Table 2 Descriptive statistics for PM2.5 mass and species in California counties, 2000–2003

Pollutants	Obs* (µg/m³)	Mean (µg/m³)	IQR percentile detection	95th percentile	Below detection (%)
PM2.5	1878	19.28	14.63	46.91	0.00
Elemental carbon	1879	0.966	0.795	2.57	0.05
Organic carbon	1879	7.129	4.592	15.91	0.00
Nitrates	1817	5.415	5.524	17.46	0.00
Sulphates	1817	1.908	1.530	5.18	0.00
Calcium	1870	0.080	0.064	0.20	0.05
Chlorine	1870	0.094	0.069	0.41	2.09
Copper	1870	0.007	0.007	0.02	1.34
Iron	1870	0.124	0.099	0.34	0.00
Potassium	1870	0.117	0.081	0.26	0.00
Sulphur	1870	0.648	0.499	1.70	0.00
Silicon	1870	0.168	0.151	0.43	0.05
Titanium	1870	0.009	0.008	0.02	0.32
Zinc	1870	0.012	0.011	0.04	1.82

IQR, interquartile range; Obs, observation; PM, particulate matter.
*Total number of observations for analysis across all six counties over the 4-year study period.