Summertime total OH reactivity measurements from boreal forest during HUMPPA-COPEC 2010

Abstract

Ambient total OH reactivity was measured at the Finnish boreal forest station SMEAR II in Hyytiälä (Latitude 61°51' N; Longitude 24°17' E) in July and August 2010 using the Comparative Reactivity Method (CRM). The CRM – total OH reactivity method – is a direct, in-situ determination of the total loss rate of hydroxyl radicals (OH) caused by all reactive species in air. During the intensive field campaign HUMPPA-COPEC 2010 (<b>H</b>yytiälä <b>U</b>nited <b>M</b>easurements of <b>P</b>hotochemistry and <b>P</b>articles in <b>A</b>ir – <b>C</b>omprehensive <b>O</b>rganic <b>P</b>recursor <b>E</b>mission and <b>C</b>oncentration study) the total OH reactivity was monitored both inside (18 m) and directly above the forest canopy (24 m) for the first time. The comparison between these two total OH reactivity measurements, absolute values and the temporal variation have been analyzed here. Stable boundary layer conditions during night and turbulent mixing in the daytime induced low and high short-term variability, respectively. The impact on total OH reactivity from biogenic emissions and associated photochemical products was measured under "normal" and "stressed" (i.e. prolonged high temperature) conditions. The advection of biomass burning emissions to the site caused a marked change in the total OH reactivity vertical profile. By comparing the OH reactivity contribution from individually measured compounds and the directly measured total OH reactivity, the size of any unaccounted for or "missing" sink can be deduced for various atmospheric influences. For "normal" boreal conditions a missing OH reactivity of 58%, whereas for "stressed" boreal conditions a missing OH reactivity of 89% was determined. Various sources of not quantified OH reactive species are proposed as possible explanation for the high missing OH reactivity

Ozone deposition into a boreal forest over a decade of observations: evaluating deposition partitioning and driving variables

“…Recent in situ chemistry measurements indicate that the coniferous canopy air space possesses a large, unknown sink for OH (Nölscher et al, 2012). The study suggested that complex yet unknown reactions between the biogenic terpenes or some secondary reaction products may explain the missing OH sink.…”

A new plant chamber facility, PLUS, coupled to the atmosphere simulation chamber SAPHIR

“…Isoprene was found to suppress new particle formation from BVOC photooxidation , while constitutive and stress-induced mono-and sesquiterpene emissions contribute to photochemical particle number and mass formation (Mentel et al, , 2013Hallquist et al, 2009). Also field studies have repeatedly shown that the total reactivity of air masses towards OH is larger than can be explained, based on measurements of individual compounds reactive to OH (e.g., Edwards et al, 2013;Nölscher et al, 2012; Published by Copernicus Publications on behalf of the European Geosciences Union. al., 2011;Di Carlo et al, 2004).…”

Direct measurement of NO<sub>3</sub> reactivity in a boreal forest

“…This site has been focus of intensive research investigating BVOC (Rinne et al, 2005;Holzke et al, 2006;Hakola et al, 2009;Lappalainen et al, 2009;Aaltonen et al, 2011) and their influence on O 3 reactivity (Mogensen et al, 2015;Zhou et al, 5 2017) and OH-reactivity Nölscher et al, 2012) as well as simulations on NO 3 lifetimes (Hakola et al, 2003;Peräkylä et al, 2014). SMEAR II is located 49 km north-east of Tampere (pop.…”

“…In forested environments, the measured reactivity has generally been found to be significantly higher than that calculated from summing up reactivity due to individual reactive trace gases, 5 Nölscher et al, 2012;Nolscher et al, 2016) resulting in an apparent "missing reactivity". In a similar vein, O 3 flux measurements in Californian pine forests required monoterpene emissions that were 10 times higher than measured in order to explain the O 3 losses (Goldstein et al, 2004).…”