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

Abstract

Abstract. Nitrogen oxides (NO x ) have fallen steadily across the US over the last 15 years. At the same time, NO x 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 x in the nonlinear chemistry of atmospheric oxidation. In this context, we interpret interannual trends in wintertime ammonium nitrate (NH 4 NO 3 ) 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 NH 4 NO 3 . Here, we show that NO x reductions have simultaneously decreased nighttime and increased daytime NH 4 NO 3 production over the last decade. We find a substantial decrease in NH 4 NO 3 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 x are imminent in California, and nationwide, and we make a quantitative prediction of the response of NH 4 NO 3 . We show that the combination of rapid chemical production and efficient NH 4 NO 3 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 x within those individual cities. Our calculations indicate that large decreases in NO x in the future will not only lower wintertime NH 4 NO 3 concentrations but also cause a transition in the dominant NH 4 NO 3 source from nighttime to daytime chemistry.

Observational assessment of the role of nocturnal residual-layer chemistry in determining daytime surface particulate nitrate concentrations

“…B1-B2). Fast measurements of total NO − 3 (gas + particle, NO − 3 (g+p) ) were only available for a subset of flights (Pusede et al, 2016), and particulate-only NO − 3 measurements were not made with sufficient time resolution, less than about a minute, to allow for robust characterization of the NO − 3 (p) vertical profile. Therefore, NO − 3 (p) vertical profiles for each flight during Episode 1 are estimated from in situ measurements of dry particle scattering and the influence of water uptake on scattering, i.e., from the particle hygroscopicity, and are calibrated against the slower particle-into-liquid sampler (PILS) measurements (Appendix A, Fig.…”

“…This allows for a larger fraction of NO 2 to be oxidized to HNO 3 via the N 2 O 5 hydrolysis pathway (Cabañas et al, 2001;Wagner et al, 2013), and colder temperatures favor partitioning of nitrate to the particle phase (Stelson and Seinfeld, 1982). In winter, nighttime HNO 3 production can more efficiently compete with daytime photochemically driven production due to the low photolysis rates and hydroxyl radical concentrations (Wagner et al, 2013;Pusede et al, 2016). Multiday pollution events (i.e., periods with elevated particulate matter, PM, concentrations) can occur when meteorological conditions inhibit dispersion, as is the case with the persistent cold air pool formation often found in valley regions Baasandorj et al, 2017).…”

“…The balance between production, especially nighttime production, and daytime losses was identified by them as critical to understanding the multiday buildup during pollution events. Further, they concluded from DISCOVER-AQ aircraft measurements that much of the NO − 3 (p) production was localized over the cities given the sharp urban-rural gradients in NO − 3 (p) ; the spatial gradients in 2013 (from Pusede et al, 2016) seem to be sharper than gradients in 2000 (from Chow et al, 2006), likely reflecting the increasing localization of the NO…”

“…Young et al (2016) and Parworth et al (2017) observed similar behavior more than a decade later during DISCOVER-AQ in 2013. Pusede et al (2016) characterized the relationship between long-term (multiyear) surface measurements of wintertime NO − 3 (p) and NO 2 in Fresno and Bakersfield and showed that the decline in NO − 3 (p) in the SJV over time (2001)(2002)(2003)(2004)(2005)(2006)(2007)(2008)(2009)(2010)(2011)(2012) was predominately driven by reduced nocturnal NO − 3 (p) production in the residual layer. The balance between production, especially nighttime production, and daytime losses was identified by them as critical to understanding the multiday buildup during pollution events.…”

Fast particulate nitrate formation via N<sub>2</sub>O<sub>5</sub> uptake aloft in winter in Beijing

“…On nights when NO 3 cannot accumulate in the surface layer owing to high NO emissions, N 2 O 5 uptake can still be active aloft without NO titration. The N 2 O 5 uptake aloft leads to elevated pNO − 3 formation in the upper layer as well as effective NO x removal (Watson and Chow, 2002;Pusede et al, 2016;Baasandorj et al, 2017). Field observations at high-altitude sites in Kleiner Feldberg, Germany (Crowley et al, 2010a), the London British Telecommunications tower, UK (Benton et al, 2010), and Boulder, CO, USA showed elevated N 2 O 5 concentrations aloft.…”