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# The local pollution cost of flying from Sonoma
- URL: https://airport.mmalc.com/sts-pollution/
- Published: 2026-09-21T00:34:23.000Z
- Updated: 2026-09-26T17:11:16.000Z
- Description: Flying from Charles M. Schulz–Sonoma County Airport (STS) saves some passengers a drive to San Francisco or Oakland. Saving the drive doesn't mean saving on pollution — one local departure can produce 19 to 35 times the nitrogen oxide pollution of the car journeys it replaces.
- Author: Malcolm Crawford
- Tags: pollution, climate change, airport

The argument sounds straightforward: fill a plane in Sonoma, and avoid sending its passengers south in dozens of cars to SFO or OAK. But counting vehicles doesn't show how much pollution they produce. The useful comparison is what each option puts into Sonoma's air while carrying the same number of people.

Take an Embraer 175 with 60 passengers. Using local taxi-time records as a guide, published engine data give an estimate of about 3 kilograms of nitrogen oxides (NOₓ) during taxi, takeoff, and initial climb. If there were no STS departure and those passengers instead drove the Sonoma County part of the trip to SFO or Oakland in gasoline cars or light trucks, they'd release about 0.09 to 0.16 kilograms. That makes the departure roughly 19 to 35 times the NOₓ of the drive, assuming 1.5 passengers per vehicle and 30 miles inside the county \[1–5\].

Put another way, one departure emits as much NOₓ as 19 to 35 planeloads of passengers driving those 30 miles, or roughly 1,100 to 2,100 people. On nitrogen oxides, the idea that flying from STS is the cleaner choice doesn't survive that arithmetic.

The focus here is local air pollution. The aircraft estimate includes climb to about 3,000 feet, a standard airport-area boundary rather than a line drawn around the county. Emissions at SFO and OAK, and the climate effects of the journey, sit outside this comparison. The notes explain the assumptions.

*This comparison is deliberately local. Part of its point is that what seems obvious about pollution and climate isn't always so. Working out the whole trip, including the flight from SFO or Oakland, is more complicated, but the answer usually points the same way. That calculation is the subject of* [*Growing air service at STS runs against Sonoma's climate goals*](https://airport.mmalc.com/sts-climate/)*.*

## NOₓ matters most

NOₓ is the pollutant to watch here. It irritates airways on its own — nitrogen dioxide has its own EPA health standard — and it drives the formation of ozone and fine particles downwind. EPA's national modeling backs up giving it this weight: for most transport sectors in most U.S. locations studied, NOₓ has done more than organic gases to form ozone \[6–8\]. And the gap on NOₓ dwarfs anything else in this comparison: about 2.9 kilograms of it, added by a single departure.

Not every pollutant points the same way. Organic gases favor the aircraft, and so can carbon monoxide when the taxi is short. But neither comes close to offsetting that NOₓ gap. The organic-gas saving is 0.14 to 0.20 kilograms — 14 to 21 times smaller than the added NOₓ — and it depends on the mix of compounds involved, some of which, like benzene and formaldehyde, are toxic in their own right regardless of which side produces more \[2, 4, 7, 9\]. Particles aren't quantified here, for lack of matched aircraft-and-road data \[10, 11\].

Any claim that flying locally improves Sonoma's air has to reckon with the NOₓ excess. The smaller, partly offsetting differences in other pollutants don't change that.

## The gap survives more driving

The main example assumes 1.5 travelers per car. Even if every passenger drove alone in a gasoline light truck and each vehicle made an empty return, though, the aircraft would still emit more than six times as much NOₓ.

A shorter wait on the ground doesn't close the gap either. The main estimate uses STS records: reporting airlines' 2025 departures had a median gate-to-liftoff time of 15 minutes \[5\]. With only five minutes of taxi and idle, the E175 still produces about 17 to 31 times the road NOₓ. Takeoff and climb account for most of it.

Several other choices in the comparison favor flying. Every road vehicle is treated as gasoline-powered, although electric cars produce no tailpipe NOₓ. The drive passengers still make to STS is left out, along with aircraft auxiliary power and ground equipment. Counting those would widen the gap \[10\].

The E175 also gives flying a more favorable result than the 737-800 case. A Boeing 737-800, also in the airport's planning model \[1\], releases about three times as much NOₓ per departure with the engine version used here, and nearly 40% more per passenger even at 140 passengers. The notes give the inputs for both aircraft.

## Driving instead would add a few percent to traffic on US-101

Moving passengers to SFO or OAK could add to congestion even if their cars emitted much less NOₓ than a local departure. If every STS passenger switched to a car trip past Petaluma, averaging 1.5 passengers per car, those journeys would amount to roughly 2% to 5% of daily traffic on US-101 south of Petaluma, depending on passenger numbers and empty returns \[12, 13\]. Flight times spread those journeys across the day, though, so only some would land in commute hours. The notes examine those timing scenarios \[14, 15\].

Sharing more journeys can reduce even that modest addition to traffic, while cleaner vehicles can widen the road alternative's NOₓ advantage.

## The aircraft's score is close to fixed, and the alternatives' can improve

To a first approximation, the aircraft side of this comparison won't get better. Regional partners carry about three-quarters of STS's passengers \[16\], and US pilot contracts generally cap their aircraft at 76 seats and 86,000 pounds, which keeps the E175 the workhorse \[17\]. The newer E175-E2 has been held back by the same limits \[18\], and SkyWest, the largest E175 operator, has ordered 16 more E175s and secured delivery positions for 44 others through 2032 \[19\]. Larger aircraft don't help: the 737-800 emits about three times as much NOₓ per departure. The engines behind this comparison will be flying for years.

Sonoma already has a way to make some of those car journeys with fewer vehicles. Groome Transportation runs scheduled airport buses to both SFO and OAK, with stops including STS, Santa Rosa, Rohnert Park, and Petaluma \[20\]. A bus carrying 30 people replaces 20 cars at the assumed 1.5 people per car. Car sharing can reduce vehicle numbers too, although empty return trips still matter.

The road side has room to move beyond that. Zero-emission vehicles, plug-in hybrids included, were only 6.5% of California's registered vehicles in 2024 \[21\], and every increase in the electric share widens the gap: with half of road trips zero-tailpipe, the NOₓ ratio would be about 38 to 69 times, and at 80%, about 96 to 173\. Shared buses cut vehicle numbers and could go further as cleaner or electric buses arrive. Better service would help too: departures timed to flights, reliable and comfortable enough that passengers choose them, and through-ticketing and baggage handling so the ground trip feels like part of the flight. The proposed satellite terminal is one way of bundling several of these \[22, 23\]. It would have to pass the same calculation as every other option, including its own buses' emissions.

Two things would help now. Measuring the existing airport buses' emissions, loads, and empty mileage would show whether they beat the cars they replace, rather than assuming it — at the assumed load, a bus must emit less per mile than 20 cars to come out ahead. And claims that flying avoids driving should show their arithmetic: the aircraft and passengers actually flown, the journeys actually replaced, and the effects on traffic as well as emissions.

Flying from STS can save time, and that convenience is real. But this comparison gives flying several advantages and still finds far more NOₓ from the local departure than from the car journeys it replaces. That score won't move much. The alternatives' can, and they're already ahead.

*Claude (Anthropic) contributed to research and drafting. Codex (OpenAI) assisted with revisions.*

---

## Sources and calculation notes

Sources checked September 20, 2026\. The calculations are illustrative. They aren't measurements of an individual STS flight or a commissioned health-impact assessment.

**Scope.** The comparison sets emissions from an STS departure against those from the Sonoma portion of an alternative road trip. The aircraft estimate includes taxi, takeoff, and climb to about 3,000 feet, a standard boundary for estimating airport-area emissions. It is an approximation, not a calculation mapped to the county line. Flight paths and winds affect where the pollution travels. Approach, taxi-in, emissions at SFO and OAK, and the climate effects of flying sit outside this comparison.

**Emissions and health.** An emissions ratio is not a health-impact ratio: 35 times the NOₓ does not mean 35 times the harm. Exhaust released in a climb spreads differently from exhaust beside a road. Health effects depend on atmospheric chemistry, exposure, and who breathes it.

NOₓ contributes to ozone, fine particles, acidification, and nutrient pollution \[6\]. Organic gases also contribute to ozone; some help form particles, and some are directly toxic \[7, 9, 24\]. Their effects depend on the chemical mixture. There is no universal exchange rate between a kilogram of NOₓ and a kilogram of organic gases.

EPA's mobile-source study found larger ozone contributions from NOₓ for most sectors and locations, with exceptions including parts of southern California \[8\]. These are total source contributions, not equal-mass comparisons. They cannot establish that NOₓ is more harmful per kilogram. EPA separately publishes per-ton ozone benefit estimates for both NOₓ and volatile organic compounds \[25\]. Those estimates vary by sector and are not coefficients for STS aircraft or Sonoma airport-access traffic. The mass differences here therefore describe the scale of the emissions trade-off, not proof that one pollutant cancels out another.

**Aircraft.** The E175 example uses two CF34-8E5 engines and an assumed 60 passengers. Embraer identifies this engine as an E175 option \[26\]. The ICAO engine databank, Issue 32, gives these inputs for record 07P27GE226 \[2\], rounded here for readability:

- Taxi and idle: fuel flow 0.065213 kg/second; NOₓ 4.592550 g/kg of fuel; 14.3 minutes.
- Takeoff: fuel flow 0.652021 kg/second; NOₓ 14.552807 g/kg of fuel; 0.7 minutes.
- Climb: fuel flow 0.535542 kg/second; NOₓ 12.174887 g/kg of fuel; 2.2 minutes.

For each stage, multiply fuel flow by the NOₓ factor and seconds, then multiply by two engines and divide by 1,000 to convert grams to kilograms. Using the unrounded source values gives:

**0.514 kg taxi and idle + 0.797 kg takeoff + 1.721 kg climb = 3.032 kg per departure.**

The ground-time allowance is guided by the 15-minute median in the 2025 STS reporting-carrier data \[5\]. BTS measures gate departure to wheels off, which includes the takeoff roll. To avoid counting that interval twice, the example subtracts the full 0.7-minute reference takeoff allowance, leaving 14.3 minutes at the idle setting. This is an approximation, not a measured 14.3 minutes with both engines idling. It does not resolve engine-start timing, single-engine taxiing, or the airborne portion of the reference takeoff mode. Running both engines at idle for the whole allowance probably overstates idle time, since engines are usually started around pushback and some taxiing may be done on one engine. That runs against the aircraft, so the taxi assumption is not one of the comparison's allowances in flying's favor. Takeoff and climb retain the reference times \[3\]; together they account for about 2.52 kg NOₓ. Engine version, thrust setting, and conditions also matter. Approach and taxi-in are excluded.

For context, STS averaged 64.7 passengers per scheduled departure in the year to June 2026 \[16\] — a reference point for the assumed 60 passengers, not a measured E175 load.

**The E175 is the generous case.** The 737-800 calculation uses two CFM56-7B26 engines, databank record 8CM051, and 140 passengers \[2\]. Its inputs, using the same operating times, are:

- Taxi and idle: fuel flow 0.113 kg/second; NOₓ 4.7 g/kg of fuel; 14.3 minutes.
- Takeoff: fuel flow 1.221 kg/second; NOₓ 28.8 g/kg of fuel; 0.7 minutes.
- Climb: fuel flow 0.999 kg/second; NOₓ 22.5 g/kg of fuel; 2.2 minutes.

For both engines, using the same method as above:

**0.911 kg taxi and idle + 2.954 kg takeoff + 5.934 kg climb = 9.799 kg per departure.**

That's about 3.23 times the E175's departure total. Dividing each total by its assumed passenger load gives about 70 g per passenger for the 737 and 51 g for the E175 — about 38.5% more for the 737, calculated before rounding. The E175 is therefore more favorable to flying within these two examples. This is not a ranking of all E175s and 737s: other 737 engine versions have lower NOₓ figures, and different passenger loads can change the ranking \[2\].

**Road.** The Air Force's June 2025 emissions guide gives Sonoma County factors for 2026 from California's EMFAC2021 model: 0.073 g NOₓ per mile for gasoline passenger cars and 0.132 g per mile for gasoline light trucks up to 8,500 pounds \[4\]. These are annual, speed-averaged vehicle-class factors from an older model version, not a measured average for airport travelers. The formula is:

**passengers ÷ passengers per vehicle × miles × grams per mile ÷ 1,000 = kilograms.**

The 30 miles is an illustrative assumption, not an average across passenger origins.

**Exclusions.** The comparison leaves out drives to STS, aircraft auxiliary power and ground equipment, and any electric share of the road fleet. All three understate the gap.

| Road scenario for 60 passengers                                         | Road NOₓ | E175-to-road ratio |
| ----------------------------------------------------------------------- | -------- | ------------------ |
| Gasoline cars; 1.5 passengers per car; 30 miles                         | 0.088 kg | 35:1               |
| Gasoline light trucks; 1.5 passengers per vehicle; 30 miles             | 0.158 kg | 19:1               |
| Gasoline light trucks; one passenger per vehicle; 30 miles              | 0.238 kg | 13:1               |
| Same one-person vehicles; 60 miles each, allowing an equal empty return | 0.475 kg | 6.4:1              |

The last row is the scenario most favorable to the aircraft among those tested. It tests the effect of extra driving and doesn't describe every taxi or rideshare trip. Airport-wide ratios would need the aircraft and loads actually flown, passenger origins, and the vehicle mix.

The "planeloads" figure in the text divides the departure's 3.032 kg by the road NOₓ for 60 passengers: 3.032 ÷ 0.0876 = 34.6 (cars) and 3.032 ÷ 0.1584 = 19.1 (light trucks). Multiplying by 60 passengers gives about 2,080 and 1,150 people.

**Electrification scenarios.** The table assumes a stated share of road trips are zero-tailpipe (electric cars or electric buses) and the rest are gasoline cars and light trucks, using the main E175 example (NOₓ 3.032 kg, CO 2.677 kg, hydrocarbons 0.0181 kg). The zero-tailpipe share removes that share of tailpipe emissions from the road side. Electricity generation elsewhere and brake and tire particles are outside the boundary. Ranges span the car and light-truck factors.

| Zero-tailpipe share of road trips | NOₓ, aircraft ÷ road | CO, aircraft ÷ road | Organic gases, aircraft ÷ road |
| --------------------------------- | -------------------- | ------------------- | ------------------------------ |
| None (the article's baseline)     | 19–35×               | 1.4–2.0×            | 0.08–0.12×                     |
| 50%                               | 38–69×               | 2.9–3.9×            | 0.17–0.23×                     |
| 80%                               | 96–173×              | 7–10×               | 0.41–0.58×                     |

The organic-gas result reaches parity at about a 90% zero-tailpipe share (88% to 92% across the car and light-truck factors); beyond that, the aircraft is worse on organic gases too. Diesel coaches aren't modeled, because this article has no Sonoma heavy-duty emission factor. The 6.5% fleet share in the text includes plug-in hybrids, which burn gasoline part of the time; battery-electric vehicles were 1.8 million of the 2.2 million ZEVs registered in 2025 \[21, 27\].

**Bus threshold.** Thirty passengers replace 20 vehicles at 1.5 each. A bus emits less NOₓ than those vehicles if its per-mile factor is below 20 × 0.073 = 1.46 g per mile (against cars) or 20 × 0.132 = 2.64 g per mile (against light trucks), before counting collection trips and empty mileage.

**Traffic scale.** The airport's June 2026 total of 92,759 passengers \[12\] is about 3,092 arriving and departing passengers a day; the January–July 2026 average is 2,383 a day, and 2025's is 2,139\. Caltrans's 2024 traffic-volume table reports 82,000 vehicles a day on US-101 immediately south of the SR 116 junction in Petaluma, counting both directions \[13\]. At 1.5 passengers per car, replacing every STS trip with a drive past that point adds 2,061 vehicle passages a day if cars stay at the airport (2.5% of daily traffic), 3,092 if half the trips need an empty return (3.8%), and 4,123 if every trip does (5.0%), using June. The January–July 2026 average gives 1.9%, 2.9%, and 3.9%, and 2025 gives 1.7%, 2.6%, and 3.5%. Arrivals and departures are already counted, so the figures aren't doubled. These are gross replacement-trip estimates at the assumed occupancy, not an absolute upper bound. One air passenger per car would increase them by 50%; two would reduce them by 25%. Every passenger is assumed to cross this road section, and no existing STS access trips are subtracted. Bus use, other routes, and existing trips across this section would reduce the net addition; passenger-origin and travel-mode data are needed to quantify that adjustment.

For the commute hours, an illustrative schedule uses the 1,191 scheduled STS arrivals and departures in June 2026 \[15\]. It assumes replacement flights at the same times, cars passing Petaluma three hours before departure, and returning passengers passing 90 minutes after landing. Without empty returns, it puts about 135 additional southbound cars at 8 to 9 a.m. and 130 northbound at 5 to 6 p.m., roughly 4% of Caltrans's published peak-direction volumes \[14\]. Varying the timing and passenger allocation, with empty returns for up to half the passenger trips, gives additions equivalent to about 2% to 6% of the published peak flows in the busiest modeled commute hours. Allowing an empty return for every passenger trip raises the upper end to about 8%. The volumes are from 2022 and concern different hours (the published southbound peak is 5 to 6 a.m.), so this is a scale comparison, not a forecast for those hours. Where a road is near capacity, small additions can lengthen queues, so a small share isn't necessarily a small effect \[28\].

Buses: carrying 3,092 passengers a day at an average of 30 passengers per bus would require about 103 passenger-carrying bus passages a day, or 0.13% of the 82,000 daily vehicles. Provisionally counting each bus as two cars gives about one-tenth of the car-only case without empty returns. This is illustrative arithmetic, not a measured equivalence in congestion effects. It excludes trips to reach bus stops and empty bus movements, and does not establish that existing services have spare capacity.

**Passengers per departure.** From the Bureau of Transportation Statistics \[16\], covering scheduled U.S. services excluding freight and mail, with counts rounded to thousands: 411,000 ÷ 5,818 = 70.64; 409,000 ÷ 6,320 = 64.72\. That is a change of −8.39%, with departures up 8.63%. Holding average emissions per departure constant gives about +9.16% per passenger. STS's own totals \[12\] are 457,463 for January–July 2025 and 505,157 for January–July 2026 (+10.4%). They are airport totals, not BTS departing-passenger counts.

The drop in passengers per departure doesn't necessarily mean emptier planes; a shift to smaller aircraft would do the same. Nor does it establish that emissions per passenger rose, because that depends on the aircraft flown in each year. The calculated 9.16% increase assumes average emissions per departure stayed constant. The rising January–July passenger totals cover a different period and basis. Together, these figures show why an assessment needs the actual fleet and departures as well as passenger totals. The aircraft mix is discussed in the earlier article on STS operations \[29\].

**Regional carriers.** BTS's airport summary for STS \[16\] lists SkyWest (58.4% of passengers), Horizon (12.1%), and Envoy (7.1%) among the carriers for the 12 months to June 2026, about 78% together. These are regional operators; the summary doesn't say which aircraft types they fly at STS.

**Other gases.** The carbon-monoxide and organic-gas calculations use the same E175 engine record, operating times, passenger load, and Sonoma road factors as the NOₓ example \[2, 4\]. They are not additional full-trip or health-impact comparisons.

For carbon monoxide, the engine's emission factors are 22.471948 g/kg of fuel at idle, 0.879107 at takeoff, and 0.806906 in climb. Applying the fuel flows and times above gives **2.677 kg CO per departure**. The Sonoma road factors are 1.132 g/mile for gasoline cars and 1.546 g/mile for gasoline light trucks. At 1,200 vehicle-miles (60 passengers ÷ 1.5 × 30 miles), those give **1.358 kg and 1.855 kg CO**, respectively. The aircraft emits about 1.4 to 2.0 times as much under the main assumptions. This result reverses at sufficiently short taxi times, as the table below shows.

For hydrocarbons, the engine factors are 0.149735 g/kg of fuel at idle, 0.005829 at takeoff, and 0.007474 in climb, giving **0.018 kg per departure**. The road factors are 0.131 and 0.183 g/mile of reactive organic gases, giving **0.157 kg and 0.220 kg** for the same car and light-truck journeys. The reported organic-gas difference is about 0.139 to 0.201 kg in the aircraft's favor, against about 2.874 to 2.945 kg additional NOₓ. Aircraft hydrocarbons and road reactive organic gases are not identical measures; these subtractions are comparisons of reported masses, not chemically matched inventories or a health balance \[2, 4, 7\].

**Observed STS taxi times.** The 2025 BTS reporting-carrier files contain 4,668 STS-origin records. Excluding 84 cancellations leaves 4,584 departures with positive taxi-out times: 286 American, 280 Alaska, 417 Envoy, and 3,601 SkyWest flights. Avelo and other non-reporting operations are not represented. The flight-weighted mean is 17.42 minutes, the median is 15, the middle half runs from 12 to 20, and the 95th percentile is 33\. There are 295 observations above 30 minutes (6.4%), and the maximum is 119 \[5\]. These are airport records for the reporting carriers, not an E175-only distribution.

The median describes the middle flight. The mean also has a legitimate per-departure interpretation: it includes each flight's share of the long waits. Using the mean instead (17.42 minutes gate to liftoff, or 16.72 minutes of idle after the 0.7-minute takeoff allowance) raises the E175's NOₓ by about 3%, to 3.119 kg, giving ratios of 20 to 36 against the light trucks and cars, and its CO by about 16%, to 3.103 kg. The table below includes this case, so the choice of the median does not drive the headline. For an annual inventory, use the complete activity data and appropriate engine settings; substituting the median would lose part of the total waiting time. BTS's separate additional-ground-time field for gate returns also needs attention in an inventory. Neither gate delay nor time away from the gate automatically means both engines ran throughout \[5\].

The right-hand tail is consistent with airport-departure research: basic movement and procedures take time, while congestion, weather, and other delays can extend it. Researchers have used lognormal or Erlang distributions, rather than a symmetric normal distribution, for unimpeded taxi-out times \[30\]. A 2025 study of 71 U.S. hub airports also questioned the universal 26-minute combined taxi-in and taxi-out reference. Its small-hub figure was a mean of 19.82 minutes and a median of 18, calculated from quarter-hour averages in 2023, rather than individual flights. It does not supply an STS taxi-out estimate \[31\].

**Taxi-time sensitivity.** The table varies only the two-engine taxi/idle allowance, holding takeoff, climb, passengers, and road travel constant. These durations exclude the separately modeled takeoff stage; they are not directly interchangeable with BTS's gate-to-wheels-off measure. The 19-minute row is the reference case, 14.3 minutes is the main example, and 16.7 minutes is the mean-based case.

| Taxi and idle allowance     | E175 NOₓ  | E175 CO  | E175 hydrocarbons |
| --------------------------- | --------- | -------- | ----------------- |
| 5 minutes                   | 2.698 kg  | 1.042 kg | 0.0072 kg         |
| 10 minutes                  | 2.878 kg  | 1.921 kg | 0.0131 kg         |
| 14.3 minutes                | 3.032 kg  | 2.677 kg | 0.0181 kg         |
| 16.7 minutes (mean-based)   | 3.119 kg  | 3.103 kg | 0.0210 kg         |
| 19 minutes                  | 3.201 kg  | 3.503 kg | 0.0236 kg         |
| Road: gasoline cars         | 0.0876 kg | 1.358 kg | 0.1572 kg ROG     |
| Road: gasoline light trucks | 0.1584 kg | 1.855 kg | 0.2196 kg ROG     |

At 19 minutes, taxi/idle accounts for about 95% of the aircraft's CO and 94% of its hydrocarbons. At the main allowance, the shares are still about 94% and 92%. CO reaches road parity at about 6.8 minutes against the cars and 9.6 minutes against the light trucks. NOₓ changes much less: takeoff and climb alone account for 2.518 kg, or about 83% of the main departure estimate. Short taxi times therefore weaken the CO argument considerably without closing the NOₓ gap.

**Particles.** Particle emissions are not quantified in this article. A comparison would need consistent treatment of aircraft exhaust, road exhaust, brake and tire wear, and road dust, along with particle size, composition, and exposure. Electric vehicles still produce brake and tire particles \[2, 4, 10, 11\].

### Sources

1. Charles M. Schulz–Sonoma County Airport, *Airport Layout Plan Update, Appendix C: Aircraft Noise and Emissions*, May 2, 2022\. Section 1.5 and Table C11 identify the modeled fleet, including Embraer 175s and Boeing 737-800s. A planning model, not a measurement of current flights. [https://sonomacountyairport.org/wp-content/uploads/STS-ALP-Appendix-C-Noise-and-Emissions-Report.pdf](https://sonomacountyairport.org/wp-content/uploads/STS-ALP-Appendix-C-Noise-and-Emissions-Report.pdf?ref=airport.mmalc.com)
2. European Union Aviation Safety Agency, *ICAO Aircraft Engine Emissions Databank*, Issue 32, March 20, 2026\. Worksheet "Gaseous Emissions and Smoke": CF34-8E5 record 07P27GE226, row 284; CFM56-7B26 record 8CM051, row 137\. The E175 calculation uses the current record, rather than its superseded entries. [https://www.easa.europa.eu/en/domains/environment/icao-aircraft-engine-emissions-databank](https://www.easa.europa.eu/en/domains/environment/icao-aircraft-engine-emissions-databank?ref=airport.mmalc.com) Workbook: [https://www.easa.europa.eu/en/downloads/131424/en](https://www.easa.europa.eu/en/downloads/131424/en?ref=airport.mmalc.com)
3. Federal Aviation Administration, *Aviation Emissions and Air Quality Handbook*, appendices. Inventory methods and an example dividing 26 minutes of taxi and idle into 19 outbound and seven inbound. [https://www.faa.gov/sites/faa.gov/files/regulations\_policies/policy\_guidance/envir\_policy/airquality\_handbook/Air\_Quality\_Handbook\_Appendices.pdf](https://www.faa.gov/sites/faa.gov/files/regulations%5Fpolicies/policy%5Fguidance/envir%5Fpolicy/airquality%5Fhandbook/Air%5FQuality%5FHandbook%5FAppendices.pdf?ref=airport.mmalc.com)
4. Department of the Air Force, *Air Emissions Guide for Air Force Mobile Sources*, June 2025\. Table 5-40, printed page 400 (PDF page 421), Sonoma gasoline LDGV and LDGT rows; Section 5.2 and Table 5-5 describe the EMFAC2021 inputs. [https://aqhelp.com/Documents/2025-June Mobile Guide.pdf](https://aqhelp.com/Documents/2025-June%20Mobile%20Guide.pdf?ref=airport.mmalc.com)
5. Bureau of Transportation Statistics, *Reporting Carrier On-Time Performance (1987–present)*, January–December 2025\. Downloaded September 20, 2026, with the California geography filter, then restricted to origin STS. Analysis uses non-canceled records with positive `TAXI_OUT`; carrier counts and distribution statistics are calculated for this article. Download page: [https://transtats.bts.gov/DL\_SelectFields.aspx?QO\_fu146\_anzr=&gnoyr\_VQ=FGJ](https://transtats.bts.gov/DL%5FSelectFields.aspx?QO%5Ffu146%5Fanzr=&gnoyr%5FVQ=FGJ&ref=airport.mmalc.com) Field definitions: [https://www.transtats.bts.gov/Fields.asp?gnoyr\_VQ=FGJ](https://www.transtats.bts.gov/Fields.asp?gnoyr%5FVQ=FGJ&ref=airport.mmalc.com) Taxi-out definition: [https://www.transtats.bts.gov/Glossary.asp?v0qrA=g](https://www.transtats.bts.gov/Glossary.asp?v0qrA=g&ref=airport.mmalc.com) Reporting coverage: [https://www.transtats.bts.gov/ot\_reporting.asp](https://www.transtats.bts.gov/ot%5Freporting.asp?ref=airport.mmalc.com)
6. U.S. Environmental Protection Agency, *Basic Information about NO₂*. [https://www.epa.gov/no2-pollution/basic-information-about-no2](https://www.epa.gov/no2-pollution/basic-information-about-no2?ref=airport.mmalc.com)
7. U.S. Environmental Protection Agency, *Ground-level Ozone Basics*. [https://www.epa.gov/ground-level-ozone-pollution/ground-level-ozone-basics](https://www.epa.gov/ground-level-ozone-pollution/ground-level-ozone-basics?ref=airport.mmalc.com)
8. U.S. Environmental Protection Agency, *Mobile Sector Source Apportionment — Air Quality and Benefits Per Ton*. Summarizes Zawacki et al. (2018), *Mobile source contributions to ambient ozone and particulate matter in 2025*, and Wolfe et al. (2019), *Monetized health benefits attributable to mobile source emission reductions across the United States in 2025*. National modeling, not a Sonoma flight-versus-driving health assessment. [https://www.epa.gov/benmap/mobile-sector-source-apportionment-air-quality-and-benefits-ton](https://www.epa.gov/benmap/mobile-sector-source-apportionment-air-quality-and-benefits-ton?ref=airport.mmalc.com) Research article: [https://doi.org/10.1016/j.atmosenv.2018.04.057](https://doi.org/10.1016/j.atmosenv.2018.04.057?ref=airport.mmalc.com)
9. U.S. Environmental Protection Agency, *Gasoline Mobile Source Air Toxics*, and *Learn About How Mobile Source Pollution Affects Your Health*. Identify toxic organic compounds in vehicle emissions and explain their health effects. [https://www.epa.gov/gasoline-standards/gasoline-mobile-source-air-toxics](https://www.epa.gov/gasoline-standards/gasoline-mobile-source-air-toxics?ref=airport.mmalc.com) [https://www.epa.gov/mobile-source-pollution/learn-about-how-mobile-source-pollution-affects-your-health](https://www.epa.gov/mobile-source-pollution/learn-about-how-mobile-source-pollution-affects-your-health?ref=airport.mmalc.com)
10. U.S. Environmental Protection Agency, guidance on modeling electric, fuel-cell, and hybrid vehicles in MOVES. Distinguishes the absence of tailpipe emissions from brake and tire particles and upstream energy effects. [https://www.epa.gov/moves/can-i-model-emissions-electric-fuel-cell-and-hybrid-vehicles-and-equipment-moves](https://www.epa.gov/moves/can-i-model-emissions-electric-fuel-cell-and-hybrid-vehicles-and-equipment-moves?ref=airport.mmalc.com)
11. U.S. Environmental Protection Agency, *What is Particle Pollution?* Describes differences in particle size, composition, and sources. [https://www.epa.gov/pmcourse/what-particle-pollution](https://www.epa.gov/pmcourse/what-particle-pollution?ref=airport.mmalc.com)
12. Charles M. Schulz–Sonoma County Airport, *Passenger Numbers*. [https://sonomacountyairport.org/about-sts/passenger-numbers/](https://sonomacountyairport.org/about-sts/passenger-numbers/?ref=airport.mmalc.com)
13. Caltrans, *2024 Traffic Volumes*, worksheet "2024 AADT DATA." Sonoma Route 101, postmile 3.664, back leg, immediately south of the SR 116 (Lakeville Highway) junction in Petaluma: 82,000 annual average daily vehicles, both directions. [https://dot.ca.gov/-/media/dot-media/programs/traffic-operations/documents/census/2024/2024-traffic-volumes-ca-a11y.xlsx](https://dot.ca.gov/-/media/dot-media/programs/traffic-operations/documents/census/2024/2024-traffic-volumes-ca-a11y.xlsx?ref=airport.mmalc.com) Definitions: [https://dot.ca.gov/programs/traffic-operations/census/traffic-volumes](https://dot.ca.gov/programs/traffic-operations/census/traffic-volumes?ref=airport.mmalc.com)
14. Caltrans, *2024 Peak Hours Report*, worksheet "2024 PEAK HOURS REPORT." Sonoma Route 101, postmile 3.664, control station 11, back leg. Underlying count year 2022: southbound AM peak, 3,666 vehicles from 5 to 6 a.m.; northbound PM peak, 3,391 vehicles from 3 to 4 p.m. [https://dot.ca.gov/-/media/dot-media/programs/traffic-operations/documents/census/2024/2024-ca-peak-hours-a11y.xlsx](https://dot.ca.gov/-/media/dot-media/programs/traffic-operations/documents/census/2024/2024-ca-peak-hours-a11y.xlsx?ref=airport.mmalc.com)
15. Bureau of Transportation Statistics, *Reporting Carrier On-Time Performance*, June 2026\. Downloaded September 20, 2026, filtered to California, then restricted to STS origins or destinations: 595 scheduled departures and 596 scheduled arrivals. All scheduled records, including four cancellations and three diversions, are retained as a timing template. Monthly airport passengers are divided equally between arrivals and departures and, in the central case, equally between flights within each direction. Sensitivity cases vary the road crossing from two to four hours before departure and from half an hour to two and a half hours after arrival, and vary the relative passenger weights by operating carrier. Empty legs are placed two hours after a southbound drop-off journey or two hours before a northbound pickup journey. These assumptions describe scenarios, not observed passenger behavior. [https://www.transtats.bts.gov/DL\_SelectFields.aspx?gnoyr\_VQ=FGJ&QO\_fu146\_anzr=b0-gvzr](https://www.transtats.bts.gov/DL%5FSelectFields.aspx?gnoyr%5FVQ=FGJ&QO%5Ffu146%5Fanzr=b0-gvzr&ref=airport.mmalc.com)
16. Bureau of Transportation Statistics, TranStats airport summary for STS, displayed September 20, 2026\. [https://www.transtats.bts.gov/airports.asp?20=E&Nv42146=fgf&Nv42146\_anzr=fn06n+e15n%2C+PN%3A+Pun4yr5+Z.+fpu7yC+-+f101zn+P1706B&pn44vr4=SNPgf](https://www.transtats.bts.gov/airports.asp?20=E&Nv42146=fgf&Nv42146%5Fanzr=fn06n+e15n%2C+PN%3A+Pun4yr5+Z.+fpu7yC+-+f101zn+P1706B&pn44vr4=SNPgf&ref=airport.mmalc.com)
17. IBA, *SkyWest Orders 60 Embraer 175 Aircraft*. Explains that US mainline pilot "scope clauses" generally limit regional partners' aircraft to 76 or fewer seats and under 86,000 lb, and that the E175 is one of the larger aircraft that fits. [https://www.iba.aero/resources/articles/skywest-orders-60-embraer-175-aircraft/](https://www.iba.aero/resources/articles/skywest-orders-60-embraer-175-aircraft/?ref=airport.mmalc.com)
18. Simple Flying, *Why US Regional Airlines Are Not Ordering The E175-E2*, July 5, 2024\. Reports that the E175-E2's entry into service was delayed by the US scope clause. [https://simpleflying.com/embraer-e2-large-engines-analysis/](https://simpleflying.com/embraer-e2-large-engines-analysis/?ref=airport.mmalc.com)
19. SkyWest, Inc., *SkyWest Orders 60 New Embraer E175 Aircraft*, press release, June 18, 2025\. Sixteen E175s for Delta with deliveries from 2027; firm delivery positions for 44 more from 2028 to 2032, subject to flying agreements with airline partners; purchase rights on 50 more. [https://www.businesswire.com/news/home/20250618426153/en/SkyWest-Orders-60-New-Embraer-E175-Aircraft](https://www.businesswire.com/news/home/20250618426153/en/SkyWest-Orders-60-New-Embraer-E175-Aircraft?ref=airport.mmalc.com)
20. Groome Transportation, *Sonoma County*. Operator-published routes, stops, and schedules. Doesn't establish vehicle sizes, loads, or emissions. [https://groometransportation.com/sonoma-county/](https://groometransportation.com/sonoma-county/?ref=airport.mmalc.com)
21. Next10, *California Green Innovation Index* (2025 edition), Transportation. ZEVs, including plug-in hybrids, were 6.5% of registered vehicles in California in 2024\. [https://greeninnovationindex.org/2025-edition/transportation/](https://greeninnovationindex.org/2025-edition/transportation/?ref=airport.mmalc.com)
22. Sonoma Air Record, *A discussion paper offers Sonoma County more air connectivity without a bigger airport*. Describes the proposed terminal; cited as a proposal, not a validation of its emissions benefits. <https://airport.mmalc.com/satellite-terminal-discussion-paper/>
23. *A satellite terminal in Sonoma County as an alternative to STS expansion* (discussion paper, August 2026), Sonoma Air Record archive. <https://airport.mmalc.com/satellite-terminal-discussion-paper-august-2026/>
24. U.S. Environmental Protection Agency, *Secondary Organic Aerosol Research*, archived January 19, 2017\. Describes particle formation involving volatile organic compounds, sunlight, and other airborne chemicals. [https://19january2017snapshot.epa.gov/air-research/secondary-organic-aerosol-soas-research\_.html](https://19january2017snapshot.epa.gov/air-research/secondary-organic-aerosol-soas-research%5F.html?ref=airport.mmalc.com)
25. U.S. Environmental Protection Agency, *Sector-based PM2.5 and Ozone Benefit Per Ton Estimates*. Includes separate NOₓ and VOC ozone values for specified sectors. These are not a matched set of local aircraft and road-traffic damage factors. [https://www.epa.gov/benmap/sector-based-pm25-and-ozone-benefit-ton-estimates](https://www.epa.gov/benmap/sector-based-pm25-and-ozone-benefit-ton-estimates?ref=airport.mmalc.com)
26. Embraer, *E175 Airport Planning Manual*, Section 3.3, takeoff field lengths. Identifies CF34-8E5 and CF34-8E5A1 engine options. [https://www.embraercommercialaviation.com/wp-content/uploads/2017/06/E175-APM\_APM\_175-1.pdf](https://www.embraercommercialaviation.com/wp-content/uploads/2017/06/E175-APM%5FAPM%5F175-1.pdf?ref=airport.mmalc.com)
27. Center for Jobs and the Economy, *State's Progress on Zero-Emission Vehicles (ZEV) Sales: 2025 & 2026:Q1 Results*. Energy Commission data: 2.2 million ZEVs in 2025, including 1.8 million battery-electric vehicles and 471,000 plug-in hybrids. [https://centerforjobs.org/ca/zev-reports/states-progress-on-zero-emission-vehicles-zev-sales-2025-2026q1-results](https://centerforjobs.org/ca/zev-reports/states-progress-on-zero-emission-vehicles-zev-sales-2025-2026q1-results?ref=airport.mmalc.com)
28. Federal Highway Administration, *Traffic Bottlenecks: Identification and Solutions*, chapter 2, 2016\. Explains congestion when demand approaches or exceeds available capacity, and the importance of location and timing. [https://www.fhwa.dot.gov/publications/research/operations/16064/003.cfm](https://www.fhwa.dot.gov/publications/research/operations/16064/003.cfm?ref=airport.mmalc.com)
29. Sonoma Air Record, article on STS aircraft operations. <https://airport.mmalc.com/sts-operation-counts/>
30. Simaiakis, I., and Balakrishnan, H., *A Queuing Model of the Airport Departure Process*, Transportation Science 50(1), 94–109, 2016\. Section 5.1 discusses the physical lower bound, long right-hand tail, and lognormal or Erlang models. [https://doi.org/10.1287/trsc.2015.0603](https://doi.org/10.1287/trsc.2015.0603?ref=airport.mmalc.com) Author manuscript: [https://www.mit.edu/\~hamsa/pubs/SimaiakisBalakrishnan\_TS2014.pdf](https://www.mit.edu/~hamsa/pubs/SimaiakisBalakrishnan%5FTS2014.pdf?ref=airport.mmalc.com)
31. Wang, J., Gupta, S., and Johnson, M. E., *Reassessing the ICAO’s Standard Taxi/Ground Idle Time: A Statistical Analysis of Taxi Times at 71 U.S. Hub Airports*, Aerospace 12(3), 220, March 8, 2025\. Uses 2023 quarter-hour averages and combines taxi-in with taxi-out; not an STS study. [https://doi.org/10.3390/aerospace12030220](https://doi.org/10.3390/aerospace12030220?ref=airport.mmalc.com)