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TECHNICAL REPORTS

Soil Water Content and Soil Disaggregation by Disking Affects PM₁₀ Emissions

^a Dep. of Land, Air and Water Resources
^b Dep. of Plant Sciences, Univ. of California, Davis, CA. 95616

^* Corresponding author (nmmadden{at}ucdavis.edu).

Received for publication May 1, 2008. Row crop agriculture in California's San Joaquin Valley is a major contributor of particulate matter <10 µm in aerodynamic diameter (PM₁₀). The California Air Resources Board uses fixed PM₁₀ emission values for various tillage operations to monitor and design attainment strategies. However, fixed emission values do not reflect emissions produced by a single implement operating under different soil conditions. This 2-yr study evaluated how PM₁₀ mass concentrations (µg L^–1) from disking change as a function of gravimetric soil water content (GWC), number of sequential diskings (D1, D2, D3), and the soil's weighted mean ped diameter (WMPD). Results showed PM₁₀ increased logarithmically as the soil dried from a GWC of 14 to 4%. Average PM₁₀ values at the lower GWCs were six to eight times greater than at the higher GWCs. Number of diskings also increased PM₁₀, especially in drier soil. Below a GWC of 7%, PM₁₀ for D3 was about twice that for D1. Despite strong correlations between more disking and lower WMPD, a lower WMPD did not always result in an increase in PM₁₀. This underscored the role soil water plays in reducing PM₁₀ at high GWCs despite low WMPDs from multiple diskings. Three-way interactions between GWC, disking, and PM₁₀ showed, on average, that the magnitude of PM₁₀ produced by D1 was 1.3 to 1.6 times lower than by D3, despite having insignificantly different GWC. Therefore, a disking operation can yield two different PM₁₀ values under similar GWCs if the amount of soil disaggregation is different. Our results show that inclusion of soil parameters in PM₁₀ emission estimates is essential to describing agriculture's role in air quality violations and to assess the value of proposed mitigation measures, such as conservation tillage.

Abbreviations: CARB, California Air Resources Board • CT, conservation tillage • GWC, gravimetric soil water content • PM₁₀, particulate matter <10 µm in aerodynamic diameter • SJV, San Joaquin Valley • WMPD, weighted mean ped diameter