Sulfuric Acid and OH Concentrations in a Boreal Forest Site
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Atmos. Chem. Phys., 9, 7435–7448, 2009 www.atmos-chem-phys.net/9/7435/2009/ Atmospheric © Author(s) 2009. This work is distributed under Chemistry the Creative Commons Attribution 3.0 License. and Physics Sulfuric acid and OH concentrations in a boreal forest site T. Petaj¨ a¨1,2, R. L. Mauldin, III2, E. Kosciuch2, J. McGrath2,3, T. Nieminen1, P. Paasonen1, M. Boy1, A. Adamov4, T. Kotiaho4,5, and M. Kulmala1 1Division of Atmospheric Sciences and Geophysics, Department of Physics, P.O. Box 64, 00014 University of Helsinki, Finland 2Atmospheric Chemistry Division, Earth and Sun Systems Laboratory, National Center for Atmospheric Research, P.O. Box 3000, Boulder, CO 80307-5000, USA 3Department of Atmospheric and Oceanic Sciences, University of Colorado, Boulder, CO 80309-0311, USA 4Laboratory of Analytical Chemistry Department of Chemistry, A.I. Virtasenaukio 1, P.O. Box 55, 00014 University of Helsinki, Finland 5Faculty of Pharmacy, Division of Pharmaceutical Chemistry, P.O. Box 56, Viikinkaari 5E, 00014 University of Helsinki, Finland Received: 11 September 2008 – Published in Atmos. Chem. Phys. Discuss.: 3 December 2008 Revised: 21 August 2009 – Accepted: 15 September 2009 – Published: 7 October 2009 Abstract. As demonstrated in a number of investigations, 1 Introduction gaseous sulfuric acid plays a central role in atmospheric aerosol formation. Using chemical ionization mass spec- An important phenomenon associated with the atmospheric trometer the gas-phase sulfuric acid and OH concentration aerosol system is the formation of new atmospheric aerosol were measured in Hyytial¨ a,¨ SMEAR II station, Southern Fin- particles. Atmospheric aerosol formation consists of a com- land during 24 March to 28 June 2007. Clear diurnal cycles plicated set of processes that include the production of were observed as well as differences between new particle nanometer-size clusters from gaseous vapors, the growth of formation event days and non-event days. Typically, the daily these clusters to detectable sizes, and their simultaneous re- maximum concentrations of gas phase sulfuric acid varied moval by coagulation with the pre-existing aerosol parti- from 3×105 to 2×106 molec cm−3 between non-event and cle population (Kerminen et al., 2001; Kulmala, 2003). It event days. Noon-time OH concentrations varied from 3- has been proposed, and confirmed by observations, that at- 6×105 molec cm−3 and not a clear difference between event mospheric new particle formation depends on the sulfuric and non-events was detected. The measured time series were acid concentration (Weber et al., 1996, 1997; Kulmala et al., also used as a foundation to develop reasonable proxies for 2006). Recent theoretical and observational results predict sulfuric acid concentration. The proxies utilized source and that atmospheric clusters (Kulmala et al., 2007) are activated sink terms, and the simplest proxy is radiation times sulfur and sulfuric acid is an important player in this process. In ad- dioxide divided by condensation sink. Since it is still chal- dition, large amounts of neutral ammonium bisulfate clusters lenging to measure sulfuric acid in ambient concentrations, have been predicted theoretically (Vehkamaki¨ et al., 2004). and due to its significant role in atmospheric particle forma- As demonstrated in a number of investigations (Kulmala tion, reasonable proxies are needed. We use all together three et al., 2004; Kulmala and Kerminen , 2009), gaseous sulfuric different proxies and one chemical box model and compared acid plays a central role in atmospheric aerosol formation. A their results to the measured data. The proxies for the sulfuric technique for measuring the gas-phase sulfuric acid concen- acid concentration worked reasonably well, and will be used tration even down to about 104 molec cm−3 has already been to describe sulfuric acid concentrations in SMEAR II station, available for more than a decade (Eisele and Tanner, 1993; when no measured sulfuric acid data is available. With cau- Berresheim et al., 2000). As a result, a number of field cam- tion the proxies could be applied to other environments as paigns have been performed that allow us to look at connec- well. tions between the gas-phase sulfuric acid concentration and aerosol formation rate, both in laboratory and atmospheric conditions. Correspondence to: T. Petaj¨ a¨ In the laboratory experiments (Viisanen et al., 1997; (tuukka.petaja@helsinki.fi) Berndt et al., 2005), particle number concentration is found Published by Copernicus Publications on behalf of the European Geosciences Union. 7436 T. Petaj¨ a¨ et al.: Sulfuric acid and OH in Hyytial¨ a¨ to have a power-law dependency on the sulfuric acid concen- they can be used to describe average variability in the sul- trations having exponents ranging from 4 to 10. On the other furic acid concentrations. With the aid of these proxies we hand in the atmospheric conditions this dependency is not as are able to produce a time series of a proxy-sulfuric acid for strong (Weber et al., 1996, 1997; Kulmala et al., 2006; Sihto the time periods without direct measurements of gas phase et al., 2006; Riipinen et al., 2007), with an exponent of only sulfuric acid. 1–2. In addition to initial formation, sulfuric acid contributes to the growth of aerosol particles. However, there are strong 2 Experimental setup indications that condensing vapors other than sulfuric acid Measurements presented in this study were conducted are frequently needed to explain the observed particle growth at SMEAR II (Station for measuring Forest Ecosystem- rates (Kulmala et al., 2001b; Held et al., 2004; Fiedler et al., Atmosphere Relations) located in Hyytial¨ a,¨ Southern Fin- 2005; Boy et al., 2005; Sihto et al., 2006). Oxidation prod- land. The site is surrounded by 42-year-old pine domi- ucts of volatile organic compounds (VOCs) are viable can- nated forest (Pinus Sylvestris L.). Detailed information about didates to explain the missing growth component (Kulmala the continuous measurements and the infrastructure can be et al., 2001a). A few methods are available to estimate source found elsewhere (Vesala et al., 1998; Kulmala et al., 2001b; strengths of these compounds. It can be calculated from Hari and Kulmala, 2005). These observations were done observed particle growth rates (Kulmala et al., 2005) or by as a part of “European Integrated project on Aerosol Cloud measuring their concentrations directly (Sellegri et al., 2005). Climate and Air Quality Interactions” (EUCAARI) project Concentrations of potential condensable vapors can also be (Kulmala et al., 2009) field campaign. The measurements estimated based on measured VOC emission rates (Tarvainen presented in this study were conducted between 24 March et al., 2005) then modeling their subsequent oxidation in the and 28 June 2007. boundary layer (Boy et al., 2006). This latter method would benefit from measured hydroxyl radical concentrations as it 2.1 Selected Ion Chemical Ionization Mass is together with ozone the main driver of atmospheric oxida- Spectrometer, SICIMS tion of VOCs during daytime. In order to obtain reliable information on the relationship Sulfuric acid and hydroxyl radical concentrations in the gas of in situ sulfuric acid with new particle formation events phase were measured with a technique utilizing selected and their subsequent growth, atmospheric measurements of chemical ionization and subsequent detection with a mass sulfuric acid concentrations and also OH concentrations are spectrometer. The technique is described in more detail in needed. In the case when no measured data of sulfuric acid Tanner et al. (1997); Mauldin III et al. (1998) and references concentrations are available also physically sound proxies therein. The CIMS was operated inside a seatainer, approxi- are needed. The aim of this study is to present results from mately 400 m southwest of the main SMEAR II measurement a field campaign conducted in Hyytial¨ a,¨ Finland, and to de- station. velop physically sound proxies, and investigate their capabil- ities to predict ambient sulfuric acid concentrations. 2.1.1 H2SO4 We measured gaseous phase sulfuric acid with a Chem- ical Ionization Mass Spectrometer (CIMS). With the same Measurement of sulfuric acid with the CIMS consists of sev- instrument we also monitored hydroxyl radical concentra- eral steps. First the sample is drawn inside through an inlet tions, which is the main day-time oxidizing agent in the at- (inner diameter 10.2 cm) with a high flow rate (∼1400 l per mosphere. Measured concentrations were compared with min, lpm) to minimize wall losses. From the main inlet flow modeled sulfuric acid and OH abundances. Several prox- a sample (typically 10 lpm) is extracted coaxially via a thin ies for sulfuric acid concentrations were explored. Similar walled 1.91 cm stainless steel tube. measurements of sulfuric acid and proxy estimates have been The sample is then directed to an ion reaction region, − performed e.g. at the Hohenpeissenberg meteorological ob- where ambient sulfuric acid molecules react with NO3 - servatory (Rohrer and Berresheim, 2006; Berresheim et al., reagent ions. The reagent ions are generated by adding a 2000). The derived proxy in Rohrer and Berresheim (2006) small amount of nitric acid to nitrogen flow and then expos- uses the photolysis rate of ozone as a more direct source term ing them to an alpha-active Am241 (activity 6.67 MBq) ra- in production of sulfuric acid. The measurement of UV-B ra- dioactive source. diation is, however, more common, and the aim here is to ex- Inside an ion reaction region the sample flow is surrounded amine a proxy that can be more readily applied. Furthermore, by a concentric flow containing the reagent ions. Since the − Rohrer and Berresheim (2006) considered in their proxy only two flows are laminar, the air containing NO3 in the outer the source term of OH. For H2SO4 proxy also the sink term core of the flow and sample with atmospheric air containing is important (e.g.