Nocturnal isoprene oxidation over the Northeast United States in summer and its impact on reactive nitrogen partitioning and secondary organic aerosol

Abstract

Abstract. Isoprene is the largest single VOC emission to the atmosphere. Although it is primarily oxidized photochemically during daylight hours, late-day emissions that remain in the atmosphere at sunset undergo oxidation by NO 3 in regionally polluted areas with large NO x levels. A recent aircraft study examined isoprene and its nocturnal oxidants in a series of night flights across the Northeast US, a region with large emissions of both isoprene and NO x . Substantial amounts of isoprene that were observed after dark were strongly anticorrelated with measured NO 3 and were the most important factor determining the lifetime of this radical. The products of photochemical oxidation of isoprene, methyl vinyl ketone and methacrolein, were more uniformly distributed, and served as tracers for the presence of isoprene at sunset, prior to its oxidation by NO 3 . A determination of the mass of isoprene oxidized in darkness showed it to be a large fraction (>20%) of emitted isoprene. Organic nitrates produced from the NO 3 +isoprene reaction, though not directly measured, were estimated to account for 2-9% of total reactive nitrogen. The mass of isoprene oxidized by NO 3 was comparable to and correlated with the organic aerosol Correspondence to: S. S. Brown (steven.s.brown@noaa.gov) loading for flights with relatively low organic aerosol background. The contribution of nocturnal isoprene oxidation to secondary organic aerosol was determined in the range 1-17%, and isoprene SOA mass derived from NO 3 was calculated to exceed that due to OH by approximately 50%.

Peroxy radical chemistry and OH radical production during the NO<sub>3</sub>-initiated oxidation of isoprene

“…Thus, the isoprene:NO 3 ratios during the experiment, while variable, are likely on the order of ∼ 1000 when most of the reaction takes place. Apel et al (2002) and Brown et al (2009) find isoprene concentrations ∼ 1 ppb at sunset in Northern Michigan and New England, respectively, so NO 3 levels of ∼ 1 ppt would yield similar ratios. Studies of daytime NO 3 oxidation find even higher ratios, with isoprene concentrations ∼ 10 ppb and NO 3 concentrations ∼ 0.5 ppt, or a ratio of ∼ 20 000 (Brown et al, 2005;Fuentes et al, 2007).…”

“…Globally, it is estimated that the isoprene + NO 3 reaction is responsible for ∼ 6-7 % of total isoprene oxidation (Horowitz et al, 2007;Ng et al, 2008) and ∼ 15 % of oxidized nitrogen consumption . Field studies in the northeastern United States, which has a mix of NO x and isoprene sources, find that ∼ 22 % of isoprene oxidation in the residual daytime boundary layer, ∼ 40 % of isoprene oxidation in airmasses advected offshore within the marine boundary layer, and ∼ 73 % of NO 3 consumption can be attributed to this reaction (Warneke et al, 2004;Brown et al, 2009). In addition, the isoprene + NO 3 reaction is likely an important source of isoprene nitrates, which are significant NO x -reservoir compounds affecting regional ozone formation (von Kuhlmann et al, 2004;Fiore et al, 2005;Horowitz et al, 1998Horowitz et al, , 2007.…”

“…Although nighttime isoprene emissions are negligible (Sharkey et al, 1996;Harley et al, 2004), isoprene emitted late in the day, as OH concentrations drop, remains in the nighttime atmosphere (Starn et al, 1998;Stroud et al, 2002;Warneke et al, 2004;Steinbacher et al, 2005;Brown et al, 2009). The rate constant for isoprene's reaction with NO 3 is ∼ 50 000 times higher than that of its reaction…”

Radical chemistry at night: comparisons between observed and modelled HO<sub>x</sub>, NO<sub>3</sub> and N<sub>2</sub>O<sub>5</sub> during the RONOCO project

“…When primary production of OH by solar photolysis cannot occur, other oxidants dominate, notably O 3 and NO 3 (Mihelcic et al, 1993;Carslaw et al, 1997;Salisbury et al, 2001;Fleming et al, 2006;Warneke et al, 2004;Brown et al, 2009Brown et al, , 2011.…”

Speciation of organic aerosols in the Saharan Air Layer and in the free troposphere westerlies

“…At nighttime, the VOC-NO 3 interaction dominates, with SOA yields greater than those for OH or O 3 oxidation (Ng et al, 2017, and references therein). Previous modelling studies by Hoyle et al (2007) suggested that, during twilight conditions, ∼ 21 % of the global average SOA may be due to oxidation of SOA precursors by NO 3 , and measurements performed by Brown et al (2009) found that, during nighttime, 1-17 % of SOA was the result of NO 3 -initiated isoprene oxidation.…”