Table 1.

Overall (absolute and percent) trends for NO, NO₂, NO_x, O₃, PM₁₀ and PM_2.5 at UK AURN sites: for each site, Average Yearly Change (pre-2020) are the average annual change as determined by Theil-Sen analysis for the period 01 January 2015 to 31 December 2019, and Change Locking Down and Change while Locked Down are determined as the net sum of changes due for break-points/segments detected during the periods 10 March to 10 April 2020 and 11 April to 30 June 2020, respectively. Results are reported as site type median with 5% and 95% quantiles in parentheses.

Species	Change	Average yearly change (pre-2020) [μg m−3 yr−1]			Change locking down (10 March 10 April 2020) [μg m−3]			Change while locked down (11 April to 30 June 2020) [μg m−3]
		Rural Background	Urban Background	Urban Traffic	Rural Background	Urban Background	Urban Traffic	Rural Background	Urban Background	Urban Traffic
NO	Absolute	−0.07 [−0.37, 0.03]	−0.35 [−1.27, 0.51]	−1.55 [−6.39, 0.38]	0.41 [−0.36, 0.8]	−3.9 [−7.37, 0.54]	−9.68 [−32.96, −4.34]	−0.42 [−0.76, 0.25]	1.37 [−2.5, 13.26]	6.06 [2.28, 13.51]
	Percent	−6.91 [−16.57, 4.41]	−5.72 [−11.3, 10.59]	−5.88 [−11.48, 2.24]	80.8 [−42.58, 156.88]a	−45.79 [−59.39, 46.08]	−49.85 [−79.29, −29.88]	−36.12 [−61.93, 55.28]	26.26 [−45.62, 516.38]a	50.1 [17.07, 107.49]
NO2	Absolute	−0.19 [−0.85, 0.25]	−0.74 [−2.6, 0.04]	−1.48 [−3.74, −0.1]	1.76 [−1.35, 5.19]	−4.16 [−7.88, 4.55]	−7.58 [−20.32, 2.14]	1.49 [−2.81, 3.05]	3.06 [−6.1, 6.71]	4.99 [−7.47, 13.52]
	Percent	−3.41 [−8.38, 5.06]	−3.31 [−7.94, 0.27]	−4.35 [−8.04, −0.36]	39.61 [−126.86, 158.09]a	−25.62 [−39, 36.7]	−32.17 [−54.87, 10.54]	25.46 [−29.34, 186.37]	20.61 [−37.73, 45.09]	31.37 [−33.15, 98]
NOx	Absolute	−0.27 [−1.32, −0.04]	−1.26 [−4.61, 0.36]	−3.3 [−13.17, 0.56]	1.37 [−1.32, 4.74]	−5.66 [−15.39, 8.66]	−17.14 [−48.25, −5.27]	0.93 [−4.27, 2.34]	3.19 [−6.37, 9.54]	8.95 [2.12, 32.54]
	Percent	−3.31 [−9.62, −0.17]	−4.13 [−9.25, 0.81]	−5.29 [−10.04, 0.95]	40.21 [−68.57, 109.27]a	−28 [−42.63, 47.29]	−38.19 [−67.83, −17.42]	10.96 [−32.01, 829.57]	21.27 [−27.35, 42.62]	34.17 [5.52, 91.26]
O3	Absolute	0.48 [−1.62, 2.12]	0.66 [−2.05, 1.95]	1.37 [−0.56, 3.71]	−4.3 [−10.21, 11.59]	6.96 [−6.35, 10.12]	7.39 [6.3, 12.62]	4.25 [−8.53, 12.44]	1.89 [−7.76, 13.98]	ca. −6.82b
	Percent	0.84 [−3.06, 3.65]	1.46 [−3.81, 5.12]	8.34 [−1.52, 16]	−7.44 [−16.43, 20.81]	13.91 [−11.99, 22.07]	17.46 [16.37, 49.13]	6.77 [−13.66, 21.88]	3.55 [−15.04, 28.7]	ca. −17.24b
PM10	Absolute	−0.26 [−1.02, 0.28]	−0.13 [−1.9, 1.04]	−0.25 [−1.51, 1.06]	5.81 [1.71, 11.37]	6.16 [−0.71, 10.52]	6.26 [−3.62, 11.16]	−3.52 [−7.3, −1.64]	−2.06 [−9.55, 4.22]	−2.09 [−10.37, 5.35]
	Percent	−2.68 [−6.71, 4.85]	−0.95 [−10.88, 8.95]	−1.15 [−7.69, 6.15]	73.05 [28.4, 118.68]a	61.28 [−6.23, 98.39]	47.81 [−25.28, 80.89]	−25.91 [−33.31, −22.33]	−12.39 [−41.25, 27.25]	−13.08 [−38.01, 44.3]
PM2.5	Absolute	−0.43 [−0.85, 0.33]	−0.22 [−1.1, 0.34]	−0.43 [−0.87, 0.38]	3.94 [1.55, 7.04]	4.79 [1.47, 6.94]	5 [0.59, 8.49]	−1.08 [−1.86, 2.24]	0.46 [−6.45, 4.72]	0.18 [−6.7, 4.5]
	Percent	−5.73 [−12.09, 11.51]	−2.08 [−9.55, 4.11]	−3.05 [−7.41, 4.44]	80.46 [47.26, 114.1]	90.73 [30.92, 143]	84.81 [25.85, 134.99]	−14.19 [−22.35, 80.9]	5.51 [−43.27, 50.06]	2.46 [−45.19, 92.96]

Notes: For Average Yearly Change/Theil-Sen analyses, percent changes are calculated relative to mid-point concentration for available data time-range; For both lockdown related changes/Break-point/segment analyses, percent changes are calculated relative to concentration prior to first detected change in that time period. As a result, percent changes locking down and while locked down should not be compared directly because each is calculated relative to its start-point. (From example, a 50% reduction from 100 μg m⁻³ followed by a 50% increase from there does not return levels to 100 μg m⁻³: 100 μg m⁻³ − 50% = 50 μg m⁻³; then 50 μg m⁻³ + 50% = 75 μg m⁻³.)

^aConsidered less reliable because large uncertainty associated with change, start-point concentration or combination.

^bMedian change reported without 5% and 95% quantiles as ESTIMATE ONLY because insufficient measurements for quantile calculation.