Hannele Hakola

Direct Observations of Atmospheric Aerosol Nucleation

Aerosol Formation Most atmospheric aerosol particles result from a growth process that begins with atmospheric molecules and clusters, progressing to larger and larger sizes as they acquire other molecules, clusters, and particles. The initial steps of this process involve very small entities—with diameters of less than 2 nanometers—which have been difficult to observe. Kulmala et al. (p. 943; see the Perspective by Andreae) developed a sensitive observational protocol that allows these tiny seeds to be detected and counted, and they mapped out the process of aerosol formation in detail. Detailed aerosol measurements provide a consistent framework for the formation of particles from atmospheric gases. [Also see Perspective by Andreae] Atmospheric nucleation is the dominant source of aerosol particles in the global atmosphere and an important player in aerosol climatic effects. The key steps of this process occur in the sub–2-nanometer (nm) size range, in which direct size-segregated observations have not been possible until very recently. Here, we present detailed observations of atmospheric nanoparticles and clusters down to 1-nm mobility diameter. We identified three separate size regimes below 2-nm diameter that build up a physically, chemically, and dynamically consistent framework on atmospheric nucleation—more specifically, aerosol formation via neutral pathways. Our findings emphasize the important role of organic compounds in atmospheric aerosol formation, subsequent aerosol growth, radiative forcing and associated feedbacks between biogenic emissions, clouds, and climate.

Influence of residential wood combustion on local air quality.

The importance of wood combustion to local air quality was estimated by measuring different air pollutants and conducting chemical mass balance modelling. PM10, PM2.5, PAHs and VOC concentrations in ambient air were measured in a typical Finnish residential area. Measurements were conducted in January-March 2006. For some compounds, wood combustion was clearly the main local source at this site. The effect of wood combustion was more clearly seen for organic compounds than for fine particle mass. For fine particles, background concentrations dominated. However, very high, short-lived concentration peaks were detected, when the wind direction and other weather conditions were favourable. For organic compounds, the effect of wood combustion was seen in diurnal and in two-week average concentrations. PAH-concentrations were often several times higher at the residential area than in the background. Benzene concentrations showed similar diurnal pattern as the use of wood and benzene/toluene ratios indicated that wood combustion is the most important source. A chemical mass balance model was used for studying the effect of wood combustion on the measured concentrations of VOCs. Model results showed that the main local sources for VOCs at Kurkimäki are wood combustion and traffic. Wood combustion was clearly the most important source for many compounds (e.g., benzene).

The ambient concentrations of biogenic hydrocarbons at a northern European, boreal site.

Abstract Concentrations of monoterpenes, 1,8-cineol and light hydrocarbons were measured in Potsonvaara, Ilomantsi, Eastern Finland during two growing seasons in 1997 and 1998. The measuring site was located on the top of a hill, outside a mixed forest. The monthly average summer concentrations of isoprene were 0.3–1.7 ppbC and monoterpenes and 1,8-cineol together 1.6–3.2 ppbC. Isoprene and α-pinene were the most abundant compounds throughout the growing season, but β-pinene, Δ3-carene, camphene, 1,8-cineol, sabinene and limonene were found as well. Isoprene and sabinene concentrations started to increase later than the concentrations of other compounds, and were better correlated with each other than with other compounds. Diurnal variations of monoterpenes show a minimum in the daytime and a maximum at night, except sabinene at midsummer, that has maximum concentrations during the day. The field data support the idea that the effective temperature sum can be used to predict the initiation of emissions of isoprene and also terpene emissions from Betula pendula.

Modeling speciated terpenoid emissions from the European boreal forest.

Abstract We present the first estimates of speciated monoterpene emissions from the North European coniferous forests. Measured emission factors and emission profiles of boreal tree species ( Picea abies, Pinus sylvestris, Betula pendula, Salix phylicifolia, Populus tremula, and Alnus incana ) were used together with detailed satellite land cover information and meteorological data in an emission model based on the Guenther emission algorithms. The variation of the coniferous biomass within the boreal region (60°N to 70°N) was obtained from forest inventory data, and the seasonal variability of the deciduous biomass was taken into account through simple boreal climatology parameterisation. The annual biogenic emissions in the boreal zone are dominated by coniferous species, but in the summer months, the deciduous contribution to the monoterpene and isoprene emissions is considerable. Norway spruce ( Picea abies ) is the most important isoprene emitter in the north European boreal forests. The biogenic emission fluxes in the South boreal zone are approximately twice as high as fluxes in the North boreal zone. α - and β -pinene, carene, and cineole are the most abundant emitted terpenes, with a strong contribution of isoprene and linalool during the summer months.

Seasonal cycle of C2C5 hydrocarbons over the Baltic Sea and Northern Finland

Abstract Measurements of C2C5 hydrocarbons in ambient air from a marine and a mountain site in Scandinavia are presented in relation to observed ozone concentrations. The marine site (the island of Uto) is located in the Baltic Sea about 80 km southwest of mainland Finland, while the other site (Pallas) is 900 km to the north on a fell top in a sub-arctic environment north of the Arctic Circle. The concentrations at both stations show a seasonal cycle with a winter maximum which is typical of arctic and mid-latitude areas. In spring, the concentrations decreased in steps when the air mass changed to a cleaner oceanic or arctic types after a period with the prevailing continental air mass. The decrease of concentrations in spring was further studied in terms of the concentration ratio between Uto and Pallas. For low and moderately reactive alkan es the ratio decreased steadily from March to August, indicating a long period of photochemical transformation from a winter concentration distribution to a summer one. However, for alkenes the concentrations at the northern site, compared to the more southerly one, were lower in spring and increased in summer. This suggests an emissions source that is active in summer in the vicinity of the northern terrestrial site. Isoprene and 1-butene are only observed at the pristine northern site during the growing season. The :local photochemical reactivity of the hydrocarbon mixture was estimated by calculating Propylene-Equivalent concentrations. In summer, isoprene, 1-butene, ethene, and propene comprise 50 and 80% of the total reactive mass at Uto and Pallas, respectively. The spring decrease of total reactive mass is much less compared to the total mass of light hydrocarbons. In fact, the total reactivity-scaled mass at the site north of the Arctic circle was roughly the same in April and in July, the minimum being observed in May before the beginning of the growing season. Logarithmic alkane ratios were used to estimate photochemical age, assuming OH-initiated photochemistry as the only sink process for these species. Based on the ethane and propane concentrations, the calculations gave a seasonal cycle of photochemical activity with a concomitant increase with background ozone concentrations in spring. When the calculations were made using propane and n-butane concentrations, which have a shorter lifetime, the resulting scale of photochemical histories was more regional. Ozone concentrations were higher during continental air masses compared to background ozone concentrations when the photochemical histories showed a more aged hydrocarbon composition.

Determination of source contributions of NMHCs in Helsinki (60°N, 25°E) using chemical mass balance and the Unmix multivariate receptor models

Abstract A chemical mass balance (CMB) receptor model and a multivariate receptor model (Unmix) were used for the determination of the contributions of different non-methane hydrocarbon (NMHC) sources in Helsinki. C2–C10 hydrocarbon measurements were conducted in the Helsinki city area during four different seasons in 2001 in order to get a data set for CMB modelling. The samples were collected using 24-h passive canister sampling simultaneously with 2-h pumped adsorbent samples. The emission profiles of the NMHC sources were also determined. Source contributions were estimated for different seasons of the year and weekly and diurnal variations were examined using the CMB model. According to the CMB analysis major sources of C2–C10 NMHCs were traffic (gasoline exhaust 33%, gasoline vapour 9% and liquid gasoline 12%) and emissions from distant sources, advected to the study area in long-range transported air masses (37%). City gas (2%), solvents (3%), biogenic compounds (1%) and diesel exhaust (0.2%) were minor contributions. Unmix was used to confirm that the major sources were included in the CMB calculations. For C6–C10 hydrocarbons the major sources in both CMB and Unmix analyses were gasoline exhaust, accounting for 52% and 53% of the NMHCs, respectively.

Temporal trends of Persistent Organic Pollutants (POPs) in arctic air: 20 years of monitoring under the Arctic Monitoring and Assessment Programme (AMAP) ☆

Temporal trends of Persistent Organic Pollutants (POPs) measured in Arctic air are essential in understanding long-range transport to remote regions and to evaluate the effectiveness of national and international chemical control initiatives, such as the Stockholm Convention (SC) on POPs. Long-term air monitoring of POPs is conducted under the Arctic Monitoring and Assessment Programme (AMAP) at four Arctic stations: Alert, Canada; Stórhöfði, Iceland; Zeppelin, Svalbard; and Pallas, Finland, since the 1990s using high volume air samplers. Temporal trends observed for POPs in Arctic air are summarized in this study. Most POPs listed for control under the SC, e.g. polychlorinated biphenyls (PCBs), dichlorodiphenyltrichloroethanes (DDTs) and chlordanes, are declining slowly in Arctic air, reflecting the reduction of primary emissions during the last two decades and increasing importance of secondary emissions. Slow declining trends also signifies their persistence and slow degradation under the Arctic environment, such that they are still detectable after being banned for decades in many countries. Some POPs, e.g. hexachlorobenzene (HCB) and lighter PCBs, showed increasing trends at specific locations, which may be attributable to warming in the region and continued primary emissions at source. Polybrominated diphenyl ethers (PBDEs) do not decline in air at Canadas Alert station but are declining in European Arctic air, which may be due to influence of local sources at Alert and the much higher historical usage of PBDEs in North America. Arctic air samples are screened for chemicals of emerging concern to provide information regarding their environmental persistence (P) and long-range transport potential (LRTP), which are important criteria for classification as a POP under SC. The AMAP network provides consistent and comparable air monitoring data of POPs for trend development and acts as a bridge between national monitoring programs and SCs Global Monitoring Plan (GMP).

Aromatic hydrocarbon and methyl tert-butyl ether measurements in ambient air of Helsinki (Finland) using diffusive samplers.

The diffusive sampling method was evaluated for measuring benzene, toluene, ethylbenzene, xylenes, styrene, propylbenzene, ethyltoluenes, trimethylbenzenes and methyl tert-butyl ether (MTBE) in the urban air of Helsinki, Finland. Concentrations were measured in 2-week periods at four different sites during the year 2000. Tube type adsorbent tubes were pre-packed with Carbopack-B (60/80). Analysis was conducted using thermal desorption and gas chromatograph coupled to a mass spectrometer. In different seasons, during five diffusive sampling periods, parallel measurements were conducted using pumped and online sampling. The compared techniques agreed reasonably well for other compounds than trimethylbenzenes. Based on comparisons, diffusive uptake rates for ethyltoluenes, styrene, propylbenzene and MTBE were determined, and for trimethylbenzenes, uptake rates were revised. The concentrations of aromatic compounds in Helsinki metropolitan area were also compared to the concentrations of a rural, forested site in Central Finland.

Measurements of hydrocarbon fluxes by a gradient method above a northern boreal forest

The boreal vegetation is considered as being a major source of hydrocarbons in the atmosphere. Measurements of vertical fluxes of isoprene and monoterpenes above a northern boreal forest are reported in this paper. Measurements were conducted in northern Finland (6758 0 N, 2414 0 E) near the northern timber lines for conifers. The hydrocarbon fluxes were measured by the gradient technique. The total daytime fluxes of following four monoterpenes: a- andb-pinene, carene and camphene, were typically 30‐60 ng m 2 s 1 . These compounds represented 60‐80% of the total monoterpenes in the ambient air. The estimated uncertainty of isoprene fluxes exceeded observed fluxes several times, the mean daytime flux being 4 20 ng m 2 s 1 . The error sources of the measured hydrocarbon fluxes are discussed. The most important source of uncertainty was the hydrocarbon sampling and analysis due to low concentration gradients.

Vertical fluxes of monoterpenes above a Scots pine stand in the Boreal vegetation zone

Abstract The vertical fluxes of monoterpenes, measured above a boreal forest canopy by a micrometeorological gradient method during two days in summer 1997, are presented. The forest, composed mainly of Scots pine ( Pinus sylvestris ), emitted significant amounts of monoterpenes, α-pinene and δ 3 -carene being the major species. The correlations of measured monoterpene fluxes with several meteorological parameters are presented. The best correlation was found with the IR radiation temperature of the canopy. The temperature dependency of the monoterpene fluxes, obtained from the measurements, was β =0.20 °C −1 . The mean total monoterpene emission rates, normalized to 30°C with temperature coefficients β =0.09 °C −1 and gb =0.20 °C −1 , were 190 ng m −2 s −1 and 570 ng m −2 s −1 , respectively.