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
The overall rate constant for the reaction OH + CH 3 COOH f products in the temperature range of 229-300 K was determined using a chemical ionization mass spectrometer coupled to a high-pressure turbulent flow reactor (∼200 Torr of carrier gas N 2 ). A strong negative temperature dependence of the rate constant was found in this range which can be expressed in Arrhenius form as k 1 (T) ) ((2.2 ( 0.2) × 10 -14 ) exp((1012 ( 80)/T) cm 3 molecule -1 s -1 with k 1 ) 6.6 × 10 -13 cm 3 molecule -1 s -1 at 298 K. When these results are combined with previous measurements in the range of 298-446 K, 8 a three-parameter expression can be derived: k 1 (T) ) (2.45 × 10 -16 )(T/298) 5.24(0.68 exp((2358 ( 189)/T) cm 3 molecule -1 s -1 , describing the curvature of the Arrhenius plot observed at T > 300 K. A branching fraction of (64 ( 17)% was determined between 300 and 249 K for the H-atom abstraction from the carboxyl group OH + CH 3 COOH f CH 3 + CO 2 + H 2 O. This latter parameter was measured as the yield of CO 2 , formed as a result of the fast decomposition of the primary CH 3 C(O)O radical. Atmospheric implications of the obtained results are discussed. The obtained k 1 value provides a lifetime of CH 3 COOH in the upper troposphere (UT) that is a factor of 2 lower than that calculated so far from existing recommendations. The data also show that acetic acid could be as significant as methane in influencing the oxidative capacity of the UT considering that concentrations of CH 3 COOH from hundreds of pptv to a few ppbv have been measured during several campaigns.
A high-pressure turbulent flow reactor coupled with a chemical ionization mass spectrometer was used to investigate the minor channel (1b) producing nitric acid, HNO3, in the HO2 + NO reaction for which only one channel (1a) is known so far: HO2 + NO --> OH + NO2 (1a), HO2 + NO --> HNO3 (1b). The reaction has been investigated in the temperature range 223-298 K at a pressure of 200 Torr of N2 carrier gas. The influence of water vapor has been studied at 298 K. The branching ratio, k1b/k1a, was found to increase from (0.18(+0.04/-0.06))% at 298 K to (0.87(+0.05/-0.08))% at 223 K, corresponding to k1b = (1.6 +/- 0.5) x 10(-14) and (10.4 +/- 1.7) x 10(-14) cm3 molecule(-1) s(-1), respectively at 298 and 223 K. The data could be fitted by the Arrhenius expression k1b = 6.4 x 10(-17) exp((1644 +/- 76)/T) cm3 molecule(-1) s(-1) at T = 223-298 K. The yield of HNO3 was found to increase in the presence of water vapor (by 90% at about 3 Torr of H2O). Implications of the obtained results for atmospheric radicals chemistry and chemical amplifiers used to measure peroxy radicals are discussed. The results show in particular that reaction 1b can be a significant loss process for the HO(x) (OH, HO2) radicals in the upper troposphere.
Abstract. This paper describes the background, instrumentation, goals, and the regional influences on the HUMPPA-COPEC intensive field measurement campaign, conducted at the Boreal forest research station SMEAR II (Station for Measuring Ecosystem-Atmosphere Relation) in Hyytiälä, Finland from 12 July-12 August 2010. The prevailing meteorological conditions during the campaign are examined and contrasted with those of the past six years. Back trajectory analyses show that meteorological conditions at the site in 2010 were characterized by a higher proportion of southerly flow than in the other years studied. As a result the summer Correspondence to: J. Williams (jonathan.williams@mpic.de) of 2010 was anomalously warm and high in ozone making the campaign relevant for the analysis of possible future climates. A comprehensive land use analysis, provided on both 5 and 50 km scales, shows that the main vegetation types surrounding the site on both the regional and local scales are: coniferous forest (Scots pine and/or Norway spruce); mixed forest (Birch and conifers); and woodland scrub (e.g. Willows, Aspen); indicating that the campaign results can be taken as representative of the Boreal forest ecosystem. In addition to the influence of biogenic emissions, the measurement site was occasionally impacted by sources other than vegetation. Specific tracers have been used here to identify the time periods when such sources have impacted the site namely: biomass burning (acetonitrile and CO), urbanPublished by Copernicus Publications on behalf of the European Geosciences Union. 10600 J. Williams et al.: An overview of meteorological and chemical influences anthropogenic pollution (pentane and SO 2 ) and the nearby Korkeakoski sawmill (enantiomeric ratio of chiral monoterpenes). None of these sources dominated the study period, allowing the Boreal forest summertime emissions to be assessed and contrasted with various other source signatures.
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