A wintertime PM2.5 episode at the fresno, CA, supersite

作者：Watson JG^*; Chow JC

来源：Atmospheric Environment, 2002, 36(3): 465-475.

DOI：10.1016/S1352-2310(01)00309-0

• supersite
• PM2.5
• nitrate
• black carbon
• conceptual model
• Fresno

A winter PM2.5 episode that achieved a maximum 24-h average of 138 mug m(-3) at the Fresno Supersite in California's San Joaquin Valley between 2 and 12 January, 2000 is examined using 5-min to 1-h continuous measurements of mass, nitrate, black carbon, particle-bound PAH, and meteorological measurements. Every day PM2.5 sampling showed that many episodes. including this one, are missed by commonly applied sixth-day monitoring, even though quarterly averages and numbers of US air quality standard exceedances are adequately estimated. Simultaneous measurements at satellite sites show that the Fresno Supersite represented PM2.5 within the city, and that half or more of the urban concentrations were present at distant, non-urban locations unaffected by local sources. Most of the primary particles accumulated during early morning and nighttime, decreasing when 'surface temperatures increased and the shallow radiation inversion coupled to a valleywide layer. When this coupling occurred, nitrate levels increased rapidly over a 10-30 min period as black carbon and gaseous concentrations dropped. This is consistent with a conceptual model in which secondary aerosol forms above the surface layer and is effectively decoupled from the surface for all but the late-morning and early afternoon period. Primary pollutants, such as organic and black carbon. accumulate within the shallow surface layer in urban areas where wood burning and vehicle exhaust emissions are high. Such a model would explain why earlier studies find nitrate concentrations to be nearly the same among widely separated sites in urban areas, as winds aloft of 1 to 6 ms(-1) could easily disperse the elevated aerosol throughout the valley.

• 出版日期2002-1