The 50 most-cited articles in Atmospheric chemistry and physics, 2020

, HCl, and organic acids and bases as well as chemical reaction rates. It has implications for the atmospheric lifetime of pollutants, deposition, and human health. Despite its fundamental role in atmospheric processes, only recently has this field seen a growth in the number of studies on particle acidity. Even with this growth, many fine particle pH estimates must be based on thermodynamic model calculations since no operational techniques exist for direct measurements. Current information indicates acidic fine particles are ubiquitous, but observationally-constrained pH estimates are limited in spatial and temporal coverage. Clouds and fogs are also generally acidic, but to a lesser degree than particles, and have a range of pH that is quite sensitive to anthropogenic emissions of sulfur and nitrogen oxides, as well as ambient ammonia. Historical measurements indicate that cloud and fog droplet pH has changed in recent decades in response to controls on anthropogenic emissions, while the limited trend data for aerosol particles indicates acidity may be relatively constant due to…

Abstract. Surface ozone data from the Chinese Ministry of Ecology and Environment (MEE) network show sustained increases across the country over the 2013–2019 period. Despite Phase 2 of Clean Air Action targeting ozone pollution, ozone was higher in 2018–2019 than in previous years. The mean summer 2013–2019 trend of maximum 8-h average (MDA8) ozone was 1.9 ppb a−1 across China and 3.3 ppb a−1 in the North China Plain (NCP). Fitting ozone to meteorological variables with a multiple linear regression model shows that meteorology played a significant but not dominant role in the 2013–2019 ozone trend, contributing 0.70 ppb a−1 across China and 1.4 ppb a−1 in the NCP. Higher June–July temperatures over the NCP were the main meteorological driver, particularly in recent years (2017–2019), and were associated with increased foehn winds. NCP data for 2017–2019 show a 15 % continuing decrease in fine particulate matter (PM2.5) and flat emissions of volatile organic compounds (VOCs), which would explain the continued anthropogenic increase in ozone. VOC emission controls, as targeted by…

Abstract. Fine particulate matter with aerodynamic diameters ≤ 2.5 μm (PM2.5) shows adverse effects on human health and atmospheric environment. Satellite-derived aerosol products have been intensively adopted in estimating surface PM2.5 concentrations, but most previous studies failed to monitor air pollution over small-scale areas limited by coarse spatial-resolution (3–50 km) and low data-quality aerosol optical depth (AOD) products. Therefore, a new space-time extremely randomized trees (STET) model is developed that integrates spatiotemporal information to improve PM2.5 estimates at both spatial resolution and overall accuracy across China. To this end, the newly released MODIS MAIAC AOD product, meteorological and other auxiliary data are inputs to the STET model. Daily 1-km PM2.5 maps in 2018 across mainland China are produced. The STET model performs well with a high out-of-sample (out-of-station) cross-validation coefficient of 0.89 (0.88), a low root-mean-square error of 10.35 (10.97) μg/m3, a small mean absolute error of 6.71 (7.17) μg/m3, and a small mean relative error…

A long-term historical emission inventory of air and climate pollutants in East, Southeast, and South Asia during 1950–2015 was developed as the Regional Emission inventory in ASia version 3 (REASv3). REASv3 provides details of emissions from major anthropogenic sources for each country and its sub-regions and also provides monthly gridded data with 0.25 ∘ × 0.25 ∘ resolution. The average total emissions in Asia during 1950–1955 and during 2010–2015 (growth rates in these 60 years estimated from the two averages) are as follows: SO 2 : 3.2 Tg, 42.4 Tg (13.1); NO x : 1.6 Tg, 47.3 Tg (29.1); CO: 56.1 Tg, 303 Tg (5.4); non-methane volatile organic compounds: 7.0 Tg, 57.8 Tg (8.3); NH 3 : 8.0 Tg, 31.3 Tg (3.9); CO 2 : 1.1 Pg, 18.6 Pg (16.5) (CO 2 excluding biofuel combustion 0.3 Pg, 16.8 Pg (48.6)); PM 10 : 5.9 Tg, 30.2 Tg (5.1); PM 2.5 : 4.6 Tg, 21.3 Tg (4.6); black carbon: 0.69 Tg, 3.2 Tg (4.7); and organic carbon: 2.5 Tg, 6.6 Tg (2.7). Clearly, all the air pollutant emissions in Asia increased significantly during these 6 decades, but situations were different among countries and…

Abstract. China has suffered from increasing levels of ozone pollution in urban areas despite the implementation of various stringent emission reduction measures since 2013. In this study, we conducted numerical experiments with an up-to-date regional chemical transport model to assess the contribution of the changes in meteorological conditions and anthropogenic emissions to the summer ozone level from 2013 to 2017 in various regions of China. The model can faithfully reproduce the observed meteorological parameters and air pollutant concentrations and capture the increasing trend in the surface maximum daily 8-hour average (MDA8) ozone (O3) from 2013 to 2017. The emission control measures implemented by the government induced a decrease in MDA8 O3 levels in rural areas but an increase in urban areas. The meteorological influence on the ozone trend varied by region and by year and could be comparable to or even more significant than the impact of changes in anthropogenic emissions. Meteorological conditions can modulate the ozone concentration via direct (e.g., increasing reaction…

Abstract. The effective radiative forcing, which includes the instantaneous forcing plus adjustments from the atmosphere and surface, has emerged as the key metric of evaluating human and natural influence on the climate. We evaluate effective radiative forcing and adjustments in 13 contemporary climate models that are participating in CMIP6 and have contributed to the Radiative Forcing Model Intercomparison Project (RFMIP). Present-day (2014) global mean anthropogenic forcing relative to pre-industrial (1850) from climate models stands at 1.97 (± 0.26) W m−2, comprised of 1.80 (± 0.11) W m−2 from CO2, 1.07 (± 0.21) W m−2 from other well-mixed greenhouse gases, −1.04 (± 0.23) W m−2 from aerosols and −0.08 (± 0.14) W m−2 from land use change. Quoted uncertainties are one standard deviation across model best estimates, and 90 % confidence in the reported forcings, due to internal variability, is typically within 0.1 W m−2. The majority of the remaining 0.17 W m−2 is likely to be from ozone. As determined in previous studies, cancellation of tropospheric and surface adjustments means…

Abstract. The Chinese government launched the Air Pollution Prevention and Control Action Plan in 2013, and various stringent measures have since been implemented, which have resulted in significant decreases in emissions and ambient concentrations of primary pollutants such as SO2, NOx, and particulate matter (PM). However, surface ozone (O3) concentrations have still been increasing in urban areas across the country. In a previous analysis, we examined in detail the roles of meteorological variation during 2013–2017 in the summertime surface O3 trend in various regions of China. In this study, we evaluated the effect of changes in multi-pollutant emissions from anthropogenic activities on O3 concentrations during the same period, by using an up-to-date regional chemical transport model (WRF-CMAQ) driven by an interannual anthropogenic emission inventory. The CMAQ model was improved with regard to heterogeneous reactions of reactive gases on aerosol surfaces, which led to better model performance in reproducing the ambient concentrations of those gases. The model simulations showed…

Abstract. Aerosols from biomass burning (BB) emissions are poorly constrained in global and regional models, resulting in a high level of uncertainty in understanding their impacts. In this study, we compared six BB aerosol emission datasets for 2008 globally as well as in 14 sub-regions. The six BB emission datasets are: (1) GFED3.1 (Global Fire Emissions Database version 3.1); (2) GFED4s (Global Fire Emissions Database version 4 with small fires); (3) FINN1.5 (Fire INventory from NCAR version 1.5); (4) GFAS1.2 (Global Fire Assimilation System version 1.2); (5) FEER1.0 (Fire Energetics and Emissions Research version 1.0), and (6) QFED2.4 (Quick Fire Emissions Dataset version 2.4). Although biomass burning emissions of aerosols from these six BB emission datasets showed similar spatial distributions, their global total emission amounts differed by a factor of 3–4, ranging from 13.76 to 51.93 Tg for organic carbon and from 1.65 to 5.54 Tg for black carbon. In most regions, QFED2.4 and FEER1.0, which are based on the satellite observations of fire radiative power (FRP) and utilize the…

Abstract. Ozone pollution in China is influenced by meteorological processes on multiple scales. Using multiple linear regression and weather classification, we statistically assess the impacts of local and synoptic meteorology on daily variability of surface ozone in eastern China in summer during 2013–2018. In this period, summertime surface ozone in eastern China (110–130° E, 20–42° N) is among the highest in the world with regional means of 73.1 and 114.7 µg m−3, respectively, in daily mean and daily maximum 8-hour average. By developing a multiple linear regression (MLR) model driven by local and synoptic weather factors, we establish a quantitative linkage between the daily ozone concentrations and meteorology in the study region. The meteorology described by the MLR model can explain ~ 46 % of the daily variability in summertime surface ozone across eastern China. The model shows that synoptic factors contribute to ~ 37 % of the overall meteorological effects on daily variability of surface ozone in eastern China. Among local meteorological factors, relative humidity is the…

Abstract. Poor air quality is currently responsible for large impacts on human health across the world. In addition, the air pollutants, ozone (O3) and particulate matter less than 2.5 microns in diameter (PM2.5), are also radiatively active in the atmosphere and can influence Earth’s climate. It is important to understand the effect of air quality and climate mitigation measures over the historical period and in different future scenarios to ascertain any impacts from air pollutants on both climate and human health. The 6th Coupled Model Intercomparison Project (CMIP6) presents an opportunity to analyse the change in air pollutants simulated by the current generation of climate and Earth system models that include a representation of chemistry and aerosols (particulate matter). The shared socio-economic pathways (SSPs) used within CMIP6 encompass a wide range of trajectories in precursor emissions and climate change, allowing for an improved analysis of future changes to air pollutants. Firstly, we conduct an evaluation of the available CMIP6 models against surface observations of…

Abstract. Satellite observations of tropospheric NO2 columns are extensively used to infer trends in anthropogenic emissions of nitrogen oxides (NOx ≡ NO + NO2), but this may be complicated by trends in NOx lifetime. Here we use 2004–2018 observations from the OMI satellite-based instrument (QA4ECV and POMINO v2 retrievals) to examine the seasonality and trends of tropospheric NO2 columns over central-eastern China, and we interpret the results with the GEOS-Chem chemical transport model. The observations show a factor of 3 increase in NO2 columns from summer to winter, which we explain in GEOS-Chem as reflecting a longer NOx lifetime in winter than in summer (21 h versus 5.9 h in 2017). The 2005–2018 summer trends of OMI NO2 closely follow the trends in the Multi-resolution Emission Inventory for China (MEIC), with a rise over the 2005–2011 period and a 25 % decrease since. We find in GEOS-Chem no significant trend of the NOx lifetime in summer, supporting the emission trend reported by MEIC. The winter trend of OMI NO2 is steeper than in summer over the entire period, which we…

Abstract. Secondary ice production (SIP) plays a key role in the formation of ice particles in tropospheric clouds. Future improvement of the accuracy of the weather predictions and climate models relies on a proper description of SIP in numerical simulations. For now, laboratory studies remain a primary tool for developing physically based parameterizations for cloud modeling. Over the past seven decades, six different SIP-identifying mechanisms have emerged: (1) shattering during droplet freezing; (2) the rime splintering (Hallett-Mossop) process; (3) fragmentation due to ice-ice collision; (4) ice particle fragmentation due to thermal shock; (5) fragmentation of sublimating ice; (6) activation of ice nucleating particles in transient supersaturation around freezing drops. This work presents a critical review of the laboratory studies related to secondary ice production. While some of the six mechanisms have received little research attention, others consist of contradictory results obtained by different research groups. Unfortunately, despite past investigative efforts, the lack…

Abstract. Future trends in air pollution and greenhouse gas (GHG) emissions for China are of great concern to the community. A set of global scenarios regarding future socioeconomic and climate developments, combining shared socioeconomic pathways (SSPs) with climate forcing outcomes as described by the representative concentration pathways (RCPs), were created by the Intergovernmental Panel on Climate Change (IPCC). Chinese researchers have also developed various emission scenarios by considering detailed local environmental and climate policies. However, a comprehensive scenario set connecting SSP-RCP scenarios with local policies and representing dynamic emission changes under local policies is still missing. In this work, to fill this gap, we developed a dynamic projection model, the Dynamic Projection model for Emissions in China (DPEC), to explore China’s future anthropogenic emission pathways. The DPEC model is designed to integrate the energy system model, emission inventory model, dynamic projection model, and parameterized scheme of Chinese policies. The model contains two…

A vast area in China is currently going through severe haze episodes with drastically elevated concentrations of PM 2.5 in winter. Nitrate and sulfate are the main constituents of PM 2.5 , but their formations via NO 2 and SO 2 oxidation are still not comprehensively understood, especially under different pollution or atmospheric relative humidity (RH) conditions. To elucidate formation pathways of nitrate and sulfate in different polluted cases, hourly samples of PM 2.5 were collected continuously in Beijing during the wintertime of 2016. Three serious pollution cases were identified reasonably during the sampling period, and the secondary formations of nitrate and sulfate were found to make a dominant contribution to atmospheric PM 2.5 under the relatively high RH condition. The significant correlation between NOR, NOR = NO3-/(NO3-+NO2), and [NO 2 ] 2 × [O 3 ] during the nighttime under the RH≥60 % condition indicated that the heterogeneous hydrolysis of N 2 O 5 involving aerosol liquid water was responsible for the nocturnal formation of nitrate at the extremely high RH levels.…

Abstract. This study presents airborne in-situ and satellite remote sensing climatologies of cirrus clouds and humidity. The climatologies serve as a guide to the properties of cirrus clouds, with the new in-situ data base providing detailed insights into boreal mid-latitudes and the tropics, while the satellite-borne data set offers a global overview. To this end, an extensive, quality checked data archive, the Cirrus Guide II in-situ data base, is created from airborne in-situ measurements during 150 flights in 24 campaigns. The archive contains meteorological parameters, IWC, Nice, Rice, RHice and H2O for each of the flights (IWC: ice water content, Nice: number concentration of ice crystals, Rice: ice crystal mean mass radius, RHice: relative humidity with respect to ice, H2O: water vapor mixing ratio). Depending on the specific parameter, the data base has extended by about a factor of 5–10 compared to the previous studies of Schiller et al. (2008), JGR, and Krämer et al. (2009), ACP. One result of our investigations is, that across all latitudes, the thicker liquid origin…

Abstract. To better understand the effects of wildfires on air quality and climate, it is important to assess the occurrence of chromophoric compounds in smoke and characterize their optical properties. This study explores the molecular composition of light-absorbing organic aerosol, or brown carbon (BrC), sampled at the Missoula Fire Sciences laboratory as a part of the FIREX Fall 2016 lab intensive. Twelve biomass fuels from different plant types were tested, including gymnosperm (coniferous) and angiosperm (flowering) plants, and different ecosystem components such as duff, litter, and canopy. Emitted particles were collected onto Teflon filters and analyzed offline using high performance liquid chromatography/ photodiode array/high resolution mass spectrometry (HPLC/PDA/HRMS). Separated BrC chromophores were classified by their retention times, absorption spectra, integrated PDA absorbance in the near-UV and visible spectral range (300–700 nm), and chemical formulas from the accurate m/z measurements. BrC chromophores were grouped into the following classes and subclasses:…

Abstract. We investigated the fluorescence and chemical-structural characteristics of dissolved brown carbon (BrC) in smoke particulates emitted from the combustion of biomass and fossil fuels (coal and vehicle exhaust) by excitation-emission matrix (EEM) spectroscopy and Fourier-transform ion cyclotron resonance mass spectrometry (FT-ICR MS) coupled with electrospray ionization (ESI). Six components were resolved by parallel factor analysis (PARAFAC) of the water-soluble and methanol-soluble organic carbon (MSOC) fractions, respectively. These fluorescent components varied among sources. Combined with FT-ICR MS ion groups, we found that the fluorescent components agreed well with the functional groups, particularly with nitrogen (N)- and sulfur (S)-containing groups. Among the six PARAFAC components (P1–6) retrieved from the water-soluble organic carbon (WSOC) fraction, except for the P3 component, the other components exhibited different values among the three types of emission sources tested. Vehicle exhaust was characterized by high P1 and P6 components, which are mainly…

Abstract. The formation of inorganic nitrate is the main sink for nitrogen oxides (NOx = NO + NO2). Due to the importance of NOx for the formation of tropospheric oxidants such as the hydroxyl radical (OH) and ozone, understanding the mechanisms and rates of nitrate formation is paramount for our ability to predict the atmospheric lifetimes of most reduced trace gases in the atmosphere. The oxygen isotopic composition of nitrate (Δ17O(nitrate)) is determined by the relative importance of NOx sinks, and thus can provide an observational constraint for NOx chemistry. Until recently, the ability to utilize Δ17O(nitrate) observations for this purpose was hindered by our lack of knowledge about the oxygen isotopic composition of ozone (Δ17O(O3)). Recent and spatially widespread observations of Δ17O(O3) have greatly reduced this uncertainty, and allow for an updated comparison of modeled and observed Δ17O(nitrate) and a reassessment of modeled nitrate formation pathways. Model updates based on recent laboratory studies of heterogeneous reactions renders dinitrogen pentoxide (N2O5)…

Abstract. Carbonaceous aerosols significantly affect global radiative forcing and climate through absorption and scattering of sunlight. Black carbon (BC) and brown carbon (BrC) are light-absorbing carbonaceous aerosols. The direct radiative effect (DRE) of BrC is uncertain. A recent study suggests that BrC absorption is comparable to BC in the upper troposphere over biomass burning regions and that the resulting radiative heating tends to stabilize the atmosphere. Yet current climate models do not include proper physical and chemical treatments of BrC. In this study, we derived a BrC global biomass burning emission inventory on the basis of the Global Fire Emissions Database 4 (GFED4), developed a BrC module in the Community Atmosphere Model version 5 (CAM5) of Community Earth System Model (CESM) model, and investigated the photo-bleaching effect and convective transport of BrC on the basis of Studies of Emissions, Atmospheric Composition, Clouds and Climate Coupling by Regional Surveys (SEAC4RS) and Deep Convective Clouds and Chemistry Project (DC-3) measurements. The model…

Abstract. Chemical transport models have historically struggled to accurately simulate the magnitude and variability of observed organic aerosol (OA), with previous studies demonstrating that models significantly underestimate observed concentrations in the troposphere. In this study, we explore two different model OA schemes within the standard GEOS-Chem chemical transport model and evaluate the simulations against a suite of 15 globally-distributed airborne campaigns from 2008–2017. These include the ATom, KORUS-AQ, GoAmazon, FRAPPE, SEAC4RS, SENEX, DC3, CalNex, OP3, EUCAARI, ARCTAS and ARCPAC campaigns and provide broad coverage over a diverse set of atmospheric-composition regimes – anthropogenic, biogenic, pyrogenic and remote. The schemes include significant differences in their treatment of the primary and secondary components of OA – a simple scheme that models primary OA (POA) as non-volatile and takes a fixed-yield approach to secondary OA (SOA) formation, and a complex scheme that simulates POA as semi-volatile and uses a more sophisticated volatility basis set approach…

Satellite instruments provide a vantage point for studying aerosol loading consistently over different regions of the world. However, the typical lifetime of a single satellite platform is on the order of 5–15 years; thus, for climate studies, the use of multiple satellite sensors should be considered. Discrepancies exist between aerosol optical depth (AOD) products due to differences in their information content, spatial and temporal sampling, calibration, cloud masking, and algorithmic assumptions. Users of satellite-based AOD time-series are confronted with the challenge of choosing an appropriate dataset for the intended application. In this study, 16 monthly AOD products obtained from different satellite sensors and with different algorithms were inter-compared and evaluated against Aerosol Robotic Network (AERONET) monthly AOD. Global and regional analyses indicate that products tend to agree qualitatively on the annual, seasonal and monthly timescales but may be offset in magnitude. Several approaches were then investigated to merge the AOD records from different satellites…

Abstract. Nitrogen oxides (NOx) and ammonia (NH3) from anthropogenic and biogenic emissions are central contributors to particulate matter (PM) concentrations worldwide. The response of PM to changes in the emissions of both compounds is typically studied on a case-by-case basis, owing in part to the complex thermodynamic interactions of these aerosol precursors with other PM constituents. Here we present a simple but thermodynamically consistent approach that expresses the chemical domains of sensitivity of aerosol particulate matter to NH3 and HNO3 availability in terms of aerosol pH and liquid water content. From our analysis, four policy-relevant regimes emerge in terms of sensitivity: (i) NH3-dominated, (ii) HNO3-dominated, (iii) combined NH3 and HNO3 sensitive, and, (iv) a domain where neither NH3 and HNO3 are important for PM levels (but only nonvolatile precursors such as NVCs and sulfate). When this framework is applied to ambient measurements or predictions of PM and gaseous precursors, the chemical regime of PM sensitivity to NH3 and HNO3 availability is directly…

A regional fully coupled meteorology–chemistry model, Weather Research and Forecasting model with Chemistry (WRF-Chem), was employed to study the seasonality of ozone (O 3 ) pollution and its sources in both China and India. Observations and model results suggest that O 3 in the North China Plain (NCP), Yangtze River Delta (YRD), Pearl River Delta (PRD), and India exhibit distinctive seasonal features, which are linked to the influence of summer monsoons. Through a factor separation approach, we examined the sensitivity of O 3 to individual anthropogenic, biogenic, and biomass burning emissions. We found that summer O 3 formation in China is more sensitive to industrial and biogenic sources than to other source sectors, while the transportation and biogenic sources are more important in all seasons for India. Tagged simulations suggest that local sources play an important role in the formation of the summer O 3 peak in the NCP, but sources from Northwest China should not be neglected to control summer O 3 in the NCP. For the YRD region, prevailing winds and cleaner air from the…

Abstract. The spread of the new coronavirus (COVID-19) forced the Spanish Government to implement extensive lockdown measures to reduce the number of hospital admissions, starting on March 14th 2020. Over the following days and weeks, strong reductions of nitrogen dioxide (NO2) pollution were reported in many regions of Spain. A substantial part of these reductions is obviously due to decreased local and regional anthropogenic emissions. Yet, the confounding effect of meteorological variability hinders a reliable quantification of the lockdown impact upon the observed pollution levels. Our study uses machine learning (ML) models fed by meteorological data along with other time features to estimate the business-as-usual NO2 mixing ratios that would have been observed in the absence of the lockdown. We then quantify the so-called meteorology-normalized NO2 reductions induced by the lockdown measures by comparing the business-as-usual with the actually observed NO2 mixing ratios. We applied this analysis for a selection of urban background and traffic stations covering the more than 50…

Abstract. Gas-phase auto-oxidation of organics can generate highly-oxygenated organic molecules (HOMs) and thus increase secondary organic aerosol production and enable new-particle formation. Here we present a new implementation of the Volatility Basis Set (VBS) that explicitly resolves peroxy radicals (RO2) formed via auto-oxidation. The model includes a strong temperature dependence for auto oxidation as well as explicit termination of RO2, including reactions with NO, HO2, and other RO2. The RO2 cross reactions can produce dimers (ROOR). We explore the temperature and NOx dependence of this chemistry, showing that temperature strongly influences the intrinsic volatility distribution and that NO can suppress auto-oxidation under conditions typically found in the atmosphere.

Abstract. We present particle optical properties of stratospheric smoke layers observed over Punta Arenas (53.2° S, 70.9° W), Chile, at the southernmost tip of South America in January 2020. The smoke originated from the record-breaking bushfires in Australia. The stratospheric aerosol optical thickness reached values up to 0.7 at 532 nm in mid January 2020. The measured smoke extinction-to-backscatter ratios (lidar ratios) and linear depolarization ratios at 355 and 532 nm wavelength indicate shape, size, and light-absorption properties and are important input parameters in the analysis of spaceborne lidar observations of the CALIPSO and Aeolus missions. They are also of key importance regarding the homogenization of the overall Aeolus (355 nm wavelength) and CALIPSO (532 nm wavelength) smoke data sets and interpretation of the observations with respect to the spread of the smoke particles across the southern hemisphere and decay of the stratospheric perturbation. We found typical values and spectral dependencies of the lidar ratio and linear depolarization ratio for aged…

Abstract. Recent studies have recognized highly oxygenated organic molecules (HOM) in the atmosphere as important in the formation of secondary organic aerosol (SOA). A large number of studies have focused on HOM formation from oxidation of biogenically emitted monoterpenes. However, HOM formation from anthropogenic vapours has so far received much less attention. Previous studies have identified the importance of aromatic volatile organic compounds (VOC) for SOA formation. In this study, we investigated several aromatic compounds, benzene (C6H6), toluene (C7H8), and naphthalene (C10H8), for their potential to form HOM upon reaction with hydroxyl radicals (OH). We performed flow tube experiments with all three VOC, and focused in detail on benzene HOM formation in the Jülich Plant Atmosphere Chamber (JPAC). In JPAC, we also investigated the response of HOM to NOx and seed aerosol. Using a nitrate-based chemical ionization mass spectrometer (CI-APi-TOF), we observed the formation of HOM in the flow reactor oxidation of benzene from the first OH attack. However, in the oxidation of…

Abstract. Volatile organic compounds (VOCs) play important roles in the tropospheric atmosphere. In this study, VOCs were measured at an urban site in Guangzhou, one of the mega-cities in Pearl River Delta (PRD) using a gas chromatograph mass spectrometer/flame ionization detection (GC-MS/FID) and a proton transfer reaction time-of-flight mass spectrometer (PTR-ToF-MS). Diurnal profile analyses show that stronger chemical removal by OH radicals for more reactive hydrocarbons during the daytime. Diurnal profiles of OVOCs indicate evidence of contributions from secondary formation. Detailed source analyses of OVOCs using a photochemical age-based parameterization method suggest important contributions from both primary emissions and secondary formation for measured OVOCs. During the campaign, around 1700 ions were detected in PTR-ToF-MS mass spectra, among of which 462 ions with noticeable concentrations. VOCs signals from these ions without calibration in PTR-TOF-MS are quantified based on sensitivities of available VOCs species. OVOC-related ions dominated PTR-ToF-MS mass spectra…

Abstract. The switch from the use of coal to natural gas or oil for the energy generation potentially reduces the greenhouse gas emissions and thus the impact on global warming and climate change because of the larger energy content per CO2 molecule emitted. However, the climate benefit over coal is offset by methane (CH4) leakage from natural gas and petroleum systems, which reverses the climate impact mitigation if the rate of fugitive emissions exceeds the compensation point at which the global warming resulting from the leakage and the benefit from the reduction of coal combustion coincide. Consequently, an accurate quantification of the CH4 emissions from the oil and gas industry is essential to evaluate the suitability of natural gas and petroleum as bridging fuels on the way to a carbon-neutral future. We show that regional CH4 release from large oil and gas fields can be monitored from space by using dense daily recurrent measurements of the TROPOspheric Monitoring Instrument (TROPOMI) onboard the Sentinel-5 Precursor satellite to quantify emissions and leakage rates. The…

Abstract. Wintertime in situ measurements of OH, HO2 and RO2 radicals and OH reactivity were made in central Beijing during November and December 2016. Exceptionally elevated NO was observed on occasions, up to ~ 250 ppbv, believed to be the highest mole fraction for which there have then co-located radical observations. The daily maximum mixing ratios for radical species varied significantly day-to-day over the range 1–8 × 106 cm−3 (OH), 0.2–1.5 × 108 cm−3 (HO2) and 0.3–2.5 × 108 cm−3 (RO2). Averaged over the full observation period, the mean daytime peak in radicals was 2.7 × 106 cm−3, 0.39 × 108 cm−3 and 0.88 × 108 cm−3 for OH, HO2 and total RO2, respectively. The main daytime source of new radicals via initiation processes (primary production) was the photolysis of HONO (~ 83 %), and the dominant termination pathways were the reactions of OH with NO and NO2, particularly under polluted, haze conditions. The Master Chemical Mechanism (MCM) v3.3.1 operating within a box model was used to simulate the concentrations of OH, HO2 and RO2. The model underpredicted OH, HO2 and RO2,…

Abstract. Two consecutive cruises in the Weddell Sea, Antarctica, in winter 2013 provided the first direct observations of sea salt aerosol (SSA) production from blowing snow above sea ice, thereby validating a model hypothesis to account for winter time SSA maxima in polar regions not explained otherwise. Blowing or drifting snow always lead to increases in SSA during and after storms. Observed aerosol gradients suggest that net production of SSA takes place near the top of the blowing or drifting snow layer. The observed relative increase of SSA concentrations with wind speed suggests that on average the corresponding aerosol mass flux during storms was equal or larger above sea ice than above the open ocean, demonstrating the importance of the blowing snow source for SSA in winter and early spring. For the first time it is shown that snow on sea ice is depleted in sulphate relative to sodium with respect to sea water. Similar depletion observed in the aerosol suggests that most sea salt originated from snow on sea ice and not the open ocean or leads, e.g. on average 93 % during…

Abstract. This study attempts identification of mechanisms of secondary ice production (SIP) based on the observation of small faceted ice crystals (hexagonal plates or columns) with characteristic sizes smaller than 100 μm. Due to their young age, such small ice crystals can be used as tracers for identifying the conditions for SIP. Observations reported here were conducted in oceanic tropical mesoscale convective systems (MCS) and mid-latitude frontal clouds in the temperature range from 0 °C to −15 °C heavily seeded by aged ice particles. It was found that both in MCSs and frontal clouds, SIP was observed right above the melting layer and extended to the higher altitudes with colder temperatures. It is proposed that the initiation of SIP above the melting layer is related to the circulation of liquid drops through the melting layer. Liquid drops formed via melting ice particles are advected by the convective updrafts above the melting layer, where they impact with aged ice, freeze and shatter. The ice splinters generated by shattering initiate the chain reaction of SIP. The size…

Abstract. In order to track progress towards the global climate targets, the parties that signed the Paris Climate Agreement will regularly report their anthropogenic carbon dioxide (CO2) emissions based on energy statistics and CO2 emission factors. Independent evaluation of this self-reporting system is a fast-growing research topic. Here, we study the value of satellite observations of the column CO2 concentrations to estimate CO2 anthropogenic emissions with five years of the Orbiting Carbon Observatory-2 (OCO-2) retrievals over and around China. With the detailed information of emission source locations and the local wind, we successfully observe CO2 plumes from 60 cities and industrial regions over China and quantify their CO2 emissions from the OCO-2 observations, which add up to a total of 1.6 Gt CO2 yr−1 that account for 17 % of mainland China's annual emissions. The number of cities whose emissions are constrained by OCO-2 here is three to ten times larger than previous studies that only focused on large cities and power plants in different locations around the world. Our…

A large data set of rate coefficients for H migration in peroxy radicals is presented and supplemented with literature data to derive a structure–activity relationship (SAR) for the title reaction class. The SAR supports aliphatic RO 2 radicals; unsaturated bonds and β -oxo substitutions both endocyclic and exocyclic to the transition state ring; and α -oxo (aldehyde), –OH, –OOH, and –ONO 2 substitutions, including migration of O-based hydrogen atoms. Also discussed are –C(=O)OH and –OR substitutions. The SAR allows predictions of rate coefficients k ( T ) for a temperature range of 200 to 450 K, with migrations spans ranging from 1,4 to 1,9-H shifts depending on the functionalities. The performance of the SAR reflects the uncertainty of the underlying data, reproducing the scarce experimental data on average to a factor of 2 and the wide range of theoretical data to a factor of 10 to 100, depending also on the quality of the data. The SAR evaluation discusses the performance in multi-functionalized species. For aliphatic RO 2 , we also present some experimental product…

Abstract. This article, the seventh in the series, presents kinetic and photochemical data sheets evaluated by the IUPAC Task Group on Atmospheric Chemical Kinetic Data Evaluation. It covers an extension of the gas phase and photochemical reactions related to Criegee intermediates previously published in ACP in 2006, and implemented on the IUPAC website up to 2020. The article consists of an introduction, description of laboratory measurements, discussion of rate coefficients for reactions of O3 with alkenes producing Criegee intermediates, rate coefficients of unimolecular and bimolecular reactions and photochemical data for reactions of Criegee intermediates, and overview of the atmospheric chemistry of Criegee intermediates. Summary tables of the recommended kinetic and mechanistic parameters for the evaluated reactions are provided. Data sheets summarizing information upon which the recommendations are based are given in two files, provided as a Supplement to this article.

Abstract. Highly-oxygenated organic molecules (HOMs) contribute substantially to the formation and growth of atmospheric aerosol particles, which affect air quality, human health and Earth's climate. HOMs are formed by rapid, gas-phase autoxidation of volatile organic compounds (VOCs) such as α-pinene, the most abundant monoterpene in the atmosphere. Due to their abundance and low volatility, HOMs can play an important role for new-particle formation (NPF) and the early growth of atmospheric aerosols, even without any further assistance of other low-volatility compounds such as sulfuric acid. Both the autoxidation reaction forming HOMs and their new-particle formation rates are expected to be strongly dependent on temperature. However, experimental data on both effects are limited. Dedicated experiments were performed at the CLOUD (Cosmics Leaving OUtdoor Droplets) chamber at CERN to address this question. In this study, we show that a decrease in temperature (from +25 to −50 °C) results in a reduced HOM yield and reduced oxidation state of the products, whereas the new-particle…

Abstract. In response to increasing greenhouse gases, the subtropical edges of Earth's Hadley circulation shift poleward in global climate models. Recent studies have found that reanalysis trends in the Hadley cell edge over the past 30–40 years are within the range of trends simulated by Coupled Model Intercomparison Project Phase 5 (CMIP5) models, and have documented seasonal and hemispheric asymmetries in these trends. In this study, we evaluate whether these conclusions hold for the newest generation of models (CMIP6). Overall, we find similar characteristics of Hadley cell expansion in CMIP5 and CMIP6 models. In both CMIP5 and CMIP6 models, the poleward shift of the Hadley cell edge in response to increasing greenhouse gases is 2–3 times larger in the Southern Hemisphere (SH), except during September–November. The trends from CMIP5 and CMIP6 models agree well with reanalyses, although prescribing observed coupled atmosphere-ocean variability allows the models to better capture reanalysis trends in the Northern Hemisphere (NH). We find two notable differences between CMIP5 and…

Abstract. Fires and the aerosols that they emit impact air quality, health, and climate, but the abundance and properties of carbonaceous aerosol (both black carbon and organic carbon) from biomass burning (BB) remain uncertain and poorly constrained. We aim to quantify the uncertainties associated with fire emissions and their air quality and radiative impacts from underlying dry matter consumed and emissions factors. To explore this, we compare model simulations from a global chemical transport model, GEOS-Chem, driven by a variety of fire emission inventories with surface and airborne observations of black carbon (BC) and organic aerosol (OA) concentrations and satellite-derived aerosol optical depth (AOD). We focus on two fire detection/burned area-based (FD/BA) inventories using burned area and active fire counts, respectively: the Global Fire Emissions Database version 4 (GFED4s) with small fires and the Fire INventory from NCAR version 1.5 (FINN1.5) and two fire radiative power (FRP)-based approaches: the Quick Fire Emission Dataset version 2.4 (QFED2.4) and the Global Fire…

Abstract. We report a 40-year history of SF6 atmospheric mole fractions measured at the Advanced Global Atmospheric Gases Experiment (AGAGE) monitoring sites, combined with archived air samples to determine emission estimates from 1978–2018. Previously we reported a global emission rate of 7.3 ± 0.6 Gigagrams (Gg) yr−1 in 2008 and over the past decade emissions have continued to increase by about 24 % to 9.04 ± 0.35 Gg yr−1 in 2018. We show that changing patterns in SF6 consumption from developed (Kyoto Protocol Annex-1) to developing countries (non-Annex-1) and the rapid global expansion of the electric power industry, mainly in Asia, have increased the demand for SF6-insulated switchgear, circuit breakers and transformers. The large bank of SF6 sequestered in this electrical equipment provides a substantial source of emissions from maintenance, replacement and continuous leakage. Other emissive sources of SF6 occur from the magnesium, aluminium, electronics industries and more minor industrial applications. More recently, reported emissions, including those from electrical…

Abstract. As has been the case in North America and Western Europe, the SO2 emissions substantially reduced in North China Plain (NCP) in recent years. A dichotomy of reductions in SO2 and NOx concentrations result in the frequent occurrences of nitrate (pNO3−)-dominated particulate matter pollution over NCP. In this study, we observed a polluted episode with the nitrate mass fraction in non-refractory PM1 (NR-PM1) up to 44 % during wintertime in Beijing. Based on this typical pNO3−-dominated haze event, the linkage between aerosol water uptake and pNO3− formation, further impacting on visibility degradation, have been investigated based on field observations and theoretical calculations. During haze development, as ambient relative humidity (RH) increased from ~ 10 % up to 70 %, the aerosol particle liquid water increased from ~ 1 μg/m3 at the beginning to ~ 75 μg/m3 at the fully-developed haze period. Without considering the water uptake, the particle surface area and the volume concentrations increased by a factor of 4.1 and 4.8, respectively, during the development of haze…

Abstract. The present work aims at better understanding regional climate-aerosol interactions by studying the relationships between aerosols and synoptic atmospheric circulation over the Euro-Mediterranean region. Two 40-year simulations (1979–2018) have been carried out with the CNRM-ALADIN64 regional climate model, one using interactive aerosols and the other one without any aerosol. The simulation with aerosols has been evaluated in terms of different climate and aerosol parameters. This evaluation shows a good agreement between the model and observations, significant improvements compared to the previous model version, and consequently the relevance of using this model for the study of climate-aerosol interactions over this region. A first attempt to explain the climate variability of aerosols is based on the use of the North-Atlantic Oscillation (NAO) index, which explains a significant part of the interannual variability, notably in winter for the export of dust aerosols over the Atlantic Ocean and the Eastern Mediterranean, and in summer for the positive anomalies of…

Abstract. The Earth's equilibrium climate sensitivity (ECS) to a doubling of atmospheric CO2, along with the transient 35 climate response (TCR) and greenhouse gas emissions pathways, determines the amount of future warming. Coupled climate models have in the past been important tools to estimate and understand ECS. ECS estimated from Coupled Model Intercomparison Project Phase 5 (CMIP5) models lies between 2.0 and 4.7 K (mean of 3.2 K), whereas in the latest CMIP6 the spread has increased: 1.8–5.5 K (mean of 3.7 K), with 5 out of 25 models exceeding 5 K. It is thus pertinent to understand the causes underlying this shift. Here we compare the CMIP5 and CMIP6 model ensembles, and find a systematic shift between CMIP eras to be unexplained as a process of random sampling from modeled forcing and feedback distributions. Instead, shortwave feedbacks shift towards more positive values, in particular over the Southern Ocean, driving the shift towards larger ECS values in many of the models. These results suggest that changes in model treatment of mixed-phase cloud processes and changes to…

Abstract. The Twomey effect describes the radiative forcing associated with a change in cloud albedo due to an increase in anthropogenic aerosol emissions. It is driven by the perturbation in cloud droplet number concentration (ΔNd,ant) in liquid-water clouds and is currently understood to exert a cooling effect on climate. The Twomey effect is the key driver in the effective radiative forcing due to aerosol–cloud interactions which also comprises rapid adjustments. These adjustments are essentially the responses of cloud fraction and liquid water path to ΔNd,ant and thus scale approximately with it. While the fundamental physics of the influence of added aerosol particles on the droplet concentration (Nd) is well described by established theory at the particle scale (micrometres), how this relationship is expressed at the large scale (hundreds of kilometres) ΔNd,ant remains uncertain. The discrepancy between process understanding at particle scale and insufficient quantification at the climate-relevant large scale is caused by co-variability of aerosol particles and vertical wind…

Abstract. We analyse historical (1850–2014) atmospheric hydroxyl (OH) and methane lifetime data from CMIP6/AerChemMIP simulations. Global OH changed little from 1850 up to around 1980, then increased by around 10 %, with an associated reduction in methane lifetime. The model-derived OH trend since 1980 differs from trends found in several studies that infer OH from inversions of methyl chloroform measurements; however, these inversions are poorly constrained and contain large uncertainties that do not rule out the possibility of recent positive OH trends. The recent increases in OH that we find are consistent with one previous study that assimilated global satellite-derived carbon monoxide (CO) over the period 2002–2013. The upward trend in modelled OH since 1980 was mainly driven by changes in anthropogenic Near-Term Climate Forcer emissions (increases in anthropogenic nitrogen oxides and decreases in CO). Increases in halocarbon emissions since 1950 have made a small contribution to the increase in OH, whilst increases in aerosol-related emissions have slightly reduced OH.…

Abstract. Particle acidity is crucial to understand secondary formation processes in pollution events because of its substantial impacts on the physiochemical properties of PM2.5. Recently, particle pH predicted by thermodynamic modeling were used to elucidate the sulfate formation mechanism in China, but the results were biased and controversial. In this article, particle pH was found to be increase as a result of effective sulfur emission control. Benefit from strict pollution control actions, average PM2.5 concentration reduced to a low level (39.7 μg/m3) in urban Beijing during winter of 2017. Compare to history record (2014–2017), SO2 gradually decreased to a low level (3.2 ppbv in 2017 winter) while NO2 kept increasing (21.4 ppbv in 2017 winter). As a response, nitrate's contribution (23.0 μg/m3) to PM2.5 become dominant over sulfate (13.1 μg/m3) during the PM2.5 pollution. The nitrate to sulfate molar ratio significantly increased from 1 to 2.7 (value of 1999 and 2017). As particulate nitrate fraction significantly elevated, particle pH was also found to increase in winter…

Abstract. Aerosol-planetary boundary layer (PBL) interaction was proposed as an important mechanism to stabilize atmosphere and exacerbate surface air pollution. Despite the tremendous progress made in understanding this process, its magnitude and significance still bear large uncertainties and vary largely with aerosol distribution and meteorological conditions. In this study, we particularly focus on the role of aerosol vertical distribution on thermodynamic stability and PBL development by jointly using the micropulse lidar, sun-photometer, and radiosonde measurements over Beijing. Despite complex aerosol vertical distributions, the cloud-free aerosol structures can be classified into three types: well-mixed, decreasing with height, and the inversed. Under these different aerosol vertical structures, the aerosol-PBL relationships and the diurnal cycles of the PBLH and PM2.5 show distinct characters. The vertical distribution of aerosol radiative forcing differs drastically with strong heating in the lower, mid, and upper PBL respectively. Such a discrepancy in heating rate…

Abstract. Modern-Era Retrospective analysis for Research and Applications v.2 (MERRA-2), Copernicus Atmosphere Monitoring Service Operational Analysis (CAMS-OA) data assimilation products, and a regional Weather Research and Forecasting model (10 km resolution) coupled with Chemistry (WRF-Chem) were used to evaluate natural and anthropogenic aerosol air pollution in the ME during 2015–2016. Satellite and ground-based AOD observations, as well as in-situ Particulate Matter (PM) measurements for 2016, were used for validation. WRF-Chem code was modified to correct the calculation of dust gravitational settling and aerosol optical properties. The dust emission in WRF-Chem is calibrated to fit Aerosol Optical Depth (AOD) and aerosol volume size distributions obtained from Aerosol Robotic Network (AERONET) observations. MERRA-2 was used to construct WRF-Chem initial and boundary conditions both for meteorology and chemical/aerosol species. SO2 emissions in WRF-Chem are based on the novel NASA SO2 emission dataset that reveals unaccounted sources over the ME. Although aerosol fields in…

Abstract. Primary biogenic organic aerosols (PBOA) represent a major fraction of coarse organic matter (OM) in air. Despite their implication in many atmospheric processes and human health problems, we surprisingly know little about PBOA characteristics (i.e., composition, dominant sources, and contribution to airborne-particles). In addition, specific primary sugar compounds (SCs) are generally used as markers of PBOA associated with Bacteria and Fungi but our knowledge of microbial communities associated with atmospheric particulate matter (PM) remains incomplete. This work aimed at providing a comprehensive understanding of the microbial fingerprints associated with SCs in PM10 (particles smaller than 10 µm) and their main sources in the surrounding environment (soils and vegetation). An intensive study was conducted on PM10 collected at rural background site located in an agricultural area in France. We combined high-throughput sequencing of Bacteria and Fungi with detailed physicochemical characterization of PM10, soils and plant samples, and monitored meteorology and…

Abstract. An observation-based model coupled to the Master Chemical Mechanism (V3.3.1) and constrained by a full suite of observations was developed to study atmospheric oxidation capacity (AOC), OH reactivity, OH chain length, and HOx (= OH + HO2) budget for three different ozone (O3) concentration levels in Shanghai, China. Five months of observation from 1 May to 30 September 2018 showed that 10 days with ozone as the primary pollutant occurred and the days with good air quality (AQI

Abstract. Oil and natural gas (O&NG) exploration presents a significant source of atmospheric volatile organic compounds (VOCs), which are central players of tropospheric chemistry and contribute to formations of ozone (O3) and secondary organic aerosols. The impacts of O&NG extraction on regional air quality have been investigated in recent years in North America, but have long been overlooked in China. To assess the impacts of O&NG exploration on tropospheric O3 and regional air quality in China, intensive field observations were conducted during February–March and June–July 2017 in the Yellow River Delta, an oil extraction region in northern China. Very high concentrations of ambient VOCs were observed at a rural site, with the highest alkane mixing ratios reaching 2498 ppbv. High O3 episodes were not encountered during wintertime but were frequently observed in summer. The emission profiles of VOCs from the oil fields were directly measured for the first time in China. The chemical budgets of ROx radicals and O3 were dissected with a detailed chemical box model constrained by…