On the effectiveness of nitrogen oxide reductions as a control over ammonium nitrate aerosol

Sally E Pusede ,R C Cohen,L C Valin,A J Beyersdorf,R M Hoff,J H Crawford,P J Wooldridge,A J Weinheimer,J Walega,J Olson,Joshua L Laughner ,Caroline Parworth ,Christopher D Cappa ,Alexis A Shusterman ,Qi Zhang ,Shannon L Capps ,Hwajin Kim ,Timothy A Berkoff ,K Duffey ,A K Md Ehsanes Saleh ,Alan Fried ,John B Nowak

doi:10.5194/acp-16-2575-2016

Abstract

Abstract. Nitrogen oxides (NOx) have fallen steadily across the US over the last 15 years. At the same time, NOx 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 NOx 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 NOx 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 NOx 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 NOx within those individual cities. Our calculations indicate that large decreases in NOx 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.

Highlights

Aerosol abundances are decreasing across the US, improving air quality and affecting climate
We found that reductions in nitrogen oxides (NOx) have both decreased and increased NH4NO3 formation rates by the various chemical pathways, but the net downward trend in nitric oxide (NO)−3 has been driven by local changes in nighttime chemistry in residual layers decoupled from fresh surface emissions
We showed that high NH4NO3 abundances were a combined function of active chemical production of NO−3 (P NO−3) and rapid atmospheric loss by deposition of gas-phase HNO3; in contrast, the total aerosol mass lifetime was controlled by cold fronts that turnover valley air on average every 5 ± 1.5 days

Summary

Introduction

Aerosol abundances are decreasing across the US, improving air quality and affecting climate. These decreases have been broadly attributed to regulatory controls on the emissions of gas-phase precursors; it has proven difficult to link precursor reductions to observed changes in aerosol concen-. The effect is that weekday NOx levels equal weekend NOx years earlier in the record (Fig. 1). We use this NOx constraint to interpret trends in observed wintertime ammonium nitrate (NH4NO3) concentrations over the last decade in the San Joaquin Valley (SJV) of California

Results

Discussion

Conclusion