High Natural Aerosol Loading over Boreal Forests

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Science  14 Apr 2006:
Vol. 312, Issue 5771, pp. 261-263
DOI: 10.1126/science.1123052


Aerosols play a key role in the radiation balance of the atmosphere. Here, we present evidence that the European boreal region is a substantial source of both aerosol mass and aerosol number. The investigation supplies a straightforward relation between emissions of monoterpenes and gas-to-particle formation over regions substantially lacking in anthropogenic aerosol sources. Our results show that the forest provides an aerosol population of 1000 to 2000 particles of climatically active sizes per cubic centimeter during the late spring to early fall period. This has important implications for radiation budget estimates and relevancy for the evaluation of feedback loops believed to determine our future climate.

The boreal forest plays an important role in both climate regulation and carbon cycling (13). Boreal forests represent one-third of all forested land and cover 15 million square kilometers of land. The boreal region is characterized by large seasonal variations in temperature, and the flora is dominated by different pine and spruce species. Most boreal forests contain few large sources of anthropogenic pollution.

In the northern European boreal region, long-term studies of aerosol formation and transformation processes have been performed at several measurement sites. One of the major research aims of these studies, including those performed at the Finnish background station Hyytiälä (47), has been to investigate the role of particle formation. This is important because homogeneous or ion-induced nucleation can provide substantial numbers of aerosols in an environment otherwise deficient of primary sources contributing to the fine-particle mode. This has clear relevance for understanding radiation budgets. Similar issues have been addressed at other locations in northern Europe. Particle-formation events over the boreal forest are well-studied phenomena at stations located in Finnish Lapland [Pallas and Värriö (810)] and at the Scandinavian rim of the boreal region in Sweden [Aspvreten (11)]. The mechanisms responsible for the formation and growth of these particles are still uncertain. Although sulfuric acid is one of the most likely candidates thought to be responsible for the formation of the initial nanometer-sized particles, sulfur chemistry does not sustain enough sulfuric acid in the atmosphere to explain more than a small fraction of the observed particle-size growth rate. To explain the observed growth, which is up to a diameter of 50 to 100 nm, other compounds are required (12).

Organic constituents comprise a large fraction of the global aerosol burden (13, 14) and there is growing evidence that naturally emitted terpenes contribute notably to gas-to-particle formation (15). In the boreal region of northern Europe, monoterpenes are abundant at concentrations ranging from some tens of parts per trillion up to parts per billion depending on season, boundary layer conditions, and temperature (1618). When released into the atmosphere, monoterpenes undergo oxidation by ozone, hydroxyl radical, and nitrate radical to yield numerous compounds, the most interesting of which are oxygenated carbon compounds such as mono- and dicarboxylic acids (19). Similar compounds may contribute considerably, either directly or through polymerization in the particle phase (20), to the gas-to-particle conversion rate over forested areas.

Here, we present evidence of substantial contributions to the aerosol mass and abundance from natural emissions of aerosol precursor gases, most likely terpenes. The approach utilizes a statistical method including long-term observations of the submicrometer aerosol number-size distribution at three different stations in the Finnish boreal zone. The three locations selected for this purpose are the station for Measuring Forest Ecosystem-Atmosphere Relations (SMEAR I) located at Värriö [67°46′N, 29°35′E, 390 m above sea level (asl)], the Pallas-Sodankylä Global Atmospheric Watch (GAW) station located close to Pallas (68°01′N, 24°10′E, 303 m asl), and the SMEAR II station (Hyytiälä, 61°51′N, 24°17′E, 170 m asl). The database used includes 5 years (1999 to 2004) of aerosol number-size distribution data from the stations. The study considers only the period from April to September.

The current ambition is to investigate the characteristic changes of the aerosol population in air masses undergoing marine to continental transition over forested areas in northern Norway, Sweden, and Finland. Previous investigations (11, 21) have shown that substantial sources affect the aerosol during this transition. However, these investigations only account for a limited number of cases. By combining previous findings with the current long-term study, we derive a mechanistic explanation of the seemingly rapid changes in aerosol properties during the marine-to-continental transition.

Trajectories were calculated [with the HYSPLIT4 model (22)] for each station with a resolution of 1 hour, throughout the measurement period. From this data set, we extracted trajectories that described transport during 90% of the time in an 80° sector west-north over land relative to each station (Fig. 1). In total, 3700 trajectories described transport to Värriö and Pallas stations, and 4400 trajectories described transport in the Hyytiälä sector. Furthermore, for each single trajectory, we calculated the time of transport over land and measured number-size distribution. Observations were averaged 1 hour around the time of arrival for each trajectory (23).

Fig. 1.

The transport sectors of the trajectories used in the study. To qualify, each trajectory must spend 90% of the time in the sector. The selection of sectors assures minimum input from anthropogenic sources. The northerly stations share the same sector. Red, Hyytiälä; blue, Värriö and Pallas.

The approach is conceptual. Any change in source profile will result in either buildup or depletion of aerosol mass (assuming constant sink processes). The accumulation of mass is in turn closely related to the time the air parcel spent over the sources. Our results show that an equally simple model can be used to describe the marine-to-continental transition, which evidently involves a change in source profiles large enough to result in an apparent accumulation of mass (i.e., integrated effect of sources and sinks of particles smaller than 0.450 μm, assuming a density of ρ = 1500 kg m–3). In Fig. 2A, the average binned mass increase per hourly increment in time spent over land is displayed for the northerly and southerly stations (in blue and red, respectively). The increase in mass is linear for both receptor stations. On average, the two northerly stations gain approximately 0.014 μg m–3 per hour over land with a correlation coefficient of r2 = 0.83. The corresponding fit for the Hyytiälä data shows that the average mass increase is twice as high, 0.028 μg m–3 per hour (r2 = 0.86).

Fig. 2.

(A) Average observed mass (particle diameter Dp < 0.45 μm, ρ = 1500 kg m–3) versus time over land for Hyytiälä (red) and Värriö+Pallas (blue). Data are fitted linearly. The slope and intersect are displayed in the figure for each combination of stations. Only bins contributing more than 10 observational points were considered in the analysis. (B) Size distribution as dN/dlogDp (cm–3) plotted against time over land for Värriö and Pallas. (C) Same as (B) but for Hyytiälä. h, hours.

An apparent mass increase could result from either primary or secondary aerosol sources. The average evolution of the aerosol size distribution and a clear relation between observed size distribution properties and the time spent over land is present at both the northerly stations (Fig. 2B) and at Hyytiälä (Fig. 2C). A continuous growth as time over land increases is evident. The average growth rate for the dominating nuclei/Aitken mode was calculated to be 0.69 nm per hour at the northerly stations and 0.5 nm per hour at the southerly station Hyytiälä (fig. S1). Based on the observed growth rate, we estimated that the vapor generation rate necessary to support this growth of particles less than 80 nm (23) was 6.3 × 103 molecules cm–3 s–1 at Pallas and Värriö and 104 molecules cm–3 s–1 at Hyytiälä. Compared with the overall mass increase, this result indicates that at least 26% (21% at Hyytiälä) of the condensed material has vapor pressure low enough to be partitioned in this size range.

An increase in mass dominated by primary emissions would not result in an apparent growth of the size distribution modes as we observed. This means that the sources of nucleating and/or condensing gases are much larger over land they are over the marine environment. Based on the selection of trajectories, this source is most likely the boreal forest. Because emission of monoterpenes along the transport path is the most promising explanation for substantial gas-to-particle formation over the boreal forest, we tried to estimate the emission of each trajectory. We adopted an approach for monoterpene emission calculations (24) that uses relevant values of emission potential (25) along with estimates of foliar biomass density for the northern parts of Scandinavia and Finland. Using this approach, we calculated emissions in the range of 0.5 to 15 μg m–3 for the transport region of Värriö and Pallas and emissions reaching 30 μg m–3 in the fetch area of the Hyytiälä station (23). We assume that the emissions are confined and well mixed in the lowermost 1000 m of the atmosphere.

The previous calculations were repeated with the use of the accumulated monoterpene emissions as the input variable. In Fig. 3A, the result is shownasmass(μgm–3) versus estimated terpene emissions (μg m–3). Recall that the emission equals the total amount that has been emitted during transport, and not the emission rate. When calculated mass was compared with time over land (Fig. 2A), the slope of the Hyytiälä data set was twice that of the northerly stations. However, the slope of the aerosol mass versus the total emitted monoterpenes (Fig. 3A) is almost identical for the northerly and the southerly stations. The curve is best described by a straight line. The slope at both stations is ∼0.075 and correlation of r2 = 0.79 and r2 = 0.93 for the northerly and southerly stations, respectively. Given that monoterpenes show a high reactivity toward the major atmospheric oxidants (ozone, hydroxyl radical, and nitrate radical), the lifetime during the studied seasons (16) (April to September) will be short enough (at most a couple of hours) to allow us to assume that most terpenes have undergone primary oxidation steps before reaching the receptors. Because the slope roughly describes the aerosol mass gained per mass of the oxidized biogenic volatile organic carbon (BVOC), this is an indirect estimate of the aerosol mass yield resulting from oxidation of terpenes (26). Assuming a reasonably good representation of terpene emission, this apparent mass yield (apparent because we omit the possible effect of the sinks that are also present during transport) would correspond to ∼7.5% [5 to 10% as fits of 25th and 75th percentiles (supporting online material text)]. Although the monoterpene emission estimates are crude, it is clear that latitude- and temperature-dependent emissions of similar compounds are sufficient to support the observed growth. The constant slope of the aerosol mass increase compared with the total terpene emissions indicates that an irreversible and continuous formation of secondary organic aerosol (SOA) from terpenes takes place during transport over forested areas.

Fig. 3.

(A) Average observed mass (Dp < 0.45μm, ρ = 1500 kg m–3) versus binned total emissions of terpenes for Hyytiälä (red) and Värriö+Pallas (blue). Data are fitted linearly. The slope and intersect are displayed for each combination of stations. Only bins contributing more than 10 observational points were considered in the analysis. (B) 50th to 75th percentile ranges for observed and averaged number concentration (cm–3) in the size range from 10 to 30 nm (green), 30 to 90 nm (red), and >90 nm (black) plotted against calculated total emission of terpenes, Värriö and Pallas. (C) Same as (B) but for Hyytiälä.

Figure 3, B and C, gives the evolution of the number concentration in different size ranges per 0.2 μg m–3 increment in emitted terpenes. The size ranges described are 10 to 30 nm (green), 30 to 90 nm (red), and 90 to 450 nm (black), shown as 50th to 75th percentile range of the mass in each bin. Both figures clearly show that the highest concentrations of very small particles are present when accumulated emissions are low. As emissions increase, more mass is added to the existing particles and they start to grow into the size range between 30 to 90 nm. Throughout the transport over land and the associated emissions, the accumulation mode concentration (>90 nm) increases almost linearly. This means that emissions of monoterpenes (most likely in combination with other trace gases such as H2SO4, H2O, and NH3) serve as an efficient way of contributing to an increased aerosol number concentration. The formation of particles remains high only as long as the balance between production and removal of condensable gases supports a high enough concentration of condensable gases for nucleation to occur. Because formation of particles adds to the overall sink of precursor gases, this balance becomes distorted, disfavoring further nucleation as condensable gases are preferentially partitioned on existing larger particles. The nucleation quenches itself. Nucleation thus serves as a regulatory mechanism for building up and maintaining high number concentrations over land. Only during the first stages of the transition does nucleation contribute to a substantial increase in number. This means that the forest at high latitudes can typically support 1000 to 2000 cm–3, reflecting a steady-state aerosol population representative of the typical source strength in this region. This equilibrium in particle number is established rapidly (Fig. 3, B and C).

Our results clearly show that a substantial gas-to-particle formation of BVOC to SOA takes place over the boreal forest in northern Europe. These BVOCs are most likely emitted from the forest itself and, based on previous findings, are most likely constituted of terpenes. Based on modeled monoterpene emission, our analysis suggests an apparent mass yield in the range of 5 to 10%. The investigations further show that the boreal forest typically sustains 1000 to 2000 particles cm–3 in a climatic relevant size range (∼40 to 100 nm). This notable level of particle number is established rapidly, making estimates of the natural aerosol loadings over the boreal forest comparably easy. Because we provided a similar mechanistic and quantitative behavior for two widely separate locations (more than 700 km apart), the derived relations suggest that similar mechanisms control the aerosol number and mass evolution over large areas in the boreal regions of the Northern Hemisphere.

The high natural aerosol loading in the boreal forest environment should be contrasted with other natural aerosol systems, such as the remote marine environment, in which substantially lower number densities of climate-active particles have been observed (27).

Recent findings (28) further support the role of aerosol production over the boreal forest in contributing to the cloud condensation nuclei (CCN) and cloud droplet populations in remote areas. These findings together with our study clearly establish that the forest is a major source of climate-relevant aerosol particles, most likely also capable of competing with the anthropogenic CCN loadings transported over forested areas.

Supporting Online Material

Materials and Methods

SOM Text

Figs. S1 to S6


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