Radiative Forcing Efficiencies Research Articles

Atmospheric aerosol size distribution impacts radiative effects over the Himalayas via modulating aerosol single-scattering albedo

The single-scattering albedo (SSA) of atmospheric aerosols is a key parameter that controls aerosol radiative effects. The variation of SSA is thought to be mainly regulated by aerosol absorption in the Himalayas and South Asia, but observations contradict this idea. In situ field campaigns conducted over two Himalayan sites revealed that SSA was strongly dependent on scattering but weakly correlated with absorption. Observational results combined with the Mie theory further illustrated that SSA was primarily modulated by size distribution rather than absorption. Aerosol Robotic Network (AERONET) data showed similar impacts of size distribution on SSA and that aerosol radiative forcing efficiencies were significantly dependent on SSA. Aerosol size distribution therefore considerably affects radiative forcing by modulating aerosol SSA over the Himalayas. This study highlighted the influence of aerosol size distribution on radiative forcing over the Himalayas, which has important implications for understanding aerosol radiative effects globally.

Broadband spectrum characteristics and radiative effects of primary brown carbon from wood pyrolysis

Brown carbon (BrC), known as light-absorbing organic aerosol in the near-ultraviolet (UV) and short visible region, plays a significant role in the global and regional climate change. A detailed understanding of the spectral optical properties of BrC is beneficial for reducing the uncertainty in radiative forcing calculation. In this work, the spectral properties of primary BrC were investigated by using a four-wavelength broadband cavity-enhanced albedometer with central wavelengths at 365, 405, 532 and 660 nm. The BrC samples were generated by the pyrolysis of three types of wood. During the pyrolysis process, the measured average single scattering albedo (SSA) at 365 nm was about 0.66 to 0.86, where the average absorption Ångström exponent (AAE) was between 5.8 and 7.8, and the average extinction Ångström exponent (EAE) was within 2.1 to 3.5. The full spectral measurement of SSA (300–700 nm) was realized by an optical retrieval method and the retrieved SSA spectrum was directly applied to evaluate aerosol direct radiative forcing (DRF) efficiency. The DRF efficiency over ground of various primary BrC emissions increased from 5.3 % to 68 % as compared to the non-absorbing organic aerosol assumption. A decrease of about 35 % in SSA would cause the DRF efficiency over ground to change from cooling effect to warming effect (from −0.33 W/m2 to +0.15 W/m2) in the near-UV band (365–405 nm). The DRF efficiency over ground of strongly absorptive primary BrC (lower SSA) contributed 66 % more than weakly absorptive primary BrC (higher SSA). These findings proved the importance of broadband spectral properties of BrC, which are substantial for radiative forcing evaluation of BrC and should be considered in global climate models.

Relative Humidity Dependence of Growth Factor and Real Refractive Index for Sea Salt/Malonic Acid Internally Mixed Aerosols

AbstractSea salt aerosols (SSAs) often mix with organic matter via sea‐to‐air emission, photochemical oxidation, and atmospheric transport, which change particle hygroscopicity and real refractive index (RRI), thereby the radiative effects. However, the influence of water uptake and organic matters on radiative effects is not well understood. In this study, relative humidity (RH)‐dependent growth factor (GF), light scattering enhancement factor, RRI (nmea), and radiative forcing efficiency (RFE) in the visible spectrum of SSAs/malonic acid (MA) internally mixed aerosols were investigated. Our results showed that MA‐mediated SSAs readily absorbed water even at low RH but suppressed hygroscopic growth at high RH. The discrepancy between Zdanovskii‐Stokes‐Robinson predictions and measured GF indicated that the suppressing effect on size caused by produced malonate salts in chloride depletion became more evident as MA content increased. RRI in the visible spectrum decreased as MA got enriched and approached toward water's RRI at high RH, resulting in a less cooling effect. The increasing RH reduced RFE both by decreasing RRI and increasing particle size substantially. Ignoring the effect of MA on particle size and RRI under varied RH, errors in RFE at 525 nm up to 25% could be induced. Volume mixing rule performs well in predicting the RRI of mixed aerosols in dry conditions, while the retrieved difference of 0.061 between volume mixing RRI nvol and nmea would make the error in RFE up to 1.15 W·m−2·Aerosol optical depth−1 at 87% RH. Our study helps understand the impact of water content and dicarboxylic acids on the direct radiative effects of SSAs.

Influence of aerosol properties and surface albedo on radiative forcing efficiency of key aerosol types using global AERONET data

According to the particle size and absorptivity as determined by the fine mode fraction and the single-scattering albedo (SSA) retrievals from AErosol RObotic NETwork (AERONET) sites around the world, aerosols are classified into four key categories: coarse and absorptive aerosol (Type I), mixed aerosol (Type II), fine and absorptive aerosol (Type III), fine and non-absorptive aerosol (Type IV). Seasonal variations of aerosol types with their corresponding direct radiative forcing efficiency (RFE) are observed on different continents. The RFE at the surface (RFEsfc) and top of the atmosphere (RFEtoa) reach their maximum (minimum) values over Asia and North America (Europe, Oceania, and South America) from June to August. The effects of solar zenith angle (SZA), surface albedo (SA), and SSA on RFEsfc and RFEtoa are investigated. The absolute values of RFE at TOA of all types of aerosols are largest at cos(SZA) =0.3 to 0.4. The increased SA reduces the absolute value of RFE both at SFC and TOA for all types of aerosols, and when SA reaches a specific threshold, depending on the type of aerosol, the RFEtoa turns positive. RFEtoa increases while RFEsfc decreases with decreasing SSA. The RFEsfc of the four categories of aerosol varies slightly in the same SZA, SSA and SA conditions, while RFEtoa is aerosol type dependent. It is found that larger particles reflect more solar energy into space per optical depth, resulting in an enhanced cooling effect under similar SZA, SSA, and SA conditions.

Spectral dependence of light absorption and direct radiative forcing of the TSP, PM10, PM2.5 and PM0.1 in a rural region of northwestern China

Black carbon (BC) and brown carbon (BrC) are major light absorbing components of aerosol, affecting visibility, radiative forcing balance and human health. In this study, we investigated the light absorption and radiative forcing of carbonaceous aerosol in the total suspended particle (TSP), coarse particle (PM10: particulate matter with an aerodynamic diameter less than 10 μm, Dp ≤ 10 μm), fine particle (PM2.5: Dp ≤ 2.5 μm), and nanoparticle (PM0.1: Dp ≤ 0.1 μm) in a rural area of Guanzhong Plain, China. Similar variations of the absorption Ångstrom exponent (AAE) of TSP, PM10, and PM2.5 were observed. Lower light absorption coefficients and higher AAEs were obtained for PM0.1 compared with other particle sizes. The direct radiative forcing (DRE) efficiency of BC decreased with size bins of TSP, PM10, PM2.5, and PM0.1, respectively. The DRE of BCs for all particle sizes at top atmosphere (TOA), surface atmosphere (SUF) and the whole atmosphere (ATM) were estimated. The optical properties of primary and secondary BrC (PBrC and SBrC) in PM0.1 were further analyzed. The AAE values indicated that the light absorbing of SBrC was more wavelength dependent than PBrC in PM0.1. The DRE of BC, PBrC, and SBrC in PM0.1 were estimated firstly with the values of 19.9 Wm-2, 2.1 Wm-2, and 1.1 Wm-2 in the ATM, respectively.

Scavenging of Black Carbon Aerosols by Radiation Fog in Urban Central China

Radiation fog episodes are characterized by aerosol radiative properties measured at Hefei in urban central China, which hopefully benefits numerical weather prediction and air quality improvement for local governments. In this study, a high mean aerosol optical depth (AOD) is seen over Hefei during the sampling period, whereas an AOD of ~3.0 at 550 nm is observed during the fog episodes. We redefine the fog scavenging coefficient based on its starting and ending points in time, and a black carbon (BC) scavenging coefficient of 30% is observed. Meanwhile, the fog process cannot reduce aerosol number concentrations at size bins between 0.5 and 0.6 μm, whereas a mean particle scavenging coefficient of 21% at sizes within 0.6–1 μm is seen. Significantly large median aerosol scattering coefficient (2690 Mm−1) and absorption coefficient (446 Mm−1) at 550 nm, and low scattering Angstrom exponent in fog are observed, while distinctive particle size distributions between fog and haze are shown. Particle mean size distribution in fog is lower than that in haze at size bins between 0.7 and 2.1 μm, whereas the reverse is true for sizes within 0.5–0.7 μm and larger than 2.1 μm. Aerosol scattering during fog episodes undergoes a bigger increase than particle absorption, and this increase of scattering in fog is even higher than in haze. Median single scattering albedos of 0.86, 0.82, and 0.76 at 550 nm and aerosol radiative forcing efficiencies of −15.0, −14.0, and −10.0 W/m2 are seen for fog, haze and clear periods, respectively, and more negative radiative forcing efficiency emphasizes the significance of fog episodes on climate forcing. Our study clearly reveals the changes of aerosol radiative properties during radiation fog, particularly a synchronous variation of fog aerosol backscattering ratio with the visibility, indicating that more large particles are formed with fog becoming thicker and are scavenged with the dissipation of fog.

Impact of the Atmospheric Photochemical Evolution of the Organic Component of Biomass Burning Aerosol on Its Radiative Forcing Efficiency: A Box Model Analysis

We present the first box model simulation results aimed at identification of possible effects of the atmospheric photochemical evolution of the organic component of biomass burning (BB) aerosol on the aerosol radiative forcing (ARF) and its efficiency (ARFE). The simulations of the dynamics of the optical characteristics of the organic aerosol (OA) were performed using a simple parameterization developed within the volatility basis set framework and adapted to simulate the multiday BB aerosol evolution in idealized isolated smoke plumes from Siberian fires (without dilution). Our results indicate that the aerosol optical depth can be used as a good proxy for studying the effect of the OA evolution on the ARF, but variations in the scattering and absorbing properties of BB aerosol can also affect its radiative effects, as evidenced by variations in the ARFE. Changes in the single scattering albedo (SSA) and asymmetry factor, which occur as a result of the BB OA photochemical evolution, may either reduce or enhance the ARFE as a result of their competing effects, depending on the initial concentration OA, the ratio of black carbon to OA mass concentrations and the aerosol photochemical age in a complex way. Our simulation results also reveal that (1) the ARFE at the top of the atmosphere is not significantly affected by the OA oxidation processes compared to the ARFE at the bottom of the atmosphere, and (2) the dependence of ARFE in the atmospheric column and on the BB aerosol photochemical ages almost mirrors the corresponding dependence of SSA.

Reaction of •OH with CHCl=CH-CHF2 and its atmospheric implication for future environmental-friendly refrigerant

Abstract In order to understand the atmospheric implication of the chlorinated hydrofluoroolefin (HFO), the geometrical structures and the IR absorption cross sections of the stereoisomers 1-chloro-3,3-difluoropropene were studied using the B3LYP/6-31G(3df) and M06-2X/6-31G(3df) methods in the gas phase. The cis-trans isomerization was assessed using the M06-2X/6-311++G(3df,p)//6-31+G(3df,p) method. The latter method was also employed for thermochemistry and the rate coefficients of the reactions of •OH with the cis- and trans-isomers in the temperature ranging from 200 to 400 K. The computational method CCSD/cc-pVTZ//M06-2X/6-31+G(3df,p) was used to benchmark the rate coefficients. It turns out that, the trans-isomer is more stable than cis-isomer and the trans- to cis-isomerization is thermodynamically unfavorable. The rate coefficient follows the Gaussian law with respect to the inverse of temperature. At the global temperature of stratosphere, the calculated rate coefficients served to estimate the atmospheric lifetime along with the photochemical ozone creation potential (POCP). This yielded lifetimes of 4.31 and 7.31 days and POCPs of 3.80 and 2.23 for the cis- and trans-isomer, respectively. The radiative forcing efficiencies gave 0.0082 and 0.0152 W m−2 ppb−1 for the cis- and trans-isomer, respectively. The global warming potential approached zero for both stereoisomers at 20, 100, and 500 years time horizons.

Particle Shape Impact on the Radiative Forcing Efficiency Estimated from Single Levitated (NH4)2SO4 Particles

Levitation of single trapped particles enables the exploration of fundamental physicochemical aerosol properties never previously achieved. Experimental measurements showed that (NH4)2SO4’s particle shape deviated from sphericity during the crystallization process. Despite that, salt aerosols are assumed to be spheres even in low relative humidity (RH) in most climate models. In the analysis performed here, Mie and T-Matrix codes were operated to simulate crucial parameters needed to estimate the radiative forcing efficiency: extinction efficiency, asymmetry parameter and backscattering fraction. The incorporation of non-spherical effects in (NH4)2SO4 particles can cause a difference of up to 46% radiative forcing efficiency compared to the assumption of sphericity in the 0.3–0.6 µm particle radius range.

A global study of hygroscopicity-driven light-scattering enhancement in the context of other in situ aerosol optical properties

Abstract. The scattering and backscattering enhancement factors (f(RH) and fb(RH)) describe how aerosol particle light scattering and backscattering, respectively, change with relative humidity (RH). They are important parameters in estimating direct aerosol radiative forcing (DARF). In this study we use the dataset presented in Burgos et al. (2019) that compiles f(RH) and fb(RH) measurements at three wavelengths (i.e., 450, 550 and 700 nm) performed with tandem nephelometer systems at multiple sites around the world. We present an overview of f(RH) and fb(RH) based on both long-term and campaign observations from 23 sites representing a range of aerosol types. The scattering enhancement shows a strong variability from site to site, with no clear pattern with respect to the total scattering coefficient. In general, higher f(RH) is observed at Arctic and marine sites, while lower values are found at urban and desert sites, although a consistent pattern as a function of site type is not observed. The backscattering enhancement fb(RH) is consistently lower than f(RH) at all sites, with the difference between f(RH) and fb(RH) increasing for aerosol with higher f(RH). This is consistent with Mie theory, which predicts higher enhancement of the light scattering in the forward than in the backward direction as the particle takes up water. Our results show that the scattering enhancement is higher for PM1 than PM10 at most sites, which is also supported by theory due to the change in scattering efficiency with the size parameter that relates particle size and the wavelength of incident light. At marine-influenced sites this difference is enhanced when coarse particles (likely sea salt) predominate. For most sites, f(RH) is observed to increase with increasing wavelength, except at sites with a known dust influence where the spectral dependence of f(RH) is found to be low or even exhibit the opposite pattern. The impact of RH on aerosol properties used to calculate radiative forcing (e.g., single-scattering albedo, ω0, and backscattered fraction, b) is evaluated. The single-scattering albedo generally increases with RH, while b decreases. The net effect of aerosol hygroscopicity on radiative forcing efficiency (RFE) is an increase in the absolute forcing effect (negative sign) by a factor of up to 4 at RH = 90 % compared to dry conditions (RH < 40 %). Because of the scarcity of scattering enhancement measurements, an attempt was made to use other more commonly available aerosol parameters (i.e., ω0 and scattering Ångström exponent, αsp) to parameterize f(RH). The majority of sites (75 %) showed a consistent trend with ω0 (higher f(RH = 85 %) for higher ω0), while no clear pattern was observed between f(RH = 85 %) and αsp. This suggests that aerosol ω0 is more promising than αsp as a surrogate for the scattering enhancement factor, although neither parameter is ideal. Nonetheless, the qualitative relationship observed between ω0 and f(RH) could serve as a constraint on global model simulations.

Estimating the CMIP6 Anthropogenic Aerosol Radiative Effects with the Advantage of Prescribed Aerosol Forcing

The prescribed anthropogenic aerosol forcing recommended by Coupled Model Intercomparison Project Phase 6 (CMIP6) was implemented in an atmospheric model. With the reduced complexity of anthropogenic aerosol forcing, each component of anthropogenic aerosol effective radiative forcing (ERF) can be estimated by one or more calculation methods, especially for instantaneous radiative forcing (RF) from aerosol–radiation interactions (RFari) and aerosol–cloud interactions (RFaci). Simulation results show that the choice of calculation method might impact the magnitude and reliability of RFari. The RFaci—calculated by double radiation calls—is the definition-based Twomey effect, which previously was impossible to diagnose using the default model with physically based aerosol–cloud interactions. The RFari and RFaci determined from present-day simulations are very robust and can be used as offline simulation results. The robust RFari, RFaci, and corresponding radiative forcing efficiencies (i.e., the impact of environmental properties) are very useful for analyzing anthropogenic aerosol radiative effects. For instance, from 1975 to 2000, both RFari and RFaci showed a clear response to the spatial change of anthropogenic aerosol. The global average RF (RFari + RFaci) has enhanced (more negative) by ~6%, even with a slight decrease in the global average anthropogenic aerosol, and this can be explained by the spatial pattern of radiative forcing efficiency.

Theoretical investigation of the atmospheric implication for the reaction of •OH radical with CF2[dbnd]C(CH3)-CX3, X = H, F

The atmospheric implication of the hydrofluoroolefines (HFO) CF2C(CH3)-CX3, X=(H,F), through the reactions with •OH radical were assessed using the M06-2X/6-311++G(df,p)//6-31+G(df,p) method. The rate coefficient was calculated over the temperature range 200–500 K, and was accurately expressed in non-Arrhenius form exp(a+bT−1+cT−2) cm3molecule−1s−1, where a, b, and c are real constants. This served to estimate the atmospheric lifetime along with the photochemical ozone creation potential (POCP), which yielded lifetimes of 0.39 and 3.53 days and POCPs of 51.00 and 3.57 for X = H and F, respectively. The radiative forcing efficiencies (RFEs) were also estimated at G96LYP/6-311G(df,p) along with the global warming potentials (GWPs). The results showed negligible impact towards global warming for the HFOs.

Evidence of anthropization of aerosols in the Saharan and peri-Saharan regions: Implications for the atmospheric transfer of solar radiation

In this study, we use the level-2 Aeronet inversion products of 19 stations of North Africa, the Middle East, or downwind of them. The objective is to characterize the degree of anthropization of the corresponding sub-regions. For this, we define a new dust-index (Idust) based on the statistical analysis of the frequency distributions of the aerosol optical depth at 440 nm and its Fine Mode Fraction (FMF440). Idust is found to be more than one order of magnitude larger at the remote Saharan site of Tamanrasset (Idust = 6.3) and at the 4 Sahelian stations (2.1<Idust < 5.8) than along the Mediterranean coast (Idust = 0.06, 0.08, and 0.09, in Blida, Tunis, and Lampedusa, respectively) or in the Middle East (Idust = 0.11 in Sede Boker and 0.05 in Cairo). In spite of the different degrees of anthropization of the 19 stations, the Aeronet inversions do not reveal any differentiation of the real (n) and imaginary (k) parts of the refractive index, asymmetry parameter (g), and single scattering albedo (SSA) of the coarse mineral dust component of the aerosol. As a result of these commonalities, the inter-site variability of the instantaneous dust radiative forcing efficiencies at TOA and BOA (relative to the AOD440, and for solar zenith angles in the range 60 ± 5°) is found to be entirely explainable by the differences of albedo (α) of the underlying surfaces: FEBOA = 161α-145 (R2 = 0.91, n = 19), and FETOA = 192 α-103 W m−2 AOD440−1 (R2 = 0.93, n = 19). Conversely, the characteristics of the fine anthropogenic aerosols are found to be more variable. In particular, the large k (0.010, 0.012, 0.013, and 0.017 at Banizoumbou, Dakar, Cinzana, and Cabo Verde, respectively) and low SSA of the pollution aerosols of the Sahelian region denote a strong absorbing character to be linked to their most probable bio-mass burning origin. Generally, under pollution episodes, the forcing efficiency is found to be more negative (cooling) at BOA than during dust episodes, but less negative at TOA, or even close to 0 over the brightest surfaces. These results stress the importance of accounting for the surface albedo in the quantification of the climate impact of the aerosols.

DFT Study of Photochemical Properties and Radiative Forcing Efficiency Features of the Stereoisomers cis- and trans-CHCl═CH-CF3.

The accurate assessment of radiative forcing efficiency (RFE) of a greenhouse gas is based on the precise knowledge of its structure and infrared absorption spectrum. The present work investigates the UV-vis absorption spectra and IR absorption spectra that are used for the determination of RFE of the short-lived compounds, cis- and trans-CHCl═CHCF3 (CTFP). These investigations were carried out with six different density functional theory (DFT) methods B3LYP, CAM-B3LYP, M06, M06-2X, TPSS0, and ωB97X-D associated to the basis set 6-31G(3df). Therefrom, the relative populations of the two states cis and trans for temperatures over the range 220-370 K at 1 atm and along the atmospheric altitude were assessed. It turns out that trans-CTFP is the abundant component between the two states. This review reveals that B3LYP and M06 reproduce well the experimental results of UV-vis spectra of trans-CTFP. As for cis-CTFP, EOM-CCSD is not well fitted by DFT methods. The cis- to trans-CTFP isomerization leads to the red shift for DFT methods and to the blue shift in regard to EOM-CCSD and experimental results. The IR absorption spectra are well fitted by B3LYP over the range 500-1600 cm-1 and TPSS0 over 1300-2000 cm-1 for both stereoisomers. Moreover, the root-mean-square errors (RMSEs) of frequencies from experimental data are lower for B3LYP and TPSS0 for both systems. The computed IR absorption band strengths over 500-2000 cm-1 for cis- and over 600-1800 cm-1 for trans-CTFP are consistent with the experiment. The relevant descriptor RFEs of the climate effect were calculated using a narrow band model for a constant vertical profile and then corrected with a lifetime factor for different computational methods. The computed values correlate well with the experimental results for both stereoisomers except M06-2X and TPSS0. It is worth noting that, for both systems, the intense radiative forcing spectra are located at frequencies ranging in 1000-1200 cm-1. The lower forcings of trans-CTFP lying in the atmospheric window region 800-1000 cm-1 are greater than those of cis-CTFP. Therefore, RFE(trans-CTFP) = 1.127 RFE(cis-CTFP).

Analysis of Radiative Properties and Direct Radiative Forcing Estimates of Dominant Aerosol Clusters over an Urban-Desert Region in West Africa

ABSTRACT The strategic location of the AERONET site in Ilorin, Nigeria, makes it possible to obtain information on several aerosol types and their radiative effects. The strong reversal of wind direction occasioned by the movement of the ITCZ during the West Africa Monsoon (WAM) plays a major role in the variability of aerosol nature at this site, which is confirmed by aerosol optical depth (AOD) (675 nm) and Angstrom exponent (AE) (440–870 nm) values with 1st and 99th percentile values of 0.08 and 2.16, and 0.11 and 1.47, respectively. The direct radiative forcing (DRF) and radiative forcing efficiency (RFE) of aerosol, as retrieved from the AERONET sun-photometer measurements, are estimated using radiative transfer calculations for the periods of 2005–2009 and 2011–2015. The DRF and RFE of the dominant aerosol classes—desert dust (DD), biomass burning (BB), urban (UB) and gas flaring (GF)—have been estimated. The median (± standard deviation) values of the DRF at the top of the atmosphere (TOA) for the DD, BB, UB and GF aerosol classes are –27.5 ± 13.2 Wm–2, –27.1 ± 8.3 Wm–2, –11.5 ± 13.2 Wm–2 and –9.6 ± 8.0 Wm–2, respectively, while those of the RFE are –26.2 ± 4.1 Wm–2 δ–1, –35.2 ± 4.6 Wm–2 δ–1, –31.0 ± 8.4 Wm–2 δ–1 and –37.0 ± 10.3 Wm–2 δ–1, respectively. Arguably due to its high SSA and assymetric values, the DD aerosol class shows the largest DRF but the smallest RFE. Its smallest AOD notwithstanding, the GF class can cause greater perturbation of the earth-atmosphere system in the sub-region both directly and indirectly, possibly due to the presence of black carbon and other co-emitted aerosol and the ageing of the GF aerosols. This study presents the first estimate of DRF for aerosols of gas flaring origin and shows that its radiative potential can be similar in magnitude to that of biomass burning and urban aerosol in West Africa.

Accuracy Required in Measurements of Refractive Index and Hygroscopic Response to Reduce Uncertainties in Estimates of Aerosol Radiative Forcing Efficiency

AbstractThe magnitude of aerosol radiative forcing resulting from the scattering and absorption of radiation is still uncertain. Sources of uncertainty include the physical and optical properties of aerosol, reflected in uncertainties in real and imaginary refractive indices (n and k) and relative humidity (RH). The effect of RH on the geometrical size of aerosol particles is often reported as a hygroscopic kappa parameter (κ). The objective of this study is to explore the sensitivity of radiative forcing efficiency (RFE) to changes in particle properties in order to better define the accuracy with which optical and hygroscopic measurements must be made to reduce uncertainties in RFE. Parameterizations of precise values of n and k are considered as functions of RH for ammonium sulfate (AS) and brown carbon (BrC). The range of the RFE estimated for typical uncertainties of n and κ for AS of 0.1 μm dry radius is less than ±7% and is not affected by an increase of RH. For typical sizes of AS in the atmosphere (0.35 μm dry radius), the range of the RFE increases to ±20% at 90% RH and ±15% at 99% RH. Absorbing small BrC particles (0.1 μm dry radius) cause cooling at the top of the atmosphere, and as RH and κ increase, the RFE is more negative compared to the usual assumptions of dry unhygroscopic BrC. For larger BrC particles (0.35 μm dry radius), the change in RFE for RHs ~100% compared to dry conditions can take values around −100%.

Historical greenhouse gas concentrations for climate modelling (CMIP6)

Abstract. Atmospheric greenhouse gas (GHG) concentrations are at unprecedented, record-high levels compared to the last 800 000 years. Those elevated GHG concentrations warm the planet and – partially offset by net cooling effects by aerosols – are largely responsible for the observed warming over the past 150 years. An accurate representation of GHG concentrations is hence important to understand and model recent climate change. So far, community efforts to create composite datasets of GHG concentrations with seasonal and latitudinal information have focused on marine boundary layer conditions and recent trends since the 1980s. Here, we provide consolidated datasets of historical atmospheric concentrations (mole fractions) of 43 GHGs to be used in the Climate Model Intercomparison Project – Phase 6 (CMIP6) experiments. The presented datasets are based on AGAGE and NOAA networks, firn and ice core data, and archived air data, and a large set of published studies. In contrast to previous intercomparisons, the new datasets are latitudinally resolved and include seasonality. We focus on the period 1850–2014 for historical CMIP6 runs, but data are also provided for the last 2000 years. We provide consolidated datasets in various spatiotemporal resolutions for carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O), as well as 40 other GHGs, namely 17 ozone-depleting substances, 11 hydrofluorocarbons (HFCs), 9 perfluorocarbons (PFCs), sulfur hexafluoride (SF6), nitrogen trifluoride (NF3) and sulfuryl fluoride (SO2F2). In addition, we provide three equivalence species that aggregate concentrations of GHGs other than CO2, CH4 and N2O, weighted by their radiative forcing efficiencies. For the year 1850, which is used for pre-industrial control runs, we estimate annual global-mean surface concentrations of CO2 at 284.3 ppm, CH4 at 808.2 ppb and N2O at 273.0 ppb. The data are available at https://esgf-node.llnl.gov/search/input4mips/ and http://www.climatecollege.unimelb.edu.au/cmip6. While the minimum CMIP6 recommendation is to use the global- and annual-mean time series, modelling groups can also choose our monthly and latitudinally resolved concentrations, which imply a stronger radiative forcing in the Northern Hemisphere winter (due to the latitudinal gradient and seasonality).

Analysis of Aerosol Radiative Forcing over Beijing under Different Air Quality Conditions Using Ground-Based Sun-Photometers between 2013 and 2015

Aerosol particles can strongly affect both air quality and the radiation budget of the atmosphere. Above Beijing, the capital city of China, large amounts of aerosols within the atmospheric column have caused the deterioration of local air quality and have influenced radiative forcings at both the top and the bottom of the atmosphere (BOA and TOA). Observations of aerosol radiative forcing and its efficiency have been made using two sun-photometers in urban Beijing between 2013 and 2015, and have been analyzed alongside two air quality monitoring stations’ data by dividing air quality conditions into unpolluted, moderately polluted, and heavily polluted days. Daily average PM2.5 concentrations varied greatly in urban Beijing (5.5–485.0 µg/m3) and more than one-third of the analyzed period is classified as being polluted according to the national ambient air quality standards of China. The heavily polluted days had the largest bottom of atmosphere (BOA) and top of atmosphere (TOA) radiative forcings, but the smallest radiative forcing efficiencies, while the unpolluted days showed the opposite characteristics. On heavily polluted days, the averaged BOA aerosol radiative forcing occasionally exceeded −150 W/m2, which represents a value about three-times greater than that for unpolluted days. BOA aerosol radiative forcing was around two-to-three times as large as TOA aerosol radiative forcing under various air quality conditions, although both were mostly negative, suggesting that aerosols had different magnitudes of cooling effects at both the surface and the top of the atmosphere. Unpolluted days had the largest average values of aerosol radiative forcing efficiencies at BOA (and TOA) levels, which exceeded −190 W/m2 (−70 W/m2), compared with the lowest average values in heavily polluted days of around −120 W/m2 (−55 W/m2). These results suggest that the high concentrations of particulate matter pollution in the urban Beijing area had a strong cooling effect at both BOA and TOA levels.

Synergistic use of Lagrangian dispersion and radiative transfer modelling with satellite and surface remote sensing measurements for the investigation of volcanic plumes: the Mount Etna eruption of 25–27 October 2013

Abstract. In this paper we combine SO2 and ash plume dispersion modelling with satellite and surface remote sensing observations to study the regional influence of a relatively weak volcanic eruption from Mount Etna on the optical and micro-physical properties of Mediterranean aerosols. We analyse the Mount Etna eruption episode of 25–27 October 2013. The evolution of the plume along the trajectory is investigated by means of the FLEXible PARTicle Lagrangian dispersion (FLEXPART) model. The satellite data set includes true colour images, retrieved values of volcanic SO2 and ash, estimates of SO2 and ash emission rates derived from MODIS (MODerate resolution Imaging Spectroradiometer) observations and estimates of cloud top pressure from SEVIRI (Spinning Enhanced Visible and InfraRed Imager). Surface remote sensing measurements of aerosol and SO2 made at the ENEA Station for Climate Observations (35.52° N, 12.63° E; 50 m a.s.l.) on the island of Lampedusa are used in the analysis. The combination of these different data sets suggests that SO2 and ash, despite the initial injection at about 7.0 km altitude, reached altitudes around 10–12 km and influenced the column average aerosol particle size distribution at a distance of more than 350 km downwind. This study indicates that even a relatively weak volcanic eruption may produce an observable effect on the aerosol properties at the regional scale. The impact of secondary sulfate particles on the aerosol size distribution at Lampedusa is discussed and estimates of the clear-sky direct aerosol radiative forcing are derived. Daily shortwave radiative forcing efficiencies, i.e. radiative forcing per unit AOD (aerosol optical depth), are calculated with the LibRadtran model. They are estimated between −39 and −48 W m−2 AOD−1 at the top of the atmosphere and between −66 and −49 W m−2 AOD−1 at the surface, with the variability in the estimates mainly depending on the aerosol single scattering albedo. These results suggest that sulfate particles played a large role in the transported plume composition and radiative forcing, while the contribution by ash particles was small in the volcanic plume arriving at Lampedusa during this event.

The environmental impact of unsaturated fluoroesters: atmospheric chemistry towards OH radicals and Cl atoms, radiative behavior and cumulative ozone creation

Atmospheric lifetimes, reaction mechanisms, infrared spectra, radiative forcing efficiencies, global warming potentials and ozone creation potentials of the allyl and vinyl trifluoroacetate were determined to assess their environmental impact.