Table and Figure 1: Summary Stats and Annual Mean NO2 Trend

Table 1. Annual summary statistics for daily NO₂ concentrations measured at the Kearny Mesa monitoring station between 2013 and 2024, including the number of observations, annual mean concentration, and median daily concentration.

Figure 1. Annual mean daily nitrogen dioxide (NO₂) concentrations measured at the Kearny Mesa monitoring station in San Diego, California, from 2013 to 2024. Points represent the annual average of daily mean NO₂ concentrations for each year, illustrating the long-term trend in ambient NO₂ levels at this traffic-influenced urban monitoring site.

Table 1. Annual summary statistics for daily NO₂ concentrations measured at the Kearny Mesa monitoring station (2013–2024).
Year Mean_NO2 Median_NO2 Min_NO2 Max_NO2 n
2013 10.81 9.05 2.45 36.68 358
2014 9.65 7.88 1.05 33.00 346
2015 9.51 8.32 1.27 30.41 356
2016 9.15 7.86 1.50 27.27 357
2017 9.18 7.83 1.73 30.05 362
2018 7.71 6.59 -0.33 24.77 365
2019 7.79 6.86 1.36 22.18 365
2020 7.30 6.20 0.36 27.32 364
2021 7.07 5.86 1.09 47.50 366
2022 8.34 7.51 1.34 26.20 365
2023 6.06 5.25 1.13 19.40 365
2024 5.82 5.01 0.80 22.32 366

Table and Figure 2: Boxplot stats and figure: Daily NO2 by Year

Table 2. Distribution statistics for daily NO₂ concentrations by year, including the number of observations, mean, median, quartiles, and minimum and maximum values. These statistics summarize the distributions illustrated in Figure 2.

Figure 2. Boxplots showing the distribution of daily NO₂ concentrations for each year between 2013 and 2024. The horizontal line within each box represents the median, the box bounds represent the interquartile range (25th–75th percentile), whiskers indicate the typical range of observations, and points denote outliers.

Table X. Annual distribution statistics for daily NO2 concentrations at the Kearny Mesa monitoring station (2013–2024).
Year n Mean Median Q1 Q3 Min Max
2013 358 10.81 9.05 6.05 14.80 2.45 36.68
2014 346 9.65 7.88 5.45 12.64 1.05 33.00
2015 356 9.51 8.32 5.19 12.21 1.27 30.41
2016 357 9.15 7.86 5.09 12.36 1.50 27.27
2017 362 9.18 7.83 4.97 12.40 1.73 30.05
2018 365 7.71 6.59 4.00 10.19 -0.33 24.77
2019 365 7.79 6.86 4.82 10.14 1.36 22.18
2020 364 7.30 6.20 3.41 10.32 0.36 27.32
2021 366 7.07 5.86 3.91 8.83 1.09 47.50
2022 365 8.34 7.51 5.67 10.22 1.34 26.20
2023 365 6.06 5.25 3.38 8.09 1.13 19.40
2024 366 5.82 5.01 3.38 7.48 0.80 22.32

Table and Figure 3: Boxplot stats and figure: Daily NO2 by Period

Table 3. Summary statistics for daily NO₂ concentrations grouped by study period (pre-pandemic, pandemic, and post-pandemic), including the number of observations, mean, median, quartiles, and range.

Figure 3. Boxplots of daily NO₂ concentrations grouped by study period: pre-pandemic (2013–2019), pandemic year (2020), and post-pandemic period (2021–2024). The plot highlights shifts in the distribution of daily NO₂ concentrations across these periods.

Table X. Distribution statistics for daily NO2 concentrations grouped by study period.
Period n Mean Median Q1 Q3 Min Max
Pre 2509 9.11 7.77 5.05 11.90 -0.33 36.68
Pandemic 364 7.30 6.20 3.41 10.32 0.36 27.32
Post 1462 6.82 5.91 3.95 8.72 0.80 47.50

Table and Figure 4 — Monthly Mean NO2 Concentration

Table 4. Monthly summary statistics for daily NO₂ concentrations measured at the Kearny Mesa monitoring station from 2013 to 2024, including the mean and median concentrations and the number of observations for each calendar month.

Figure 4. Mean daily NO₂ concentration by month based on observations from 2013 to 2024 at the Kearny Mesa monitoring station. The figure illustrates the seasonal cycle in NO₂ concentrations, with higher values typically observed during winter months and lower concentrations during late spring and summer.

Table X. Mean and median daily NO2 concentrations by month at the Kearny Mesa monitoring station (2013–2024).
Month Mean_NO2 Median_NO2 n
Jan 12.56 11.34 372
Feb 10.06 9.39 328
Mar 7.96 7.16 372
Apr 6.51 5.87 358
May 4.64 4.20 370
Jun 5.08 4.68 360
Jul 5.11 4.77 370
Aug 5.62 5.23 359
Sep 6.83 6.00 352
Oct 9.69 8.82 370
Nov 11.80 10.73 351
Dec 12.41 11.23 373

Figure 5. Daily mean nitrogen dioxide (NO₂) concentrations measured at the Kearny Mesa monitoring station in San Diego, California, from 2013 to 2024. The black line shows daily observations, the blue line represents a 30-day rolling mean highlighting short-term seasonal variability, and the red line indicates the fitted linear regression describing the long-term trend in concentrations. The dashed vertical line marks the onset of COVID-19 pandemic restrictions in March 2020. Despite substantial day-to-day variability, the regression indicates a gradual decline in NO₂ concentrations over the study period (R² = 0.068).

## Warning in scale_x_date(): A <numeric> value was passed to a Date scale.
## ℹ The value was converted to a <Date> object.
## `geom_smooth()` using formula = 'y ~ x'
## Warning: Removed 29 rows containing missing values or values outside the scale range
## (`geom_line()`).

Statistical Analyses

  1. linear Regression: Long term trend
## 
## Call:
## lm(formula = Daily_Avg_NO2_ppb ~ Date, data = data)
## 
## Residuals:
##    Min     1Q Median     3Q    Max 
## -8.936 -3.496 -1.107  2.491 40.421 
## 
## Coefficients:
##               Estimate Std. Error t value Pr(>|t|)    
## (Intercept)  2.669e+01  1.046e+00   25.52   <2e-16 ***
## Date        -1.033e-03  5.825e-05  -17.74   <2e-16 ***
## ---
## Signif. codes:  0 '***' 0.001 '**' 0.01 '*' 0.05 '.' 0.1 ' ' 1
## 
## Residual standard error: 4.844 on 4333 degrees of freedom
## Multiple R-squared:  0.06771,    Adjusted R-squared:  0.06749 
## F-statistic: 314.7 on 1 and 4333 DF,  p-value: < 2.2e-16
  1. Mann-Kendall
## tau = -0.166, 2-sided pvalue =< 2.22e-16
  1. Sen’s Slope
## 
##  Sen's slope
## 
## data:  data$Daily_Avg_NO2_ppb
## z = -16.394, n = 4335, p-value < 2.2e-16
## alternative hypothesis: true z is not equal to 0
## 95 percent confidence interval:
##  -0.0008850116 -0.0006928850
## sample estimates:
##   Sen's slope 
## -0.0007879891
  1. Pettitt Test
## 
##  Pettitt's test for single change-point detection
## 
## data:  data$Daily_Avg_NO2_ppb
## U* = 1336147, p-value < 2.2e-16
## alternative hypothesis: two.sided
## sample estimates:
## probable change point at time K 
##                            1847
## [1] "2018-03-09"
  1. Anova
##               Df Sum Sq Mean Sq F value Pr(>F)    
## Period         2   5120    2560   106.7 <2e-16 ***
## Residuals   4332 103917      24                   
## ---
## Signif. codes:  0 '***' 0.001 '**' 0.01 '*' 0.05 '.' 0.1 ' ' 1
  1. Tukey
##   Tukey multiple comparisons of means
##     95% family-wise confidence level
## 
## Fit: aov(formula = Daily_Avg_NO2_ppb ~ Period, data = data)
## 
## $Period
##                     diff       lwr        upr     p adj
## Pandemic-Pre  -1.8101916 -2.454238 -1.1661450 0.0000000
## Post-Pre      -2.2810597 -2.658872 -1.9032471 0.0000000
## Post-Pandemic -0.4708681 -1.143499  0.2017623 0.2284481

Statistic Analyses Tables

Table X Summary of statistical analyses applied to the daily NO₂ time series (2013–2024). Linear regression, Mann–Kendall, and Sen’s slope all indicate a significant long-term decline in NO₂ concentrations, while Pettitt’s test identifies a structural change point in 2018 marking the onset of this decline.

Summary of statistical analyses applied to the daily NO₂ time series (2013–2024).
Test Result Details
Linear Regression Slope = -0.377 ppb/year R² = 0.068
Mann–Kendall Tau = -0.166 p < 0.001
Sen’s Slope Slope = -0.288 ppb/year 95% CI: [-0.323, -0.253]
Pettitt Test Change point = 2018-03-09 p < 0.001

Table X Results of one-way ANOVA and Tukey post-hoc comparisons of mean daily NO₂ concentrations across pre-pandemic, pandemic, and post-pandemic periods. NO₂ concentrations during the pandemic and post-pandemic periods were significantly lower than pre-pandemic levels, while no significant difference was observed between pandemic and post-pandemic periods.

Comparison Difference (ppb) 95% CI p-value
Pandemic-Pre 2020 vs 2013–2019 -1.810 [-2.454, -1.166] <0.001
Post-Pre 2021–2024 vs 2013–2019 -2.281 [-2.659, -1.903] <0.001
Post-Pandemic 2021–2024 vs 2020 -0.471 [-1.143, 0.202] 0.228

Tukey post-hoc comparisons of mean daily NO₂ concentrations among defined study periods. A one-way ANOVA indicated a statistically significant difference in NO₂ concentrations across pre-pandemic (2013–2019), pandemic (2020), and post-pandemic (2021–2024) periods (F(2, 4332) = 106.7, p < 0.001). Pairwise comparisons show that NO₂ concentrations during both the pandemic year (2020) and the post-pandemic period (2021–2024) were significantly lower than pre-pandemic levels, with mean differences of −1.81 ppb and −2.28 ppb, respectively (p < 0.001). No statistically significant difference was observed between the pandemic and post-pandemic periods (p = 0.228), indicating that reduced NO₂ concentrations persisted beyond the pandemic period. Values represent mean differences (ppb), 95% confidence intervals, and adjusted p-values from Tukey’s honestly significant difference test.