The result of your conversion between nanogram/cubic kilometer and grain/ounce [US] appears here
To convert between Nanogram/cubic Kilometer and Grain/ounce [US] you have to do the following:
First divide 1.0E-12/1.0E+9 / ((0.45359237/16)*(192/175)/480)/(0.0037854118/128) = 0.
Then multiply the amount of Nanogram/cubic Kilometer you want to convert to Grain/ounce [US], use the chart below to guide you.
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