Arctic fog chemistry induces the unexpected growth of Aitken mode particles to CCN-active particles

<p>New particle formation (NPF) and early growth are efficient processes producing high concentrations of cloud condensation nuclei (CCNs) precursors in the Arctic marine boundary layer (AMBL). However, due to short lifetime and lack of condensable vapors, newly formed particles do often not grow beyond 50 nm and cause low CCN particle concentrations in the AMBL. Thus, even the smallest amount of Aitken mode particle growth is capable to significantly increase the CCN budget. However, the growth mechanism of Aitken-mode particles from NPF into CCN range in the Arctic is still rather unclear and was therefore investigated during the cruise campaign PASCAL in 2017.</p> <p>During PASCAL, aerosol particles measurements were performed and an unexpected rapid growth of Aitken mode particles was observed right after fog episodes. Combined field data analyses and detailed multiphase chemistry box model simulations with the CAPRAM mechanism were performed to study the underlying processes. Resulting, a new mechanism is proposed explaining how particles with d < 50 nm are able to grow into CCN size range in the Arctic without requiring high water vapor supersaturation (SS). The investigations demonstrated that the rapid post-fog particle growth of Aitken mode is related to chemical processes within the Arctic fog. The redistribution of semi-volatile acidic (e.g., methanesulfonic acid) and basic (e.g., ammonia) compounds from processed CCN-active particles to smaller CCN-inactive particles can cause a rapid particle growth of Aitken mode particles after fog evaporation enabling them to grow towards CCN size. Comparisons of the model results with Berner impactor measurements supports the proposed growth mechanism.</p> <p>Overall, this study provided new insights on how the increasing frequency of NPF and fog-related particle processing can increase in the number of CCNs and cloud droplets leading to an increased albedo of Arctic clouds and thus affect the radiative balance in the Arctic. Since fogs will occur more frequently in the Arctic as a result of climate change, this growth mechanism and a deeper knowledge on its feedbacks can be essential to understand Arctic warming.</p>

Aitken mode particles as CCN in aerosol- and updraft-sensitive regimes of cloud droplet formation

The high variability of aerosol particle concentrations, sizes and chemical composition makes their description challenging in atmospheric models. Aerosol–cloud interaction studies are usually focused on the activation of accumulation mode particles as cloud condensation nuclei (CCN). However, under specific conditions Aitken mode particles can also contribute to the number concentration of cloud droplets (Nd), leading to large uncertainties in predicted cloud properties on a global scale. We perform sensitivity studies with an adiabatic cloud parcel model to constrain conditions under which Aitken mode particles contribute to Nd. The simulations cover wide ranges of aerosol properties, such as total particle number concentration, hygroscopicity (κ) and mode diameters for accumulation and Aitken mode particles. Building upon the previously suggested concept of updraft (w)- and aerosol-limited regimes of cloud droplet formation, we show that activation of Aitken mode particles does not occur in w-limited regimes of accumulation mode particles. The transitional range between the regimes is broadened when Aitken mode particles contribute to Nd, as aerosol limitation requires much higher w than for aerosol size distributions with accumulation mode particles only. In the transitional regime, Nd is similarly dependent on w and κ. Therefore, we analyze the sensitivity of Nd to κ, ξ(κ), as a function of w to identify the value combinations above which Aitken mode particles can affect Nd. As ξ(κ) shows a minimum when the smallest activated particle size is in the range of the “Hoppel minimum” (0.06 µm ≤ Dmin ≤0.08 µm), the corresponding (w–κ) pairs can be considered a threshold level above which Aitken mode particles have significant impact on Nd. This threshold is largely determined by the number concentration of accumulation mode particles and by the Aitken mode diameter. Our analysis of these thresholds results in a simple parametric framework and criterion to identify aerosol and updraft conditions under which Aitken mode particles are expected to affect aerosol–cloud interactions. Our results confirm that Aitken mode particles likely do not contribute to Nd in polluted air masses (urban, biomass burning) at moderate updraft velocities (w≤3 m s−1) but may be important in deep convective clouds. Under clean conditions, such as in the Amazon, the Arctic and remote ocean regions, hygroscopic Aitken mode particles can act as CCN at updrafts of w<1 m s−1.

Measurement report: The influence of traffic and new particle formation on the size distribution of 1–800 nm particles in Helsinki – a street canyon and an urban background station comparison

Most of the anthropogenic air pollution sources are located in urban environments. The contribution of these sources to the population of atmospheric particles in the urban environment is poorly known. In this study, we investigated the aerosol particle number concentrations in a diameter range from 1 to 800 nm at a street canyon site and at a background station within 1 km from each other in Helsinki, Finland. We use these number size distribution data together with complementary trace gas data and develop a method to estimate the relative contributions of traffic and atmospheric new particle formation (NPF) to the concentrations of sub-3 nm particles. During the daytime, the particle concentrations were higher at the street canyon site than at the background station in all analyzed modes: sub-3 nm particles, nucleation mode (3–25 nm), Aitken mode (25–100 nm), and accumulation mode (100–800 nm). The population of sub-3 nm and nucleation mode particles was linked to local sources such as traffic, while the accumulation mode particles were more related to non-local sources. Aitken mode particles were dominated by local sources at the street canyon site, while at the background station they were mainly influenced by non-local sources. The results of this study support earlier research showing direct emissions of the sub-3 nm particles from traffic. However, by using our new method, we show that, during NPF events, traffic contribution to the total sub-3 nm particle concentration can be small and during daytime (6:00–20:00) in spring it does not dominate the sub-3 nm particle population at either of the researched sites. In the future, the contribution of traffic to particle number concentrations in different urban environments can be estimated with a similar approach, but determining the relationships between the gas and particle concentrations from observations needs to be conducted with longer data sets from different urban environments.

Aitken mode particles as CCN in aerosol- and updraft-sensitive regimes of cloud droplet formation

The high variability of aerosol particle concentrations, sizes and chemical composition makes their description challenging in atmospheric models. Aerosol-cloud interaction studies are usually focused on the activation of accumulation mode particles as cloud condensation nuclei (CCN). However, under specific conditions also Aitken mode particles can contribute to the number concentration of cloud droplets (Nd), leading to large uncertainties in predicted cloud properties on a global scale. We perform sensitivity studies with an adiabatic cloud parcel model to constrain conditions, under which Aitken mode particles contribute to Nd. The simulations cover wide ranges of aerosol properties, such as total particle number concentration, hygroscopicity (κ) and mode diameters for accumulation and Aitken mode particles. Building upon the previously suggested concept of updraft (w)- and aerosol-limited regimes of cloud droplet formation, we show that activation of Aitken mode particles does not occur in w-limited regimes of accumulation mode particles. The transitional range between the regimes is broadened when Aitken mode particles contribute to Nd as aerosol-limitation requires much higher w than for aerosol size distributions with accumulation mode particles only. In the transitional regime, Nd is similarly dependent on w and κ. Therefore, we analyze the sensitivity of Nd to κ, ξ(κ), as a function of w to identify the value combinations, above which Aitken mode particles can affect Nd. As ξ(κ) shows a minimum when the smallest activated particle size is in the range of the Hoppel minimum (0.06 μm ≤ Dmin ≤ 0.08 μm), the corresponding (w,κ) pairs can be considered a threshold level, above which Aitken mode particles have significant impact on Nd. This threshold is largely determined by the number concentration of accumulation mode particles and by the Aitken mode diameter. Our analysis of these thresholds results in a simple parametric framework and criterion to identify aerosol and updraft conditions, under which Aitken mode particles are expected to affect aerosol-cloud interactions. Our results confirm that Aitken mode particles likely do not contribute to Nd in polluted air masses (urban, biomass burning) at moderate updraft velocities (w ≤ 3 m s−1), but may be important in deep convective clouds. Under clean conditions, such as in the Amazon, the Arctic, and remote ocean regions, hygroscopic Aitken mode particles can act as CCN at updrafts of w

The importance of Aitken mode aerosol particles for cloud sustenance in the summertime high Arctic – a simulation study supported by observational data

The potential importance of Aitken mode particles (diameters ∼ 25–80 nm) for stratiform mixed-phase clouds in the summertime high Arctic (>80∘ N) has been investigated using two large-eddy simulation models. We find that, in both models, Aitken mode particles significantly affect the simulated microphysical and radiative properties of the cloud and can help sustain the cloud when accumulation mode concentrations are low (< 10–20 cm−3), even when the particles have low hygroscopicity (hygroscopicity parameter – κ=0.1). However, the influence of the Aitken mode decreases if the overall liquid water content of the cloud is low, either due to a higher ice fraction or due to low radiative cooling rates. An analysis of the simulated supersaturation (ss) statistics shows that the ss frequently reaches 0.5 % and sometimes even exceeds 1 %, which confirms that Aitken mode particles can be activated. The modelling results are in qualitative agreement with observations of the Hoppel minimum obtained from four different expeditions in the high Arctic. Our findings highlight the importance of better understanding Aitken mode particle formation, chemical properties and emissions, particularly in clean environments such as the high Arctic.

An Aitken mode aerosol formation event in the high Arctic: evidence for aggregate breakup

&lt;p&gt;The summertime high Arctic is an extremely low-aerosol region, where even small inputs of particles can have significant impacts on cloud formation and therefore on the surface energy budget. The relative importance of new particle formation from gas phase precursors and primary sea spray production in this region remains uncertain, as does the role of atmospheric transport. We made direct, time-resolved composition measurements of Aitken mode (~20-60 nm diameter) aerosol over the high Arctic pack ice in August-September 2018, including during an intense Aitken mode formation event on August 30-31. The event particles contained both primary sea spray materials (sodium, potassium, and polysaccharide-like organics) and secondary components (non-sea-salt sulfate, methanesulfonic acid, non-sea-salt iodine, and secondary organics), most of which could be quantified on the basis of analytical standards. The composition is consistent with primary sea spray that had been atmospherically processed, and the aerosol size distribution dynamics imply the action of a process by which larger atmospheric particles or aggregates broke up to form smaller particles. Hypotheses to explain the results will be discussed.&lt;/p&gt;

Measurement report: The influence of traffic and new particle formation on the size distribution of 1–800 nm particles in Helsinki: a street canyon and an urban background station comparison

Most of the anthropogenic air pollution sources are located in urban environments. The contribution of these sources to the population of atmospheric particles in the urban environment is poorly known. In this study, we investigated the aerosol particle number concentrations in a diameter range from 1 to 800 nm at a street canyon site and at a background station within 1 km from each other in Helsinki, Finland. We use these number size distribution data together with complementary trace gas data and develop a method to estimate the relative contributions of traffic and atmospheric new particle formation (NPF) to the concentrations of sub 3 nm particles. During the daytime, the particle concentrations were higher at the street canyon site than at the background station in all analyzed modes: sub 3 nm particles, nucleation mode (3–25 nm), Aitken mode (25–100 nm), and accumulation mode (100–800 nm). The population of sub-3 nm and nucleation mode particles was linked to local sources such as traffic, while the accumulation mode particles were more related to non-local sources. Aitken mode particles were dominated by local sources at the street canyon site while at the background station they were mainly influenced by non-local sources. The results of this study support earlier research showing direct emissions of the sub 3 nm particles from traffic. However, by using our new method, we show that during NPF events, traffic contribution to the total sub-3 nm particle concentration can be small and during daytime (6:00–20:00) in spring it does not dominate the sub-3 nm particle population at either of the researched sites. In the future, this method can be applied to estimate the contribution of traffic to particle number concentrations in different urban environments. This knowledge is needed to evaluate the effects of traffic on urban air quality.

The importance of Aitken mode aerosol particles for cloud sustenance in the summertime high Arctic: A simulation study supported by observational data

The potential importance of Aitken mode particles (diameters ~ 25–80 nm) for stratiform mixed-phase clouds in the summertime high Arctic has been investigated using two large-eddy simulation models. We find that in both models Aitken mode particles significantly affect the simulated cloud properties and can help sustain the cloud when accumulation mode concentrations are low (