The contribution of black carbon to global ice nucleating particle concentrations relevant to mixed-phase clouds

Black carbon (BC) aerosol plays an important role in the Earth’s climate system because it absorbs solar radiation and therefore potentially warms the climate; however, BC can also act as a seed for cloud particles, which may offset much of its warming potential. If BC acts as an ice nucleating particle (INP), BC could affect the lifetime, albedo, and radiative properties of clouds containing both supercooled liquid water droplets and ice particles (mixed-phase clouds). Over 40% of global BC emissions are from biomass burning; however, the ability of biomass burning BC to act as an INP in mixed-phase cloud conditions is almost entirely unconstrained. To provide these observational constraints, we measured the contribution of BC to INP concentrations ([INP]) in real-world prescribed burns and wildfires. We found that BC contributes, at most, 10% to [INP] during these burns. From this, we developed a parameterization for biomass burning BC and combined it with a BC parameterization previously used for fossil fuel emissions. Applying these parameterizations to global model output, we find that the contribution of BC to potential [INP] relevant to mixed-phase clouds is ∼5% on a global average.

Black carbon (BC) is the primary light-absorbing aerosol in the atmosphere. Its short lifetime (days to weeks) relative to CO2 and methane makes it an intriguing target for near-term climate mitigation (1). Errors associated with BC climate forcing, however, obfuscate its efficacy as a climate mitigator. The largest contributions to BC’s forcing uncertainties are often attributed to its effects on clouds, in particular mixed-phase clouds [i.e., clouds containing supercooled cloud droplets and ice particles, (2)]. Efforts to reduce these uncertainties are hindered by the complexity of aerosol–cloud interactions (3). Particularly vexing is quantifying the abundance and identity of ice nucleating particles (INPs). INPs provide the only pathway for primary ice formation in mixed-phase clouds; however, they are rare [e.g., ∼1 in 106 particles are INPs at −20 °C (4)]. Despite their rarity, INPs influence mixed-phase cloud ice concentrations and precipitation and therefore alter cloud albedo and lifetime (5). Furthermore, the INP properties of aerosols, such as BC, will affect their own lifetime, vertical structure, and transport to climate-sensitive regions such as the Arctic (6). Despite its importance to the Earth’s climate and near-term climate mitigation strategies, the INP efficiency of BC relevant to mixed-phase clouds remains almost entirely unconstrained from direct observations, encumbering attempts to estimate BC’s impact on mixed-phase clouds in modeling studies (7).BC’s efficacy as an immersion-freezing INP (henceforth, INP will refer only to freezing by particles encapsulated within supercooled cloud droplets, termed immersion freezing and pertinent to mixed-phase cloud conditions) has been studied in the laboratory for decades, with starkly conflicting results. Early laboratory studies showed that acetylene and kerosene flame-generated soot can nucleate ice below −20 °C. (8, 9); after normalizing for surface area, these studies indicated that BC may be more ice active than the well-known INP mineral dust (10). Results from later laboratory studies were contradictory, suggesting that BC was not active as an INP above instrument limits of detection. These included soot aerosols from miniCAST soot generators, graphite spark generator soot, hydrocarbon flame-generated soot, and fullerene soot, as well as various lamp blacks and carbon blacks (11–15).Unfortunately, field study measurements of the contribution of BC to INP concentrations ([INP]) have also been inconclusive. For example, in-cloud measurements from the high-altitude observatory at Jungfraujoch, Switzerland saw that BC is enriched in ice-particle residuals and therefore may efficiently nucleate ice (16); later measurements at the same site, however, saw that BC is depleted in the ice phase, which suggests that BC does not play a significant role in mixed-phase cloud ice nucleation (17, 18).These contradictions in laboratory and field studies suggest that fuel type and combustion conditions determine the ice nucleation properties of BC. Such conditions prescribe BC’s physical and morphological properties as well as its coemitted and coagulated species. Major BC fuel types include fossil fuels and flammable biomass, and major combustion sources include diesel exhaust, residential fuel burning, prescribed burns, and wildfires (2). BC particles from fossil fuel combustion and anthropogenic pollution are not significant sources of INPs. For example, studies on diesel exhaust have shown that less than 1 in 109 BC particles are ice nucleation active at −30 °C (19). Furthermore, ambient [INP] in Beijing, China were relatively constant over several weeks despite BC concentrations varying by a factor of 30 and reaching values as high as 17.26 μg⋅m−3 (20).Elevated [INP] have been observed in biomass-burning smoke during laboratory and field studies (21–23); however, it is unclear from these studies if the INPs are actually BC. Some studies have shown that BC may be the dominant INP type in select biomass burning conditions. For example, soot particles were found to contribute up to 64% of the INPs in prescribed burns within a predominantly wiregrass understory (24). Furthermore, BC contributed up to 70% to [INP] in controlled laboratory burns of grasses (25). As biomass burning represents ∼40% of global BC emissions (2), BC from biomass burning could be a significant source of INP globally. In both of these studies, however, the overall ice-active fractions may be too low to influence [INP], even on the regional level (21). Thus, it remains unclear whether BC contributes to [INP] outside of thick plumes and on a global scale (26).Regional- and global-scale estimates of BC [INP] rely on models that can implement theory-based or empirical ice nucleation parameterizations. Using parameterizations based on BC INP activity from the acetylene and kerosene-burner soot studies, models have found that BC contributes ∼50% to [INP] in springtime low-level Arctic mixed-phase clouds (27), and 23 to 61% to global [INP] depending on dust loadings (28). Taking into account the aforementioned negative results, these modeling studies highlight that BC’s contribution to [INP] is poorly constrained and is estimated to vary from no contribution to being the most abundant INP globally.To assess the role of BC from biomass burning as an INP, we determined the contribution of refractory BC (rBC)-containing particles to [INP] from field measurements of both prescribed burns and wildfires using the single-particle soot photometer coupled to a continuous-flow diffusion chamber (SP2-CFDC) (29, 30). The SP2-CFDC selectively removes rBC from an aerosol stream and quantifies that effect on [INP]. From these burns, we found that rBC-containing particles contributed ≤10% to [INP]. From these results, we developed a surface-area normalized parameterization for BC INPs from biomass burning. The parameterization aligns well with other surface-area normalized parameterizations derived from laboratory proxies of BC and diesel exhaust BC (15, 19, 31). These parameterizations are over four orders of magnitude lower than the parameterization derived from acetylene and kerosene-burner soot studies and used in the aforementioned modeling studies. Assuming the INP characteristics of BC from the burns in this study can be extended to different biomass-burning fuel types and combustion conditions, this study strongly suggests that BC is not an efficient INP. Under this assumption, we assessed the global importance of BC as an INP by applying our parameterization to simulated biomass-burning aerosol from a global chemical transport model. A similar parameterization for diesel exhaust (19) was applied to simulated fossil fuel BC. From these treatments, we estimate that BC’s contribution to simulated, potential [INP] is only ∼5% on a global average.     

Practices and opinions regarding HPV vaccination among French general practitioners: evaluation through two cross-sectional studies in 2007 and 2010

Objectives

To use both quantitative and qualitative methods to investigate the evolution of practices and opinions regarding human papillomavirus (HPV) vaccination among French general practitioners.

Methods

A cross-sectional study (self-questionnaires) was performed in 2007 and repeated in 2010 among 271 general practitioners. Semi-structured interviews were conducted on 27 voluntary participants by a sociologist and analyzed according to content analysis.

Results

Acceptability of HPV vaccination had increased from 2007 to 2010 (79.9 vs. 87.1 %, respectively), just as the practice of HPV vaccination among 14-year-old girls (19.0 vs. 49.1 %, respectively). Though about 60 % reported complications associated with HPV vaccination, irrespective of year, the types of difficulties have varied: difficulties related to “questions asked by patients” had decreased, though concerns about side effects had remained stable. During interviews, difficulties related to “the reason for medical consultation” and “the target age” were often associated with addressing the issue of sexuality, especially when the parents were present.

Conclusions

Although the high level of acceptability of HPV vaccination among general practitioners, which increased from 2007 to 2010, there remain difficulties in addressing this practice.     

Home-based respiratory rehabilitation in adult patients with moderate or severe persistent asthma

Objective: We assessed retrospectively the feasibility of a home-based respiratory rehabilitation (RR) program for asthmatics under optimal pharmacological treatment, as this type of care can reduce costs and offer a more patient-friendly approach for subjects with persistent asthma. Methods: Fifty-two patients with persistent asthma were recruited to the RR program (20 males, 32 females, 54?±?11 (SD) years, forced expiratory volume in one second 71?±?33% of predicted mean value, BMI 29.9?±?7.9?kg/m²). This two-month protocol comprised education sessions, respiratory physiotherapy and an exercise training program at home and in groups supervised by an adapted physical activity instructor. Results: Thirty-nine patients completed the whole RR program, i.e. 25% dropout. The dropout rate was significantly higher with respect to younger patients in employment. The number of exacerbations decreased significantly during the year following the program, regardless of whether the patients had dropped out (p?p?p?p?Conclusions: This study demonstrates the potential of a home-based program in the treatment and rehabilitation of patients with asthma. Both functional and physiologic indices improved during the follow-up period.