Simulations of atmospheric OH, O<sub>3</sub> and NO<sub>3</sub> reactivities within and above the boreal forest

Abstract

Abstract. Using the 1-D atmospheric chemistry transport model SOSAA, we have investigated the atmospheric reactivity of a boreal forest ecosystem during the HUMPPA-COPEC-10 campaign (summer 2010, at SMEAR II in southern Finland). For the very first time, we present vertically resolved model simulations of the NO 3 and O 3 reactivity (R) together with the modelled and measured reactivity of OH. We find that OH is the most reactive oxidant (R ∼ 3 s −1 ) followed by NO 3 (R ∼ 0.07 s −1 ) and O 3 (R ∼ 2 × 10 −5 s −1 ). The missing OH reactivity was found to be large in accordance with measurements (∼ 65 %) as would be expected from the chemical subset described in the model. The accounted OH radical sinks were inorganic compounds (∼ 41 %, mainly due to reaction with CO), emitted monoterpenes (∼ 14 %) and oxidised biogenic volatile organic compounds (∼ 44 %). The missing reactivity is expected to be due to unknown biogenic volatile organic compounds and their photoproducts, indicating that the true main sink of OH is not expected to be inorganic compounds. The NO 3 radical was found to react mainly with primary emitted monoterpenes (∼ 60 %) and inorganic compounds (∼ 37 %, including NO 2 ). NO 2 is, however, only a temporary sink of NO 3 under the conditions of the campaign (with typical temperatures of 20-25 • C) and does not affect the NO 3 concentration. We discuss the difference between instantaneous and steadystate reactivity and present the first boreal forest steady-state lifetime of NO 3 (113 s). O 3 almost exclusively reacts with inorganic compounds (∼ 91 %, mainly NO, but also NO 2 during night) and less with primary emitted sesquiterpenes (∼ 6 %) and monoterpenes (∼ 3 %). When considering the concentration of the oxidants investigated, we find that OH is the oxidant that is capable of removing organic compounds at a faster rate during daytime, whereas NO 3 can remove organic molecules at a faster rate during night-time. O 3 competes with OH and NO 3 during a short period of time in the early morning (around 5 a.m. local time) and in the evening (around 7-8 p.m.). As part of this study, we developed a simple empirical parameterisation for conversion of measured spectral irradiance into actinic flux. Further, the meteorological conditions were evaluated using radiosonde observations and ground-based measurements. The overall vertical structure of the boundary layer is discussed, together with validation of the surface energy balance and turbulent fluxes. The sensible heat and momentum fluxes above the canopy were on average overestimated, while the latent heat flux was underestimated.

Simulating ozone dry deposition at a boreal forest with a multi-layer canopy deposition model

“…SOSAA has already been applied and verified in several studies (e.g. Kurtén et al, 2011;Mogensen et al, 2011Mogensen et al, , 2015Boy et al, 2013;Bäck et al, 2012;Smolander et al, 2014;Zhou et al, 2015).…”

“…The related chemical reactions of stabilised Criegee intermediates (sCIs) with updated reaction rates from Boy et al (2013) are also taken into account in current simulations. For more details about emissions and chemistry we refer to Mogensen et al (2015).…”

Contribution from biogenic organic compounds to particle growth during the 2010 BEACHON-ROCS campaign in a Colorado temperate needleleaf forest

“…The strongest gradient modeled is again a few hundred meters lower than the sounding data. This tendency of SOSAA to slightly underestimate the height of the mixed layer has also been observed in studies made in Hyytiälä, Finland (Mogensen et al, 2015). At 22:00:00 LT, the nocturnal boundary layer has built up.…”

“…After this time, the model results become higher than the observations, which should result from (1) missing sinks and (2) overestimated production. The missing sink terms have been studied previously at MEFO by Nakashima et al (2014). Based on measurements, Nakashima et al (2014) concluded a missing OH reactivity of 29.5 %, which may mainly be due to oxidized products of biogenic species.…”

“…It includes all basic aerosol processes, including nucleation, condensation, coagulation and dry deposition. The model performance has been validated against field measurements from Hyytiälä, Finland, in various studies (Boy et al, 2013;Mogensen et al, 2011Mogensen et al, , 2015Smolander et al, 2014).…”