Table 1.
Summary of Chelsea and Boston Mobile-Monitoring Modelsa
| Chelsea (adj. R2 = 0.46) (n = 508 871)b | Boston (adj. R2 = 0.43) (n = 293 475)b | |||
|---|---|---|---|---|
| variable | β coefficient | std. error | β coefficient | std. error |
| (intercept) | 11.28 | 0.011 | 9.894 | 0.019 |
| temperature (per °C) | −0.0331 | 0.0001 | −0.0652 | 0.0002 |
| wind speed (per m/s) | −0.1183 | 0.0006 | −0.1270 | 0.0008 |
| relative humidity (per %) | −0.0047 | 0.0001 | n/a | n/a |
| pressure (per mb difference from 1013.25 mb)c | −0.0171 | 0.0001 | n/a | n/a |
| cloud cover (1–10 scale; 1 = clear, 10 = overcast) | n/a | n/a | 0.0467 | 0.0007 |
| solar radiation (per kW/m2) | 0.316 | 0.005 | n/a | n/a |
| solar radiation, lagged 1 h (per kW/m2) | n/a | n/a | 1.787 | 0.010 |
| wind direction (rel. to no wind) | ||||
| NNE (0–45°) | −0.670 | 0.011 | 0.398 | 0.015 |
| ENE (45–90°) | −0.711 | 0.011 | 0.386 | 0.014 |
| ESE (90–135°) | −0.281 | 0.011 | 0.184 | 0.015 |
| SSE (135–180°) | −0.009d | 0.011 | 0.563 | 0.017 |
| SSW (180–225°) | −0.081 | 0.011 | 0.731 | 0.015 |
| WSW (225–270°) | −0.297 | 0.011 | 0.483 | 0.015 |
| WNW (270–315°) | −0.522 | 0.010 | 0.332 | 0.015 |
| NNW (315–360°) | −0.613 | 0.011 | 0.375 | 0.015 |
| downwind of Logan Airport (±15°; binary) | n/a | n/a | 0.254 | 0.006 |
| type of hour/day (rel. to weekdays 18:00–06:00) | ||||
| weekday morning rush (06:00–09:00) | 0.110 | 0.003 | 0.167 | 0.006 |
| weekday midday (09:00–15:00) | −0.055 | 0.003 | −0.360 | 0.006 |
| weekday evening rush (15:00–18:00) | −0.263 | 0.003 | −0.135 | 0.007 |
| Saturday or Sunday (all hours) | −0.437 | 0.003 | −0.306 | 0.007 |
| I-93 traffic (hourly ln(vehicles/km/h)) | 0.221 | 0.002 | 0.376 | 0.007 |
| distance from bus route (per km) | n/a | n/a | −0.974 | 0.012 |
| distance from road classes 1–4 (per km)e | −0.457 | 0.008 | −0.875 | 0.021 |
| near major-road intersection (<150 m; binary) | 0.080 | 0.002 | n/a | n/a |
| near interstate (<100 m; binary) | n/a | n/a | 0.191 | 0.007 |
| near interstate (<300 m; binary) | n/a | n/a | 0.082 | 0.005 |
| distance from elevated US-1 (per km) | −0.157 | 0.001 | n/a | n/a |
| near US-1 (<200 m; binary) | 0.077 | 0.002 | n/a | n/a |
| distance from residential land use (per km) | 0.557 | 0.015 | 0.833 | 0.024 |
| distance from transportation land use (per km) | n/a | n/a | −0.181 | 0.006 |
| distance from open spaces (any size; per km) | n/a | n/a | 0.310 | 0.011 |
| distance from open spaces (>5000 m2; per km) | 0.443 | 0.009 | n/a | n/a |
Models predict ln(PNC), where PNC is in units of particles/cm3. Variables are significant at p < 0.05 unless specified otherwise.
Number of 1-sec data points used to build the model.
Pressure was adjusted from the absolute scale to be the arithmetic difference from 1013.25 mb (standard atmosphere) to improve interpretability of the β coefficient. For example, a measured pressure of 1005 mb was expressed in the model as −8.25 mb.
Not significant at p < 0.05.
Road classes are defined by functional classification and vehicle access: limited-access highway (Class 1), nonlimited-access multilane highway (Class 2), other numbered routes (Class 3), and major arterials and collectors (Class 4). For reference, Class 5 and 6 roads are minor streets and roads.