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Nitrogen oxides (NO_<i>x</i>) have fallen steadily across the US over the last 15 years. At the same time, NO_<i>x</i> concentrations decrease on weekends relative to weekdays, largely without co-occurring changes in other gas-phase emissions, due to patterns of diesel truck activities. These trends taken together provide two independent constraints on the role of NO_<i>x</i> in the nonlinear chemistry of atmospheric oxidation. In this context, we interpret interannual trends in wintertime ammonium nitrate (NH₄NO₃) in the San Joaquin Valley of California, a location with the worst aerosol pollution in the US and where a large portion of aerosol mass is NH₄NO₃. Here, we show that NO_<i>x</i> reductions have simultaneously decreased nighttime and increased daytime NH₄NO₃ production over the last decade. We find a substantial decrease in NH₄NO₃ since 2000 and conclude that this decrease is due to reduced nitrate radical-initiated production at night in residual layers that are decoupled from fresh emissions at the surface. Further reductions in NO_<i>x</i> are imminent in California, and nationwide, and we make a quantitative prediction of the response of NH₄NO₃. We show that the combination of rapid chemical production and efficient NH₄NO₃ loss via deposition of gas-phase nitric acid implies that high aerosol days in cities in the San Joaquin Valley air basin are responsive to local changes in NO_<i>x</i> within those individual cities. Our calculations indicate that large decreases in NO_<i>x</i> in the future will not only lower wintertime NH₄NO₃ concentrations but also cause a transition in the dominant NH₄NO₃ source from nighttime to daytime chemistry.