Table S1. Site locations and date sampled. Site Date latitude Site # sampled (°N) Site longitude (°W) Site elevation (m) 1 1/10/13 47.3991 121.3886 787 2 1/28/13 45.9508 120.6700 838 3 1/29/13 44.8997 120.6104 1136 4 1/29/13 43.8038 120.8477 1576 5 1/30/13 43.4455 118.9911 1332 6 1/31/13 42.3231 118.7468 1715 7 1/31/13 41.3849 117.8184 1330 8 2/1/13 40.9014 115.8910 1949 9 10 2/1/13 2/3/13 42.2767 43.3495 116.0115 115.3968 1772 1538 11 2/3/13 43.5927 113.5894 1942 12 2/4/13 43.4010 111.2053 1727 13 2/4/13 42.9357 109.8576 2274 14 2/5/13 41.7297 109.3668 2223 15 16 17 18 2/5/13 2/7/13 2/8/13 2/9/13 40.7464 40.1316 40.4929 40.6695 109.4776 109.4711 107.8994 106.4158 2583 1538 1962 2512 19 2/13/13 42.0249 107.4167 1987 20 2/14/13 43.7016 108.8452 1963 21 2/14/13 45.0754 109.1313 1330 22 2/15/13 45.7980 108.4369 962 23 2/16/13 47.6249 108.6968 1090 Notes new snow <1 day old ; Snoqualmie pass (mountains) new snow <1 day old ; Satus pass (mountains) new snow < 1 day old drift behind sagebrush, east of Bend, Oregon in sage meadow on plateau above Burns, Oregon top layer fell ~ 3 days previous; open grassland top layer fell ~ 3 days previous; melting; grassland drift snow; many sastrugi; pass north of Elko, Nevada open grassland hilly grassland mountain foothills, near mine and lava fields new snow < 2 days old; on frozen creek in forest new snow < 2 days old; sagebrush, pasture in sagebrush; thin snow north of Rock Springs, Wyoming Uinta mountain pass; deep snow south of Vernal, Utah; thin snow sageland; juniper bushes new snow <1 day old; open grassland windy; thin, drifted snow; total snow depth varies 0-23cm new snow <2 days old; sampled in drift on north-facing slope sampled in deeper snow on slope; new snow <1 day old transect east of Billings, MT; all new snow <12 hrs old drift on north-facing slope; otherwise region largely snow-free 24 2/17/13 48.2318 105.0949 648 25 2/17/13 47.6137 103.3555 627 26 2/18/13 47.0987 101.4700 688 27 2/18/13 45.4587 99.8113 636 28 2/19/13 45.2569 98.7477 405 29 2/19/13 44.0547 97.8803 403 30 2/22/13 43.5880 97.1759 439 31 32 2/22/13 43.1963 2/22/13 42.3145 97.4784 97.8711 414 540 33 2/22/13 42.1825 97.5657 493 34 35 2/23/13 43.0561 2/23/13 44.1535 95.6485 95.7772 451 464 36 2/24/13 45.5482 95.7252 334 37 2/24/13 46.4733 96.5378 297 38 2/27/13 45.8645 96.5744 296 39 40 41 2/27/13 46.4435 2/28/13 47.2668 2/28/13 47.8796 97.4029 98.8338 101.0000 327 448 657 42 3/1/13 48.0104 102.3245 646 43 44 45 46 47 3/1/13 3/1/13 3/2/13 3/2/13 3/3/13 48.1463 48.1197 48.8649 48.8539 48.8159 102.8639 103.2260 100.9097 99.1845 97.8797 560 560 451 465 343 48A 3/4/13 47.8910 97.0761 254 48B 3/5/13 47.8285 96.8489 257 48C 3/5/13 47.8271 96.8477 246 49 3/6/13 50.7280 97.1307 244 50 3/7/13 51.2723 98.5681 245 wind-loaded slope on farm field Theo. Roosevelt Natn’l Park; deep drift on N facing slope new snow <1 day old; drift behind bushes on farm Ground mostly bare or ice; blowing new snow (<1 day old) in small drifts on frozen lake sampled deeper part of snow; on farmland new snow <1.5 days old; on frozen lake edge new snow <1.5 days old; farmland new snow <1.5 days old; farmland new snow <1.5 days old; on frozen lake new snow <2 days old; on farmland new snow <3 days old; on frozen lake new snow ~3 days old; wildlife conservation area in wildlife conservation grass area wildlife conservation grass/swamp area Sheyenne Natn’l Grasslands grassland grassland new snow <3 days old; farmland; sample between crop stubble (deeper) new snow <3 days old; on frozen lake new snow <3 days old; on frozen lake next to abandoned farmhouse on frozen lake wildlife conservation wetland new snow <12hrs old; at hotel in Grand Forks, ND new snow <12 hrs old; farmland 20km ESE of Grand Forks new snow <12 hrs old; frozen river 20km ESE of Grand Forks new snow <1 day old; on frozen bog in wildlife park new snow <2 days old; on frozen lake 51 3/7/13 52.5433 98.8883 248 52 3/8/13 53.8509 99.2559 282 53 3/8/13 55.1136 98.5094 222 54 3/11/13 56.0766 97.0474 192 55 3/11/13 56.5330 95.3282 138 56 3/12/13 58.7261 93.8281 17 57 3/13/13 54.5831 100.4714 278 58 3/14/13 53.6038 102.1222 272 59 60 61 62 63 64 3/14/13 3/15/13 3/18/13 3/19/13 3/19/13 3/20/13 51.7972 50.5768 52.7499 55.2484 54.7011 54.3954 102.7436 105.4384 106.6554 105.3427 107.8271 110.8437 521 588 514 381 458 550 65 3/20/13 52.7820 111.3175 697 66 3/21/13 50.9260 112.0593 729 67A 3/21/13 50.9713 113.4214 974 67B 3/21/13 50.9864 113.6108 1005 67C 3/21/13 50.9896 113.8211 1020 new snow <2 days old; power-line clearing in trees new snow <1 day old; clearing in trees new snow <1 day old; on frozen lake in forest new snow <3 days old; on frozen swamp in forest new snow <3 days old; on frozen lake in forest new snow <3 days old; site is in deeper area protected by trees; at forest/tundra boundary new snow <3 days old; on frozen swamp in forest new snow <2 days old; power-line clearing in trees new snow <2 days old; farmland new snow <12 hrs old; farmland new snow <3 days old; farmland new snow <3 days old; frozen lake new snow <3 days old; frozen swamp new snow <1 day old; farmland new snow <1 day old; in drift on north slope; farmland new snow <1 day old; grassland new snow <1 day old; behind farmhouse; 60km ESE of Calgary city limit new snow <1 day old; by grain elevator; bare ground in area; ~45km ESE of Calgary city limit new snow <1day old; behind house on farm field; 30km ESE of Calgary city limit Table S2. PMF analysis results, showing the fraction of 650-700nm absorption attributed to soil, biomass burning and pollution (fossil fuel) sources. The entries in italics are from a second PMF run with somewhat lower predictive capacity for light absorption (i.e. lower normalized Q), but which allowed the addition of data for five sites in the U.S. Great Plains regions. All other values are from the optimized PMF run. Surface (1), Site Sub-surface (2) FSOIL FBB FFF 2 2 -0.42 1.04 0.36 6 1 -0.54 1.06 -0.32 8 2 -0.35 0.86 0.50 9 1 -0.29 1.02 0.29 10 1 0.05 0.84 0.11 11 2 -0.14 0.34 0.13 14 2 0.34 0.32 0.34 15 1 -0.09 0.67 0.37 15 2 0.07 0.36 0.55 16 2 -0.18 0.28 0.86 17 1 0.25 0.29 0.44 17 2 0.28 0.20 0.42 18 1 0.61 0.15 0.12 19 1 0.81 -0.01 -0.02 20 1 0.46 0.19 0.25 20 2 0.46 0.19 0.31 26 1 0.70 0.06 0.04 26 2 0.85 0.13 0.01 28 1 0.74 0.09 0.17 28 2 0.68 0.17 0.10 29 2 0.87 0.02 0.07 30 2 0.21 0.23 0.53 31 1 0.22 0.39 0.32 35 1 0.33 0.27 0.39 35 2 0.94 0.02 0.02 36 1 -0.19 0.57 0.47 36 2 0.93 0.02 0.03 38 1 0.62 0.09 0.24 38 2 0.75 0.10 0.15 39 2 0.59 0.24 0.08 40 1 0.48 0.11 0.39 40 2 0.84 0.00 0.12 42 1 0.62 0.31 0.06 43 1 0.76 0.24 0.00 43 2 -0.04 0.17 0.83 44 2 0.46 0.08 0.45 45 45 46 49 49 50 50 52 53 54 54 55 57 58 59 60 61 61 62 63 63 64 65 66 1 2 1 1 2 1 2 1 2 1 2 2 2 1 2 1 1 2 2 1 2 1 2 2 0.23 0.23 0.30 0.20 0.27 -0.05 0.16 0.59 -0.16 0.36 0.04 -0.22 0.27 0.32 0.14 -0.19 0.30 -0.15 -0.24 0.02 -0.12 0.19 0.22 0.18 0.36 0.30 0.13 0.49 0.27 0.64 0.31 0.20 0.35 0.28 0.24 0.38 -0.04 0.12 0.22 0.83 0.41 0.16 0.47 0.32 0.55 0.30 0.22 0.29 0.35 0.44 0.54 0.30 0.45 0.38 0.52 0.20 0.74 0.30 0.70 0.53 0.38 0.24 0.64 0.31 0.30 0.80 0.29 0.64 0.56 0.49 0.56 0.53