed ∼3.8% of highway traffic and <5% of local traffic
in all of the study areas.41 ,42 More detailed descriptions
of the study areas are available elsewhere.5 (link),8 (link),43 ,44 Mobile monitoring with
the Tufts Air Pollution Monitoring Laboratory
(TAPL) was conducted in Somerville between September 2009 and September
2010, in Dorchester between September 2010 and July 2011, and in Chinatown
and Malden between August 2011 and July 2012 (
interannual variation in TRAPs measured at an EPA monitoring station)
were small compared to seasonal and diurnal differences in PNC, and
were therefore assumed to not play an important role in model differences.43 Monitoring was conducted under a wide range
of conditions at different times between the hours of 04:00 and 22:00
on 34–46 days per neighborhood distributed across all seasons
(∼21–70 h per season) and days of the week.43 This was more monitoring than suggested by Van
Poppel et al.,45 (link) who concluded that 3–16
h of mobile monitoring per season sufficiently characterized spatial
PNC variation in their neighborhoods. On each monitoring day the TAPL
was driven over a fixed route in one neighborhood for 2–6 h
at <20 m/s (72 km/h; mean and median = 5 m/s = 18 km/h). PNC was
monitored at 1-s intervals using a butanol condensation particle counter
(Dp,50 = 4 nm; CPC 3775, TSI, Shoreview,
MN) and matched to locations with a Garmin V GPS unit. In addition,
continuous monitoring for model performance evaluation was conducted
with a second CPC (identical to the one in the TAPL) at the Boston
Globe site ∼20 m east of I-93 in Dorchester between March and
May 2011 (Figure
The CPC used for
mobile monitoring was manufacturer-calibrated
at the start of the study in September 2009 and again in July 2011,
and the CPC used at the Globe site was received from TSI in March
2011. Side-by-side measurements by these CPCs differed by <3%.44 PNC measurements were censored for flow rate
errors (2% of observations) and for potential self-sampling of TAPL
exhaust when the TAPL speed dropped below 1.4 m/s (5 km/h; ∼14%
of observations, mainly during complete stops at intersections).44 Using the Particle Loss Calculator, we estimated
combined inlet and tubing particle losses of <10%.46 GPS coordinates >20 m from the centerline of the nearest
road (due to poor GPS reception in street canyons) were moved to the
monitoring route centerline using ArcGIS 10.1 (ESRI, Redmond, CA;
6% of data in Chinatown only).43 One-second PNC measurements were assigned spatial variables using
ArcGIS. GIS variables (e.g., road type, road features including width
and curb type, and distance and direction from I-93 or other major
roads) were obtained from MassGIS.47 ,48 Distances
from major intersections with estimated average vehicle delays ≥20
s were also calculated for Chinatown.49 Because higher-resolution covariate data were not available,
each
one-second PNC measurement was assigned hourly meteorological and
traffic values using SAS version 9.3 (SAS Institute, Inc., Cary, NC).
Hourly wind speed and direction (7.9 m above ground level) and temperature
(2 m above ground level) measurements for development of all models
were obtained from Logan International Airport.50 Hourly traffic volume and average speed on interstate highways
were provided by the Massachusetts Department of Transportation (stakeholder.traffic.com).
Neighborhood-specific real-time traffic and wind data were not available.
