Indirect Radiative Forcing Research Articles

Sensitivity of cloud microphysics to aerosol is highly associated with cloud water content: Implications for indirect radiative forcing

The sensitivity of cloud microphysics to aerosol loading, quantified by the aerosol-cloud interactions (ACI) index, plays a crucial role in calculating the radiative forcing due to ACI (RFaci). However, the dependence of the ACI index on liquid water content (LWC) and its impact on RFaci are often overlooked. This study aims to investigate this dependence and evaluate its implications for RFaci, based on ground in-situ aerosol-cloud observations on Mt. Lu in eastern China. The results demonstrate that the ACI index exhibits an initial increase, followed by a decline with increasing LWC. In the unconstrained LWC scenario, the ACI index, calculated based on cloud droplet number concentration (ACIn of 0.13), is found to be lower than the lower bound of ACIn (0.17 to 0.35) obtained from the constrained LWC scenario. Neglecting this LWC-dependence leads to a significant underestimation of the mean RFaci by 53%. Furthermore, when calculating RFaci using the ACI index from droplet effective diameter (ACId), implying the neglect of the dispersion effect by assuming a fixed droplet spectrum width, it results in an overestimation of RFaci by 22% compared to using ACIn. These findings shed new light on the assessment of RFaci and help reconcile differences between observed and simulated RFaci. Plain language summaryAerosol-cloud interactions are the major source of uncertainty in understanding human-induced climate change. This study focused on the relationship between aerosol and clouds, specifically investigating how the sensitivity of cloud properties to aerosol loading associates with the amount of liquid water present in the clouds. Using ground in-situ observations of clouds and aerosols on Mt. Lu in eastern China, this study showed that the sensitivity of cloud properties to aerosol loading initially increases and then decreases as the cloud water content increases. Neglecting this dependence can lead to a significant underestimation of the impact of aerosol-cloud interactions on climate. These findings provide valuable insights for improving our understanding of how aerosols and clouds interact and contribute to climate change.

Use of machine learning to reduce uncertainties in particle number concentration and aerosol indirect radiative forcing predicted by climate models

Use of machine learning to reduce uncertainties in particle number concentration and aerosol indirect radiative forcing predicted by climate models

Simulating the radiative forcing of oceanic dimethylsulfide (DMS) in Asia based on machine learning estimates

Abstract. Dimethylsulfide (DMS) emitted from seawater is a key precursor to new particle formation and acts as a regulator in Earth's warming climate system. However, DMS's effects are not well understood in various ocean regions. In this study, we estimated DMS emissions based on a machine learning method and used the GEOS-Chem global 3D chemical transport model coupled with the TwO Moment Aerosol Sectional (TOMAS) microphysics scheme to simulate the atmospheric chemistry and radiative effects of DMS. The contributions of DMS to atmospheric SO42- aerosol and cloud condensation nuclei (CCN) concentrations along with the radiative effects over the Asian region were evaluated for the first time. First, we constructed novel monthly resolved DMS emissions (0.5∘×0.5∘) for the year 2017 using a machine learning model; 4351 seawater DMS measurements (including the recent measurements made over the Chinese seas) and 12 relevant environment parameters were selected for model training. We found that the model could predict the observed DMS concentrations with a correlation coefficient of 0.75 and fill the values in regions lacking observations. Across the Asian seas, the highest seasonal mean DMS concentration occurred in March–April–May (MAM), and we estimate the annual DMS emission flux of 1.25 Tg (S), which is equivalent to 15.4 % of anthropogenic sulfur emissions over the entire simulation domain (which covered most of Asia) in 2017. The model estimates of DMS and methane sulfonic acid (MSA), using updated DMS emissions, were evaluated by comparing them with cruise survey experiments and long-term online measurement site data. The improvement in model performance can be observed compared with simulation results derived from the global-database DMS emissions. The relative contributions of DMS to SO42- and CCN were higher in remote oceanic areas, contributing 88 % and 42 % of all sources, respectively. Correspondingly, the sulfate direct radiative forcing (DRF) and indirect radiative forcing (IRF) contributed by DMS ranged from −200 to −20 mW m−2 and from −900 to −100 mW m−2, respectively, with levels varying by season. The strong negative IRF is mainly over remote ocean regions (−900 to −600 mW m−2). Generally, the magnitude of IRF derived by DMS was twice as large as its DRF. This work provides insights into the source strength of DMS and the impact of DMS on climate and addresses knowledge gaps related to factors controlling aerosols in the marine boundary layer and their climate impacts.

Exploring dimethyl sulfide (DMS) oxidation and implications for global aerosol radiative forcing

Abstract. Aerosol indirect radiative forcing (IRF), which characterizes how aerosols alter cloud formation and properties, is very sensitive to the preindustrial (PI) aerosol burden. Dimethyl sulfide (DMS), emitted from the ocean, is a dominant natural precursor of non-sea-salt sulfate in the PI and pristine present-day (PD) atmospheres. Here we revisit the atmospheric oxidation chemistry of DMS, particularly under pristine conditions, and its impact on aerosol IRF. Based on previous laboratory studies, we expand the simplified DMS oxidation scheme used in the Community Atmospheric Model version 6 with chemistry (CAM6-chem) to capture the OH-addition pathway and the H-abstraction pathway and the associated isomerization branch. These additional oxidation channels of DMS produce several stable intermediate compounds, e.g., methanesulfonic acid (MSA) and hydroperoxymethyl thioformate (HPMTF), delay the formation of sulfate, and, hence, alter the spatial distribution of sulfate aerosol and radiative impacts. The expanded scheme improves the agreement between modeled and observed concentrations of DMS, MSA, HPMTF, and sulfate over most marine regions, based on the NASA Atmospheric Tomography (ATom), the Aerosol and Cloud Experiments in the Eastern North Atlantic (ACE-ENA), and the Variability of the American Monsoon Systems (VAMOS) Ocean-Cloud-Atmosphere-Land Study Regional Experiment (VOCALS-REx) measurements. We find that the global HPMTF burden and the burden of sulfate produced from DMS oxidation are relatively insensitive to the assumed isomerization rate, but the burden of HPMTF is very sensitive to a potential additional cloud loss. We find that global sulfate burden under PI and PD emissions increase to 412 Gg S (+29 %) and 582 Gg S (+8.8 %), respectively, compared to the standard simplified DMS oxidation scheme. The resulting annual mean global PD direct radiative effect of DMS-derived sulfate alone is −0.11 W m−2. The enhanced PI sulfate produced via the gas-phase chemistry updates alone dampens the aerosol IRF as anticipated (−2.2 W m−2 in standard versus −1.7 W m−2, with updated gas-phase chemistry). However, high clouds in the tropics and low clouds in the Southern Ocean appear particularly sensitive to the additional aqueous-phase pathways, counteracting this change (−2.3 W m−2). This study confirms the sensitivity of aerosol IRF to the PI aerosol loading and the need to better understand the processes controlling aerosol formation in the PI atmosphere and the cloud response to these changes.

An Optical Flow Approach to Tracking Ship Track Behavior Using GOES-R Satellite Imagery

Ship emissions can form linear cloud structures, or <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ship tracks</i> , when atmospheric water vapor condenses on aerosols in the ship exhaust. These structures are of interest because they are observable and traceable examples of MCB, a mechanism that has been studied as a potential approach for solar climate intervention. Ship tracks can be observed throughout the diurnal cycle via space-borne assets like the advanced baseline imagers on the national oceanic and atmospheric administration geostationary operational environmental satellites, the GOES-R series. Due to complex atmospheric dynamics, it can be difficult to track these aerosol perturbations over space and time to precisely characterize how long a single emission source can significantly contribute to indirect radiative forcing. We propose an optical flow approach to estimate the trajectories of ship-emitted aerosols after they begin mixing with low boundary layer clouds using GOES-17 satellite imagery. Most optical flow estimation methods have only been used to estimate large scale atmospheric motion. We demonstrate the ability of our approach to precisely isolate the movement of ship tracks in low-lying clouds from the movement of large swaths of high clouds that often dominate the scene. This efficient approach shows that ship tracks persist as visible, linear features beyond 9 h and sometimes longer than 24 h.

Secondary Inorganic Ions Characteristics in PM2.5 Along Offshore and Coastal Areas of the Megacity Shanghai

AbstractSecondary inorganic ions are important constituents of PM2.5 that play an important role in global climate change through direct and indirect radiative forcing. To investigate the influence of ship emissions on coastal cities, we launched a cruise ship campaign offshore of the East China Sea (ECS) and two field observation sites in the megacity of Shanghai using 1‐hr time‐resolved water‐soluble ion chromatography from June 3 to June 27, 2017. Three main secondary inorganic soluble ions in the atmosphere of the ECS, that is, non‐sea‐salt sulfate (nss‐SO42−), NO3− and NH4+, were 8.87, 4.94, and 4.65 μg m−3, respectively. While their values at the Pudong (PD) station in urban Shanghai were 6.33, 9.47, and 4.74 μg m−3 and at the Dianshanhu (DSH) station in suburban Shanghai were 7.33, 10.01, and 6.78 μg m−3, respectively. Nss‐SO42− was dominant in the PM2.5 in the atmosphere of the ECS (nss‐SO42−/NO3− = 1.8), and mainly distributed near the Zhoushan Islands of the ECS. This indicated significant contributions from ship emissions. Based on the Community Multiscale Air Quality model, two main sources of nss‐SO42− were identified: emissions and horizontal transport. These comprised over 90% of the nss‐SO42−. The influence of ship emissions reached up to 31.6% of the total nss‐SO42− alongside the cruise ship and near coastal parking ports in the Yangtze River Delta area, suggesting that more attention should be paid to ship emissions.

Constructing Shapes and Mixing Structures of Black Carbon Particles With Applications to Optical Calculations

AbstractBlack carbon (BC) aerosols strongly absorb the solar radiation, affecting the regional and global climate through direct and indirect radiative forcing. The optical properties of BC are critical factors to estimate their radiative forcing. However, the optical absorption of BC is still under controversy partially due to the weakness in quantifying their complex morphology and mixing structures. Although a Discrete Dipole Approximation (DDA) can calculate optical properties of fine particles with arbitrary shapes, an appropriate definition of realistic BC shape models for optical simulation is essentially required. Here we present a novel Electron‐Microscope‐to‐BC‐Simulation (EMBS) tool to construct realistic BC shape models with various morphology and mixing structures for optical calculation using DDA. The optical properties of BC particles with different particle morphology, coating thickness, and embedded fraction (F) are estimated based on electron microscope. We find that absorption enhancement (Eabs) of the realistic irregular model is larger than that of the present commonly used spherical model (i.e., BC aggregate with spherical coating). The BC core morphology greatly influences Eabs of the embedded BC particles with irregular coating when the volume‐equivalent‐diameter ratio of particle to core (Dp/Dc) is larger than 1.8. The F significantly influences Eabs of BC particles, suggesting that the mixing structure between coating and core is an important factor to determine the optical absorption of aged BC particles. The study highlights that the BC core morphology, coating shape, coating thickness, and mixing structures influence their optical properties and should be considered as important variables in climate models.

Regional variation in the effectiveness of methane-based and land-based climate mitigation options

Abstract. Scenarios avoiding global warming greater than 1.5 or 2 ∘C, as stipulated in the Paris Agreement, may require the combined mitigation of anthropogenic greenhouse gas emissions alongside enhancing negative emissions through approaches such as afforestation–reforestation (AR) and biomass energy with carbon capture and storage (BECCS). We use the JULES land surface model coupled to an inverted form of the IMOGEN climate emulator to investigate mitigation scenarios that achieve the 1.5 or 2 ∘C warming targets of the Paris Agreement. Specifically, within this IMOGEN-JULES framework, we focus on and characterise the global and regional effectiveness of land-based (BECCS and/or AR) and anthropogenic methane (CH4) emission mitigation, separately and in combination, on the anthropogenic fossil fuel carbon dioxide (CO2) emission budgets (AFFEBs) to 2100. We use consistent data and socio-economic assumptions from the IMAGE integrated assessment model for the second Shared Socioeconomic Pathway (SSP2). The analysis includes the effects of the methane and carbon–climate feedbacks from wetlands and permafrost thaw, which we have shown previously to be significant constraints on the AFFEBs. Globally, mitigation of anthropogenic CH4 emissions has large impacts on the anthropogenic fossil fuel emission budgets, potentially offsetting (i.e. allowing extra) carbon dioxide emissions of 188–212 Gt C. This is because of (a) the reduction in the direct and indirect radiative forcing of methane in response to the lower emissions and hence atmospheric concentration of methane and (b) carbon-cycle changes leading to increased uptake by the land and ocean by CO2-based fertilisation. Methane mitigation is beneficial everywhere, particularly for the major CH4-emitting regions of India, the USA, and China. Land-based mitigation has the potential to offset 51–100 Gt C globally, the large range reflecting assumptions and uncertainties associated with BECCS. The ranges for CH4 reduction and BECCS implementation are valid for both the 1.5 and 2 ∘C warming targets. That is the mitigation potential of the CH4 and of the land-based scenarios is similar for regardless of which of the final stabilised warming levels society aims for. Further, both the effectiveness and the preferred land management strategy (i.e. AR or BECCS) have strong regional dependencies. Additional analysis shows extensive BECCS could adversely affect water security for several regions. Although the primary requirement remains mitigation of fossil fuel emissions, our results highlight the potential for the mitigation of CH4 emissions to make the Paris climate targets more achievable.

Simulation study on the indirect effect of sulfate on the summer climate over the eastern China monsoon region

In this study, we designed a sensitivity test using the half number concentration of sulfate in the nucleation calculation process to study the aerosol-cloud interaction (ACI) of sulfate on clouds, precipitation, and monsoon intensity in the summer over the eastern China monsoon region (ECMR) with the National Center for Atmospheric Research Community Atmosphere Model version 5. Numerical experiments show that the ACI of sulfate led to an approximately 30% and 34% increase in the cloud condensation nuclei and cloud droplet number concentrations, respectively. Cloud droplet effective radius below 850 hPa decreased by approximately 4% in the southern ECMR, while the total liquid water path increased by 11%. The change in the indirect radiative forcing due to sulfate at the top of the atmosphere in the ECMR during summer was − 3.74 W·m−2. The decreased radiative forcing caused a surface cooling of 0.32 K and atmospheric cooling of approximately 0.3 K, as well as a 0.17 hPa increase in sea level pressure. These changes decreased the thermal difference between the land and sea and the gradient of the sea-land pressure, leading to a weakening in the East Asian summer monsoon (EASM) and a decrease in the total precipitation rate in the southern ECMR. The cloud lifetime effect has a relatively weaker contribution to summer precipitation, which is dominated by convection. The results show that the ACI of sulfate was one possible reason for the weakening of the EASM in the late 1970s.

Highly significant impact of mineral dust on aerosol hygroscopicity at New Delhi

All conventional hygroscopicity parameter (κ) estimates are based on ammonium salts being the dominant salts, besides sea salt at marine locations. This grossly underestimates κ for highly polluted cities like New Delhi which is subjected to high mineral dust. To validate this hypothesis, the impact of mineral dust on κ was quantified following two mathematical approaches based on Mann-Kendall's test which differed in the sequence followed for compound formation. This leads to a remarkably high increment in κ, on an average of 57% and 63.5% during the day and night respectively, for the period Dec 2012–2014 and is based on the detailed chemical speciation of PM2.5 data. The inter-comparison of κ obtained from various analytical methods challenge the traditional way of estimating κ by including the chemical composition of a few dominant species and ignoring the others without paying any heed to the dependence of κ on Temperature (T) and Relative Humidity (RH). An increase in the ratio of water-soluble organic carbon to organic carbon lessened the impact of mineral dust on κ. The insolubility of calcium salts also decreased the effect of mineral dust, but at the same time lead to MgCl2 formation which augmented the estimated κ. The highest obtained κ was 0.82 (on 23/7/2014 during the night). The impact of mineral dust was also found to be significant for two other Indian (Durg and Kanpur) and Chinese (Xian and Beijing) cities for both PM1 and PM2.5 regimes. The high value of κ and impact of mineral dust on κ can be crucial in Cloud Condensation Nuclei (CCN) estimation at polluted sites subjected to mineral dust and thus impact precipitation quantification by Global Climatic models (GCMs), lead to visibility deterioration and affect the indirect radiative forcing.

Dispersion of Droplet Size Distributions in Supercooled Non‐precipitating Stratocumulus from Aircraft Observations Obtained during the Southern Ocean Cloud Radiation Aerosol Transport Experimental Study

AbstractThe characteristics of cloud droplet size distributions and statistical relations of the relative dispersion (ε) with the vertical velocity (w) and with the interstitial aerosol concentration (Nia) are investigated for ubiquitous supercooled shallow stratocumulus observed over the Southern Ocean (SO) using aircraft measurements obtained during the Southern Ocean Cloud Radiation Aerosol Transport Experimental Study. Distinct vertical variations have been found using 36 non‐precipitating cloud profiles. The cloud droplet effective radius (re) increases nearly monotonically from 5.3 ± 1.9 μm at cloud base to 9.4 ± 2.2 μm at cloud top. The ε decreases rapidly from cloud base (0.42 ± 0.13) and then remains relatively constant in the upper cloud layer (0.27 ± 0.09). This study also shows robust dependence of ε on both Nia and w. The ε increases (decreases) with increasing Nia (w) at a 95% confidence level when values of w (low Nia) are restricted to a small range. The important roles of aerosols and dynamics on ε are demonstrated and are crucial to estimating aerosol indirect radiative forcing, especially for pristine SO regions where models almost universally underestimate reflected radiation.

Global modeling of hydrogen using GFDL-AM4.1: Sensitivity of soil removal and radiative forcing

Hydrogen (H2) has been proposed as an alternative energy carrier to reduce the carbon footprint and associated radiative forcing of the current energy system. Here, we describe the representation of H2 in the GFDL-AM4.1 model including updated emission inventories and improved representation of H2 soil removal, the dominant sink of H2. The model best captures the overall distribution of surface H2, including regional contrasts between climate zones, when vd(H2) is modulated by soil moisture, temperature, and soil carbon content. We estimate that the soil removal of H2 increases with warming (2–4% per K), with large uncertainties stemming from different regional response of soil moisture and soil carbon. We estimate that H2 causes an indirect radiative forcing of 0.84 mW m−2/(Tg(H2)yr−1) or 0.13 mW m−2 ppbv−1, primarily due to increasing CH4 lifetime and stratospheric water vapor production.

Aerosols enhance cloud lifetime and brightness along the stratus-to-cumulus transition

Anthropogenic aerosols are hypothesized to enhance planetary albedo and offset some of the warming due to the buildup of greenhouse gases in Earth's atmosphere. Aerosols can enhance the coverage, reflectance, and lifetime of warm low-level clouds. However, the relationship between cloud lifetime and aerosol concentration has been challenging to measure from polar orbiting satellites. We estimate two timescales relating to the formation and persistence of low-level clouds over [Formula: see text] spatial domains using multiple years of geostationary satellite observations provided by the Clouds and Earth's Radiant Energy System (CERES) Synoptic (SYN) product. Lagrangian trajectories spanning several days along the classic stratus-to-cumulus transition zone are stratified by aerosol optical depth and meteorology. Clouds forming in relatively polluted trajectories tend to have lighter precipitation rates, longer average lifetime, and higher cloud albedo and cloud fraction compared with unpolluted trajectories. While liquid water path differences are found to be negligible, we find direct evidence of increased planetary albedo primarily through increased drop concentration ([Formula: see text]) and cloud fraction, with the caveat that the aerosol influence on cloud fraction is positive only for stable atmospheric conditions. While the increase in cloud fraction can be large typically in the beginning of trajectories, the Twomey effect accounts for the bulk (roughly 3/4) of the total aerosol indirect radiative forcing estimate.

High-order characteristic-finite volume methods for aerosol dynamic equations

Aerosol particles have an important effect on changing of climate and human health, where aerosols scatter and absorb the incoming solar radiation, and thus decrease the precipitation efficiency of warm clouds and can cause an indirect radiative forcing associated with changes in cloud properties. Meanwhile, it has also been recognized that the particles of aerosols in the sub-micrometer size range can be inhaled and thus pose certain health hazards. In this paper we analyze the finite volume method based on linear interpolation and Hermite interpolation combined with the method of characteristics for the nonlinear aerosol dynamic equations on time and particle size, which involve the advection process, condensation process and the nonlinear coagulation process. Numerical experiments for the multiple log-normal aerosol distributions are further given to confirm the theoretical results.

Cloud physics from space

AbstractA review of the progression of cloud physics from a subdiscipline of meteorology into the global science it is today is described. The discussion briefly touches on the important post‐war contributions of three key individuals who were instrumental in developing cloud physics into a global science. These contributions came on the heels of the post‐war weather modification efforts that influenced much of the early development of cloud physics. The review is centred on the properties of warm clouds primarily to limit the scope of the article and the connection between the early contributions to cloud physics and the current vexing problem of aerosol effects on cloud albedo is underlined. Progress toward estimating cloud properties from space and insights on warm cloud processes are described. Measurements of selected cloud properties, such as cloud liquid water path are now mature enough that multi‐decadal time series of these properties exist and this climatology is used to compare to analogous low‐cloud properties taken from global climate models. The too‐wet (and thus too bright) and the too‐dreary biases of models are called out underscoring the challenges we still face in representing warm clouds in Earth system models. We also provide strategies for using observations to constrain the indirect radiative forcing of the climate system.

The transition process of aerosol distribution state with spatio-temporal analysis based on MODIS product

Abstract. Atmospheric aerosol can have an impact on the climate through direct and indirect radiative forcing and cause air quality problems to harm human health [Charlson, R.J., et al. 1992]. Many scholars have made a lot of contributions to aerosol concentration analysis, but the analysis of chaotic aerosol concentration time series is always unable to perfectly solve the inherent randomness brought by complicated dynamic behaviors [Abarbanel, H.D.I, 1996]. This research has established the aggregation and dispersion states of aerosol optical thickness in fixed region and fixed time period by calculating spatial statistics of value-containing pixels in remote sensing images. And it has also analyzed the process of this state which is influenced by several meteorological factors. This kind of aggregation and dispersion is a conceptual method of spatial statistics in geography. The research compares the aerosol AOD value and its distribution area in the atmosphere from the signal frequency and utilizes the methods of nonlinear analysis and wavelet analysis to solve for its state transition characteristics at a regular point in time. Then the aerosol state transfer process is unfolded in space to analyze the interaction between the sequential meteorological factors and the aerosol state transfer process, as well as the comparison of the differences and similarities between the aerosol state transfer process in urban areas and non-urban areas. The research can not only provide theoretical foundation for pollution warning on the basis of long time series but also provide a new idea for analyzing the nonlinear dynamic process of the atmospheric system, which is the analysis of the change process of the chaotic system by spatial deploy at a specific time point.

Ammonium nitrate particles formed in upper troposphere from ground ammonia sources during Asian monsoons

The rise of ammonia emissions in Asia is predicted to increase radiative cooling and air pollution by forming ammonium nitrate particles in the lower troposphere. There is, however, a severe lack of knowledge about ammonia and ammoniated aerosol particles in the upper troposphere and their possible effects on the formation of clouds. Here we employ satellite observations and high-altitude aircraft measurements, combined with atmospheric trajectory simulations and cloud-chamber experiments, to demonstrate the presence of ammonium nitrate particles and also track the source of the ammonia that forms into the particles. We found that during the Asian monsoon period, solid ammonium nitrate particles are surprisingly ubiquitous in the upper troposphere from the Eastern Mediterranean to the Western Pacific—even as early as in 1997. We show that this ammonium nitrate aerosol layer is fed by convection that transports large amounts of ammonia from surface sources into the upper troposphere. Impurities of ammonium sulfate allow the crystallization of ammonium nitrate even in the conditions, such as a high relative humidity, that prevail in the upper troposphere. Solid ammonium nitrate particles in the upper troposphere play a hitherto neglected role in ice cloud formation and aerosol indirect radiative forcing. Solid ammonium nitrate particles are formed in the upper troposphere during the Asian monsoons, which bring large amounts of ground ammonia to this altitude, according to integrated analyses of measurements on ammoniated aerosol, together with model simulations.

Spatioseasonal Variations of Atmospheric Ammonia Concentrations Over the United States: Comprehensive Model‐Observation Comparison

AbstractAtmospheric ammonia plays an important role in a number of environmental issues, including new particle formation and aerosol indirect radiative forcing. Over the United States, atmospheric ammonia has seen an increasing trend due in most part to the declining SO2 and NOx emissions. We conduct the first comprehensive assessment of multiyear Goddard Earth Observing System (GEOS)‐Chem simulated ammonia concentration ([NH3]) over conterminous United States along with surface observations from all 90 National Atmospheric Deposition Program Ammonia Monitoring Network (AMoN) sites that have at least 2 years of continuous measurements. Model‐simulated [NH3] is along empirically expected lines with regard to temporal trends, seasonal variations, and spatial distribution. GEOS‐Chem‐simulated [NH3], compared to AMoN observed values, has weighted average correlation (τ) of 0.50 ± 0.15 and mean fractional bias (MFB) of −8.8 ± 56%. Most sites (63 out of 90) have −60% &lt; MFB &lt; +60%. The deviations from observed values vary spatially and seasonally, and there is significant wintertime underestimation (−44 ± 58%) across most of conterminous United States (except the Pacific states). The largest positive deviations occur in the Pacific states (101 ± 46%) and the largest negative deviations in the Southern Plain states (−73 ± 39%) and the Mountain states (−73 ± 84%), both in the winter months. Over the Great Plains region, GEOS‐Chem simulated [NH3] shows a much stronger dependence to emissions than AMoN observed [NH3], indicating scope for improved representation of emissions for the region. Over Southeast United States, there appears to be the strong effect of the changing emissions of SO2 and NOx in both modeled and observed [NH3].

Decrease in radiative forcing by organic aerosol nucleation, climate, and land use change

Organic nucleation is an important source of atmospheric aerosol number concentration, especially in pristine continental regions and during the preindustrial period. Here, we improve on previous simulations that overestimate boundary layer nucleation in the tropics and add changes to climate and land use to evaluate climate forcing. Our model includes both pure organic nucleation and heteromolecular nucleation of sulfuric acid and organics and reproduces the profile of aerosol number concentration measured in the Amazon. Organic nucleation decreases the sum of the total aerosol direct and indirect radiative forcing by 12.5%. The addition of climate and land use change decreases the direct radiative forcing (−0.38 W m−2) by 6.3% and the indirect radiative forcing (−1.68 W m−2) by 3.5% due to the size distribution and number concentration change of secondary organic aerosol and sulfate. Overall, the total radiative forcing associated with anthropogenic aerosols is decreased by 16%.

Climatology of Aerosol Optical Properties at Storm Peak Laboratory

ABSTRACT Aerosols create large uncertainty in the planetary energy balance due to both direct and indirect radiative forcing. Understanding aerosol seasonal patterns is essential for accurate climate change prediction, but mountain regions are often difficult for climate models to resolve. Therefore, long-term observations collected at high elevations are particularly useful. In-situ surface aerosol optical measurements were analyzed for the years 2011–2016 at a mountain site located in western Colorado and tied to potential sources based on relationships among the aerosol properties. The peak values for the scattering and absorption coefficients were observed during the summer, suggesting greater aerosol loading (likely due to wildfires), whereas the lowest values were observed during the winter, indicating cleaner conditions (due to less influence from the boundary layer). The scattering Angstrom exponent, a property that provides information about size distributions, revealed the predominance of coarse-mode particles during the spring, which is consistent with the presence of dust. The aerosols observed during the summer, however, were mostly composed of fine-mode particles. This increase in the fine fraction points to combustion, likely wildfires during the dry season (Hallar, 2015), as a source, which is further supported by the absorption Angstrom exponent dropping to its lowest value (close to 1) during the summer after exhibiting a slightly higher value (~1.3) during the spring. Schmeisser et al. (2017) suggests that, for in-situ aerosol, absorption Angstrom exponents larger than 1.5 may be indicative of dust if they are associated with low (< 1.3) scattering Angstrom exponents. The increase in combustion aerosols during the summer accompanied by high values for the single scattering albedo suggests that these aerosols underwent processing in the atmosphere before reaching Storm Peak Laboratory. These results are important for improving visibility and predicting future aerosol concentrations in the western U.S.