| In California's San Joaquin valley, fixed emission values for various tillage operations are used to design maintenance plans for attainment of National Ambient Air Quality Standards for PM10 (i.e., particulate matter <10 mum in aerodynamic diameter). However, fixed emission values misrepresent the dynamic nature of emissions due to constantly changing soil conditions prior to any tillage event. Our research has shown that large variability exists in tillage-generated PM10 due to changes in soil texture, gravimetric soil water content, and soil aggregate size distribution and suggests the need to incorporate these factors into current emission estimates. We have shown that: (i) PM10 emissions increase logarithmically as soil water content decreases with the largest increases beginning at a soil water potential of -1500 kPa (nominal permanent wilting point). (ii) PM10 emissions are influenced by soil aggregate size distribution, however to what extent is highly dependent on soil water content. In wet soils, differences in aggregate size distribution did not affect PM10 emissions. However, in dry soils, the lower the aggregate size distribution the higher the PM10 emission. Thus, a tillage operation can yield two different PM10 emissions under similar soil water contents if the soil aggregate size distribution is different. (iii) Coarse-textured soils begin to emit PM10 sooner than fine-textured soils. (iv) Soils with high %silt to %clay ratios have the greatest potential to produce PM10 when field-dry.Thus, in order to minimize PM10 emissions during tillage, soils should be tilled when soil water contents are at least 1.5 times their value at -1500 kPa, reductions in soil aggregate size distribution with tillage should be done at higher soil water contents, and soils with high %silt to %clay ratios should be under conservation tillage.In short, PM10 emission factors used for regulatory purposes in California's San Joaquin Valley need to be revised to reflect soil conditions prior to any tillage event. Failure to do so may compromise the design and implementation of current PM10 maintenance plans. |
