Investigation of the nighttime decay of isoprene

Abstract

Abstract. A rapid nighttime decay of isoprene (2-methyl-1,3-butadiene) has been observed at several forest sites. Data from the Program for Research on Oxidants: PHotochemistry, Emissions, and Transport (PROPHET) have been carefully examined with respect to this phenomenon. Essentially every evening (at PROPHET), isoprene concentrations fall from several ppb to levels below 100 ppt, with an average lifetime of 2.7 hours. Since this decay rate exceeds that expected from established nighttime chemistry, other possible mechanisms are suggested and discussed. Reaction with ozone will not occur at a rate consistent with the observed decay. Calculations of nitrate radical concentrations reveal that this oxidant only becomes an important sink for isoprene after the majority of the isoprene decay has taken place. The isoprene flux data were not consistent with dry deposition playing a significant role in nighttime forest loss. On the basis of ambient measurements of OH radical concentrations at the PROPHET site, calculated isoprene decay rates were compared with observations. For some nights the observed decay can be fit strictly by OH consumption; however, the reported OH data overpredict the isoprene loss rate on most nights. We estimate that vertical mixing with isoprene-depleted air probably contributes to the fast isoprene decay observed; however, the measurements needed to support this suggestion have yet to be made.

Nitrate radical oxidation of <i>γ</i>-terpinene: hydroxy nitrate, total organic nitrate, and secondary organic aerosol yields

“…10, NO 3 / HO 2 ratios are ∼ 3 at peak [HO 2 ], then decrease to ∼ 1 as the products reach steady state. In comparison, ambient nighttime NO 3 / HO 2 ratios of ∼ 1 have been measured during the PROPHET 1998 field intensive in northern Michigan (Hurst et al, 2001;Tan et al, 2001), and ∼ 0.25 at the BEARPEX field site in north central California (Bouvier-Brown et al, 2009;Mao et al, 2012). The relatively larger ratios in our chamber, initially, suggest hydroperoxy nitrates may be underrepresented compared to the atmosphere.…”

Comparison of the measured and simulated isoprene nitrate distributions above a forest canopy

“…The boundary layer height was assumed to be 100 m at night, then to linearly increase to 1000 m from 0600 to 0900 EDT, and remain constant until approximately 1800 EDT when it linearly decreased to 100 m again at 2100 EDT, and remained so through the night. A rate constant of 1.3 × 10 −4 s −1 [ Hurst et al , 2001] was used to represent isoprene “loss” (from the measurement height) by vertical mixing. This vertical mixing rate constant was applied to each isoprene nitrate isomer in the model as well.…”

Long-term study of VOCs measured with PTR-MS at a rural site in New Hampshire with urban influences

“…However, the reader is strongly cautioned that these deposition velocities are not true dry deposition velocities and should not be interpreted as such. The nocturnal depletion observed for certain species (not all species are depleted at night) is due to a combination of dry deposition, wet removal (i.e., loss of soluble species to dew), chemical loss, and dilution by vertical mixing (Hurst et al, 2001). It is not possible within the framework of mean seasonal diurnal behavior to determine the relative contribution of each process for each species to the overall loss observed.…”