Loss of isoprene and sources of nighttime OH radicals at a rural site in the United States: Results from photochemical models

Abstract

[1] A one-dimensional Lagrangian model for atmospheric transport and photochemistry has been developed and used to interpret measurements made at Pellston, Michigan, during the summer of 1998. The model represents a moving vertical column of air with vertical resolution of 25 m near the ground. Calculations have been performed for a series of trajectories, with representation of emissions, vertical mixing, and photochemistry for a 3-day period ending with the arrival of the air column at Pellston. Results have been used to identify causes of the observed decrease in isoprene at night, to investigate causes of high nighttime OH. Significant OH can be generated at night by terpenes if it is assumed that some fast-reacting monoterpenes are emitted at rates comparable to inventory emissions for terpenes. However, this nighttime OH is confined to a shallow surface layer (0 -25 m) and has little impact on nighttime chemistry. The observed decrease in isoprene at night can be reproduced in models with low OH, and is attributed primarily to vertical dilution. There is also evidence that transport from Lake Michigan contributes to low nighttime isoprene at Pellston. Model results compare well with measured isoprene, NO x , and with isoprene vertical profiles. Significant model-measurement discrepancies are found for OH, HO 2 , methylvinylketone, and formaldehyde.

Flux estimates and OH reaction potential of reactive biogenic volatile organic compounds (BVOCs) from a mixed northern hardwood forest

“…1b shows the hourly means of these measured and modeled fluxes as well as the difference (between measured and modeled fluxes) during these 20 days. The model generally overpredicts the flux during the mornings and underpredicts the fluxes in the afternoons, which is a similar finding as that reported by Sillman et al (2002).…”

Nighttime observation and chemistry of HO<sub>x</sub> in the Pearl River Delta and Beijing in summer 2006

“…In their model results, the observed loss of isoprene was mainly caused by the vertical diffusions. Nevertheless, Sillman et al (2002) also pointed out that in a shallow layer near the surface, the chemical reaction with OH might be important as diagnosed by Faloona et al (2001) for the same campaign. Furthermore, in the shallow nocturnal boundary layer, weak isoprene emitters could still play a role, such as emissions from urban traffic (Lee and Wang, 2006;Liu et al, 2008) and biogenic emissions under dark conditions, which are usually neglected compared to daytime emissions (Guenther, 1999;Shao et al, 2001).…”

“…Besides its chemical removal by OH, isoprene is also subject to transport, for which we have insufficient knowledge with respect to the spatial/vertical isoprene distribution around the measurement sites. As diagnosed by Sillman et al (2002) for the Michigan forested areas, the nighttime loss of isoprene can be attributed to three factors: chemical reaction with OH, vertical diffusion, and advection. In their model results, the observed loss of isoprene was mainly caused by the vertical diffusions.…”

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

“…Although nighttime decays of surface-level isoprene are well-documented (Hurst et al, 2001;Trainer et al, 1991), their interpretation is often complicated by the competing effects of chemistry and transport, particularly within shallow nocturnal boundary layers . For example, measurements at forested sites impacted by urban NO x emissions have in several instances found evening isoprene loss rates consistent with consumption by NO 3 radicals (Starn et al, 1998;Steinbacher et al, 2005;Stroud et al, 2002), while analysis of data from more remote locations have shown the importance of either transport or, possibly, additional oxidants such as nighttime OH (Biesenthal et al, 1998;Sillman et al, 2002). Analyses of surface level NO 3 measurements in regions downwind of forested areas have shown that biogenic VOCs, including isoprene and monoterpenes, consume a significant fraction of NO 3 radicals (Aldener et al, 2006;Ambrose et al, 2007;Geyer et al, 2001;McLaren et al, 2004).…”