Using the Quarterly Compound Fractional Grey Model to Predict the Air Quality in 22 Cities of China

Table 2

The fitting results of NO₂ concentration by three models in Tangshan (unit: μg/m³).

Quarter	Actual value	GM (1, 1)	FGM (1, 1)	QCGM (1, 1)
2015-1	68	68	68	68
2015-2	54	60	55	54
2015-3	50	59	56	49
2015-4	68	59	57	69
2016-1	61	58	58	66
2016-2	52	58	59	54
2016-3	47	57	59	48
2016-4	73	57	59	67
2017-1	68	56	58	64
2017-2	56	56	58	52
2017-3	50	55	57	46
2017-4	61	55	57	64
2018-1	56	54	56	60
2018-2	49	54	55	49
2018-3	41	53	54	43
2018-4	60	53	53	60
2019-1	56	52	52	57
2019-2	44	52	51	46
2019-3	42	51	50	41
2019-4	59	51	49	56
MAPE (%)		13.47	12.89	4.09