Table 3.

Model performance and estimated per-drive-day costs across different alternative sampling designs based on the data from the ACT-TRAP mobile monitoring campaign given a single instrument for UFP monitoring (P-Trak) in a single study area.

	Design	Version	Design component				Cost (work days for per-drive-day costs)				Model performance
			$n$ of sites	$n$ of visits per sitea	$n$ of routesb	$n$ of seasons	Regular drive	Rescheduled drivec	Quality control	Total	CV $R^2$d	Reference designe
Complete	Allf	—	309	29	7	4	203	8	21	232	0.77	—
Alternative	Fewer sites	250	250	29	6	4	174	8	18	200	0.76	Complete
		200	200	29	5	4	145	8	15	168	0.74	Complete
		150	150	29	3	4	87	8	10	105	0.70	Complete
		100	100	29	2	4	58	8	7	73	0.67	Complete
	Fewer visits	24	309	24	7	4	168	8	18	194	0.72	Complete
		20	309	20	7	4	140	8	15	163	0.72	Complete
		16	309	16	7	4	112	8	12	132	0.71	Complete
		12	309	12	7	4	84	8	9	101	0.70	Complete
		6	309	6	7	4	42	8	5	55	0.63	Complete
		4	309	4	7	4	28	8	4	40	0.60	Complete
	Fewer daysg	Weekday	309	12	7	4	84	8	9	101	0.65	Reduced
	Fewer hoursh	Business	309	12	7	4	84	8	9	101	0.55	Reduced
		Rush	309	12	7	4	84	8	9	101	0.64	Reduced
	Fewer seasonsg,i	3	309	12	7	3	84	6	9	99	0.69	Reduced
		2	309	12	7	2	84	4	9	97	0.67	Reduced

^Note:—, no data; ACT-TRAP, Adult Changes in Thought Traffic-Related Air Pollution; CV, cross-validated; UFP, ultrafine particle.

^a Maximum numbers of visits feasible given the restriction of reduced designs in a single mobile platform.

^b A single route is a schedule of driving covering on average 35 sites that provides the number of driving days.

^c Make-up driving for at least 2 d per season.

^d Median cross-validation (CV) $R^2$ across campaigns ($n=30$) for each design using predictions of UFP annual averages at all 309 sites; for the designs with fewer sites, predictions were obtained from cross-validation for 100 to 250 sites and external validation for the rest of sites not included in a given campaign.

^e Application of the complete reference design (“Complete”) to alternative designs with spatially reduction with fewer sites or temporal reduction with fewer visits, and the reduced reference design with 12 visits (“Reduced”; 309 sites but 12 visits instead of 29 visits) to alternative designs with temporal restriction including fewer days, hours, or seasons.

^f A total of 309 sites, $\sim 29$ visits per site, all days, most hours from 0500 hours to 2300 hours (5 A.M.–11 P.M.), and four seasons.

^g We did not consider the designs of weekend-only and one season because they are not realistic.

^h Business hours: weekdays 0900 hours to 1700 hours (9 A.M.–5 P.M.); Rush hours: weekdays 0700 hours to 1000 hours (7 A.M.–10 A.M.) and 1500 hours to 1800 hours (3 P.M.–6 P.M.).

ⁱ Specific combination such as summer–winter and summer–spring.