Air Pollution is here for the holidays in Stockton


By Avery Parks (1), Tanisha Raj (2), Ector Olivares (2), Fotini Katopodes Chow (1)

Civil and Environmental Engineering, University of California, BerkeleyEnvironmental Justice Program, Catholic Charities of Stockton

November 27, 2023

Fall and winter bring visions of holidays and cozy gatherings around a fire. It may be a surprise that the changing seasons also bring very high air pollution, which can last for days or weeks at a time. The air pollution comes from vehicle exhaust, industrial emissions, heavy-duty trucks, agricultural burning, wildfire smoke, and even wood smoke from fireplaces. Air pollution levels already exceeded healthy limits in Stockton over this Thanksgiving holiday weekend in 2023 (Figure 1).

This article looks at data from winter 2022 to understand trends and patterns in Stockton, to find out when, where, and why the air quality is poor – and what we can do about it.

Figure 1. Air quality index (AQI) (corrected) values in Stockton from the Purple Air sensor network at 9:45pm on November 26, 2023. AQI values above 100 are considered unhealthy.

Why is air quality important?

Long-term exposure to poor air quality can be as bad for your health as smoking a pack of cigarettes a day. Particles emitted that are smaller than 2.5 microns are called “particulate matter” or PM2.5, and they are tracked with special air quality sensors from the EPA or from sensor networks like Purple Air shown in Figure 1. These particles are small enough to enter deep into our lungs and get into the bloodstream (Figure 2). Even short term exposure to PM2.5 during these air pollution events has been found to increase infant mortality and increase hospital admissions for cardiovascular disease, COPD, and asthma.

Figure 2. PM2.5 enters deep into the lungs and into the bloodstream, leading to significant health impacts. From: Sierra Club

What can we learn from 2022?

In the fall/winter of 2022, Stockton residents experienced exceptionally poor air quality. Between November 20 and December 26, 2022, there were 19 days when at least one air quality sensor we analyzed in Stockton had a daily average PM2.5 concentration above the federal limit of 35 μg/m3 –– that’s over half the days during that time period.  On several occasions, individual sensors clocked hourly-average PM2.5 concentrations above 200 μg/m3, which is in the “very unhealthy” range. The worst air quality days were November 24-26 and December 17, where the daily average PM2.5 concentration exceeded 50 μg/m3, reaching almost 70 μg/m3 –– twice the federal limit –– on November 24, 2022 (Figure 3).

Figure 3. Daily-averaged PM2.5 levels from 6 air quality sensors in Stockton from October to December 2022 showing exceedance of the federal standard. Source: SJVAPCD

To look into these pollution events more closely, we further examined the PM2.5 data from two different air quality sensor networks. Stockton’s AB617 area has 6 air quality sensors installed by the San Joaquin Valley Air Pollution Control District (SJVAPCD). There are also a number of Purple Air sensors, which are low-cost sensors installed at individual residences. During these pollution events in 2022, the Cal Water Tank sensor consistently had the highest concentrations of PM2.5, followed by the Cal Water Office sensor and a Purple Air sensor located just south of the AB617 Boundary. All the highest PM2.5 concentrations in Stockton were recorded in South Stockton. Figure 4 shows the location of Air District sensors and the Purple Air sensors which were used to examine the air quality during this time period. A multistep quality control procedure was applied to all Purple Air data shown here.

Figure 4: Air quality sensor locations and types within the Stockton AB617 boundary (gray border) which are included in the data analysis in this article. Some sensors are overlapping on the map. AD = SJVAPCD sensors, PA = Purple Air sensors.

Why is air quality worse in winter?

Wintertime weather makes air quality worse because the air pollution gets trapped near the ground surface – where people live and breathe – rather than getting lifted high up into the atmosphere. Even places you normally associate with clean air, like San Francisco, have worse air quality in the wintertime. This is due to a meteorological phenomenon called a temperature inversion which occurs when the normal distribution of temperature in the atmosphere is inverted. Normally air gets colder as altitude increases, but with an inversion, temperature increases as you go up. That means the cold, denser air is near the ground surface, with warmer air above which acts like a lid that traps the cold air below it (Figure 5).  

Figure 5: In the wintertime, or at night, a temperature inversion can keep air pollution trapped near the surface. Background image: Stockton skyline. Adapted from: SF Chronicle.

This cold air at the surface also means that pollution is trapped near the surface, worsening the air quality. Temperature inversions occur frequently at night during the winter due to the colder ground temperatures. This means that bad air quality caused by meteorological conditions is more common during the winter. A significant temperature inversion with a depth of over 1 km is visible in the Oakland temperature data on November 23, 2022 –– one day before the dramatic increase in PM2.5 concentrations and substantial enough to have a lasting regional effect.

How do holidays affect pollution?

In addition to meteorological conditions, other factors can increase the amount of pollution, worsening the air quality. November 24, 2022, one of the peaks in Figure 3, was Thanksgiving Day. Another peak was on December 23, 2022, right before Christmas. During winter holidays, emissions from home heating, cooking, and residential wood burning are significant contributors to local air pollution. These residential sources are in addition to the usual sources from traffic, industry, and other sources. Wintertime PM2.5 is approximately 1/3 from residential wood burning! Lighting a fire at home during a wintertime inversion means all the smoke emitted from the chimney gets trapped near the ground. SJVAPCD has a “Residential Wood Smoke Reduction Program” which advises residents about whether burning is permitted that day based on weather conditions like temperature inversions.

Digging into air quality trends in Stockton

We looked further into the sensor network data to try to understand the trends in air quality throughout Stockton during this time period. Data are presented below to show the differences in PM2.5 concentrations between North and South Stockton, indoor vs. outdoor sensors, and time of day.

Figure 6: North vs. South Stockton PM2.5 at different times of day using data from sensors shown in Figure 3, averaged over data from Oct-Dec 2022.

During the winter, air pollution is worse at night. Figure 6 shows average data from North and South Stockton as a function of hour of day, so this is meant to represent a “typical” day during this time period. The highest values are at 10 pm (22:00). South Stockton shows concentrations which are 1-2 μg/m3 higher than North Stockton. Note that there are approximately twice as many sensors in South Stockton compared to North Stockton. North and South Stockton show the same time of day pattern, with PM2.5 concentrations highest at nighttime, and reaching a minimum in the afternoon. Around 4 pm, rush hour begins, as well as wintertime activities at home such as wood burning and cooking, plus the temperature gets colder, which all contribute to the rise in pollution in the late afternoon and evening. PM2.5 concentrations remain high at nighttime due to the temperature inversion. As temperatures warm up in the morning and afternoon of the next day, the inversion clears and allows the pollution to disperse. Therefore, the observed trends in air quality in Stockton are driven by both human activities and meteorological conditions.

A comparison of indoor vs. outdoor PM2.5 sensors (Figure 7) shows that air quality is clearly a lot better inside. The outdoor average follows the same pattern as the previous plot, where the air quality is worst at nighttime and best in the afternoon, and the indoor average follows a much less dramatic version of the pattern. There is a slight increase in indoor PM2.5 when there is increased concentrations outside, but only by a maximum of approximately 3 μg/m3. Several of these indoor sensors are located in environments which include some air filtration, which shows how much improvement in air quality is possible with an air purifier.  Seemingly small differences in concentrations can lead to very different health outcomes, so even differences of a few μg/m3 are significant.

Figure 7: Average PM2.5 from all indoor Purple Air sensors vs. all outdoor PM2.5 in Stockton at different hours of the day, over the Oct-Dec 2022 period. Sensors included are shown in Figure 3.

We also examined averages for different days of the week, to see what a “typical Monday” looks like compared to a “typical Saturday” (Figure 8). It turns out that weekend concentrations are starkly higher after midnight and into the early morning hours. This time period corresponds to early Saturday morning hours following Friday night, and early Sunday morning hours following Saturday night. This may be associated with activities such as evening-time wood burning on the weekends.

 Figure 8: PM2.5 at different times of day, each day of the week, averaged over Oct-Dec 2022. Sensors included are shown in Figure 3.

What have we learned?

Seasonal and time of day patterns of PM2.5 are driven by meteorological factors, but human-driven emissions matter a lot. The peak air pollution events from regional temperature inversions in 2022 coincided with holidays and weekends where residential wood burning contributed to PM2.5 levels from other sources. Sensor network data showed that South Stockton had consistently worse air quality than North Stockton. The worst air pollution levels were found in the hours just before and after midnight. These same trends are becoming clear in this year’s 2023 data too.

Here are 3 things you can do.

Filter your air. When meteorological factors and local emissions are especially bad for air quality, Stockton residents can protect themselves by staying indoors and using indoor air filtration. Resources for cleaning your indoor air are provided by the Environmental Justice Program in Stockton (click on Your Local Air in the menu).

Choose not to burn. SJVAPCD offers an incentive program to replace wood burning fireplaces with cleaner alternatives and advises that, “Choosing not to use your wood burning fireplace this winter, when possible, is critical in our pollution reduction efforts and key to public health.” Figure 9 describes the health impacts of wood smoke.

Get involved. Residents and SJVAPCD are working hard as part of the AB 617 program to improve air quality in Stockton – find out more and how to get involved here.

Figure 9. Impacts of wood smoke on human health. From: EPA 

This work was made possible by The Green Initiative Fund at UC Berkeley. For more information, please contact Fotini (Tina) Katopodes Chow at This article can also be found online here.

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