We integrated Enviro-HIRLAM (Environment-High Resolution Limited Area Model) meteorological output into FLEXPART (FLEXible PARTicle dispersion model). A FLEXPART simulation requires meteorological input from a numerical weather prediction (NWP) model. The publicly available version of FLEXPART can utilize either ECMWF (European Centre for Medium-range Weather Forecasts) Integrated Forecast System (IFS) forecast or reanalysis NWP data, or NCEP (U.S. National Center for Environmental Prediction) Global Forecast System (GFS) forecast or reanalysis NWP data. The primary benefits of using Enviro-HIRLAM are that it runs at a higher resolution and accounts for aerosol effects in meteorological fields. We compared backward trajectories generated with FLEXPART using Enviro-HIRLAM (both with and without aerosol effects) to trajectories generated using NCEP GFS and ECMWF IFS meteorological inputs, for a case study of a heavy haze event which occurred in Beijing, China in November 2018. We found that results from FLEXPART were considerably different when using different meteorological inputs. When aerosol effects were included in the NWP, there was a small but noticeable difference in calculated trajectories. Moreover, when looking at potential emission sensitivity instead of simply expressing trajectories as lines, additional information, which may have been missed when looking only at trajectories as lines, can be inferred.

Haze in Beijing is linked to atmospherically formed secondary organic aerosol, which has been shown to be particularly harmful to human health. However, the sources and formation pathways of these secondary aerosols remain largely unknown, hindering effective pollution mitigation. Here we have quantified the sources of organic aerosol via direct near-molecular observations in central Beijing. In winter, organic aerosol pollution arises mainly from fresh solid-fuel emissions and secondary organic aerosols originating from both solid-fuel combustion and aqueous processes, probably involving multiphase chemistry with aromatic compounds. The most severe haze is linked to secondary organic aerosols originating from solid-fuel combustion, transported from the Beijing-Tianjing-Hebei Plain and rural mountainous areas west of Beijing. In summer, the increased fraction of secondary organic aerosol is dominated by aromatic emissions from the Xi'an-Shanghai-Beijing region, while the contribution of biogenic emissions remains relatively small. Overall, we identify the main sources of secondary organic aerosol affecting Beijing, which clearly extend beyond the local emissions in Beijing. Our results suggest that targeting key organic precursor emission sectors regionally may be needed to effectively mitigate organic aerosol pollution.

Dust storms have a profound impact on air quality, atmospheric chemistry, and human well-being by carrying vast amounts of particles over distances of thousands of kilometers. However, the overall characteristics of these dust events and their influence on secondary pollution in the northern China region are not yet well understood, due to a lack of long-term, comprehensive observations and objective identification techniques. Based on principal component analysis combined with high-time-resolution observations of particulate matter components, here we developed a robust method to identify dust storm events and identified 14 dust events in Beijing in 2019. We further classified these 14 events into two distinct types using Lagrangian particle dispersion models and backward trajectory analysis. The first type (Type I, 9 cases) is characterized by synoptic patterns in Mongolia, originating from the north and directly impacting the Beijing area. The second type (Type II, 5 cases) involves air masses from the north or northwest that temporarily pass through polluted regions south of Beijing before being carried back into the city. Consistently, during Type I dust events, we observed a sharp decrease in secondary inorganic aerosols (SIA) from 65 % to 7 %, as well as in the sulfur oxidation ratio (SOR) and nitrogen oxidation ratio (NOR) (from 0.52 to 0.19, and 0.27 to 0.018 respectively). In contrast, during Type II dust events, SIA concentrations increased by 91 %, along with an increase in SOR (1.7 %), NOR (69 %), and f44/f43 (3.0 %), suggesting an enhancement of secondary aerosol formation resulting from the interaction between dust aerosols and gaseous anthropogenic pollutants. Our results demonstrate that dust events and the sub-type of dust events can be identified in an objective manner using the protocol developed in this study and the dynamics should be considered when discussing impacts of dust events on atmospheric chemistry.

New particle formation (NPF) is a global phenomenon that significantly influences climate. NPF also contributes to haze, with pronounced negative impacts on human health. Theory and observations both show that nucleation is favored during clean days and inhibited during haze episodes due to a high pre-existing condensation sink (CS). Here we show that the surprising occurrence of NPF during haze days in Beijing is associated with a high concentration of sulfuric acid dimers. With both field observations and model simulations, we demonstrate that downward mixing of sulfur dioxide (SO2) from the residual layer aloft enhances ground level SO2, which in turn elevates sulfuric acid dimer after rapid SO2 oxidation in the polluted air. Our results address a key gap between the source of SO2 and its atmospheric oxidation products during haze conditions in a megacity, Beijing, providing a missing link in a complete chain describing NPF in the polluted atmosphere.

The majority of new particle formation (NPF) events observed in Hada Al Sham, western Saudi Arabia, during 2013–2015 showed an unusual progression where the diameter of a newly formed particle mode clearly started to decrease after the growth phase. Many previous studies refer to this phenomenon as aerosol shrinkage. We will opt to use the term decreasing mode diameter (DMD) event, as shrinkage bears the connotation of reduction in the sizes of individual particles, which does not have to be the case. While several previous studies speculate that ambient DMD events are caused by evaporation of semivolatile species, no concrete evidence has been provided, partly due to the rarity of the DMD events. The frequent occurrence and large number of DMD events in our observations allow us to perform statistically significant comparisons between the DMD and the typical NPF events that undergo continuous growth. In our analysis, we find no clear connection between DMD events and factors that might trigger particle evaporation at the measurement site. Instead, examination of air mass source areas and the horizontal distribution of anthropogenic emissions in the study region leads us to believe that the observed DMD events could be caused by advection of smaller, less-grown particles to the measurement site after the more-grown ones. Using a Lagrangian single-particle growth model, we confirm that the observed particle size development, including the DMD events, can be reproduced by non-volatile condensation and thus without evaporation. In fact, when considering increasing contributions from a semivolatile compound, we find deteriorating agreement between the measurements and the model. Based on these results, it seems unlikely that evaporation of semivolatile compounds would play a significant role in the DMD events at our measurement site. In the proposed non-volatile explanation, the DMD events are a result of the observed particles having spent an increasing fraction of their lifetime in a lower-growth environment, mainly enabled by the lower precursor vapor concentrations further away from the measurement site combined with decreasing photochemical production of condensable vapors in the afternoon. Correct identification of the cause of the DMD events is important as the fate and the climate relevance of the newly formed particles heavily depend on it – if the particles evaporated, their net contribution to larger and climatically active particle sizes would be greatly reduced. Our findings highlight the importance of considering transport-related effects in NPF event analysis, which is an often overlooked factor in such studies.

Here we introduce a new method, termed “nanoparticle ranking analysis”, for characterizing new particle formation (NPF) from atmospheric observations. Using daily variations of the particle number concentration at sizes immediately above the continuous mode of molecular clusters, here in practice 2.5–5 nm (i.e. ΔN2.5−5), we can determine the occurrence probability and estimate the strength of atmospheric NPF events. After determining the value of ΔN2.5−5 for all the days during a period under consideration, the next step of the analysis is to rank the days based on this simple metric. The analysis is completed by grouping the days either into a number of percentile intervals based on their ranking or into a few modes in the distribution of log (ΔN2.5−5) values. Using 5 years (2018–2022) of data from the SMEAR II station in Hyytiälä, Finland, we found that the days with higher (lower) ranking values had, on average, both higher (lower) probability of NPF events and higher (lower) particle formation rates. The new method provides probabilistic information about the occurrence and intensity of NPF events and is expected to serve as a valuable tool to define the origin of newly formed particles at many types of environments that are affected by multiple sources of aerosol precursors.

Detailed knowledge on the formation of new aerosol particles in the atmosphere from precursor gases, and their subsequent growth, commonly known as new particle formation (NPF) events, is one of the largest challenges in atmospheric aerosol science. High pre-existing particle loadings are expected to suppress the formation of new atmospheric aerosol particles due to high coagulation and condensation (CS) sinks. However, NPF events are regularly observed in conditions with high concentrations of pre-existing particles and even during intense desert dust intrusions that imply discrepancies between the observations and theory. In this study, we present a multi-site analysis of the occurrence of NPF events under the presence of desert dust particles in dust-influenced areas. Characterization of NPF events at five different locations highly influenced by desert dust outbreaks was done under dusty and non-dusty conditions using continuous measurements of aerosol size distribution in both fine and coarse size fractions. Contrary to common thought, our results show that the occurrence of NPF events is highly frequent during desert dust outbreaks, showing that NPF event frequencies during dusty conditions are similar to those observed during non-dusty conditions. Furthermore, our results show that NPF events also occur during intense desert dust outbreaks at all the studied sites, even at remote sites where the amounts of precursor vapours are expected to be low. Our results show that the condensation sink associated with coarse particles (CSC) represents up to the 60 % of the total CS during dusty conditions, which highlights the importance of considering coarse-fraction particles for NPF studies in desert-dust-influenced areas. However, we did not find a clear pattern of the effect of desert dust outbreaks on the strength of NPF events, with differences from site to site. The particle growth rate (GR) did not present a clear dependence on the CS during dusty and non-dusty conditions. This result, together with the fact that desert dust has different effects on the growth and formation rates at each site, suggests different formation and growth mechanisms at each site between dusty and non-dusty conditions, probably due to differences in precursor vapours' origins and concentrations as well as changes in the oxidative capacity of pre-existing particles and their effectiveness acting as CS. Further investigation based on multiplatform measurement campaigns and chamber experiments with state-of-the-art gaseous and particulate physical and chemical properties measurements is needed to better understand the role of catalyst components present in desert dust particles in NPF. Finally, our results reveal a significant impact of NPF events on the cloud condensation nuclei (CCN) budget during desert dust outbreaks at the studied sites. Therefore, since desert dust contributes to a major fraction of the global aerosol mass load, and since there is a foreseeable increase in the frequency, duration and intensity of desert dust episodes due to climate change, it is imperative to improve our understanding of the effect of desert dust outbreaks on NPF and the CCN budget for better climate change prediction.

Abstract. During the COVID-19 lockdown, the dramatic reduction of anthropogenic emissions provided a unique opportunity to investigate the effects of reduced anthropogenic activity and primary emissions on atmospheric chemical processes and the consequent formation of secondary pollutants. Here, we utilize comprehensive observations to examine the response of atmospheric new particle formation (NPF) to the changes in the atmospheric chemical cocktail. We find that the main clustering process was unaffected by the drastically reduced traffic emissions, and the formation rate of 1.5 nm particles remained unaltered. However, particle survival probability was enhanced due to an increased particle growth rate (GR) during the lockdown period, explaining the enhanced NPF activity in earlier studies. For GR at 1.5–3 nm, sulfuric acid (SA) was the main contributor at high temperatures, whilst there were unaccounted contributing vapors at low temperatures. For GR at 3–7 nm and 7–15 nm, oxygenated organic molecules (OOMs) played a major role. Surprisingly, OOM composition and volatility were insensitive to the large change of atmospheric NOx concentration; instead the associated high particle growth rates and high OOM concentration during the lockdown period were mostly caused by the enhanced atmospheric oxidative capacity. Overall, our findings suggest a limited role of traffic emissions in NPF.

Relatively high concentrations of preexisting particles, acting as a condensation sink (CS) of gaseous precursors, have been thought to suppress the occurrence of new particle formation (NPF) in urban environments, yet NPF still occurs frequently. Here, we aim to understand the factors promoting and inhibiting NPF events in urban Beijing by combining one-year-long measurements of particle number size distributions and PM2.5 chemical composition. Our results show that indeed the CS is an important factor controlling the occurrence of NPF events, with its chemical composition affecting the efficiency of the background particles in removing gaseous H2SO4 (effectiveness of the CS) driving NPF. During our observation period, the CS was found to be more effective for ammonium nitrate-rich (NH4NO3-rich) fine particles. On non-NPF event days, particles acting as CS contained a larger fraction of NH4NO3 compared to NPF event days under comparable CS levels. In particular, in the CS range from 0.02 to 0.03 s-1, the nitrate fraction was 17% on NPF event days and 26% on non-NPF event days. Overall, our results highlight the importance of considering the chemical composition of preexisting particles when estimating the CS and their role in inhibiting NPF events, especially in urban environments.

Organic aerosol (OA) is a major component of fine particulate matter (PM), affecting air quality, human health, and the climate. The absorptive and reflective behavior of OA components contributes to determining particle optical properties and thus their effects on the radiative budget of the troposphere. There is limited knowledge on the influence of the molecular composition of OA on particle optical properties in the polluted urban environment. In this study, we characterized the molecular composition of oxygenated OA collected on filter samples in the autumn of 2018 in Beijing, China, with a filter inlet for gases and aerosols coupled to a high-resolution time-of-flight chemical ionization mass spectrometer (FIGAERO–CIMS). Three haze episodes occurred during our sampling period with daily maximum concentrations of OA of 50, 30, and 55 µg m−3. We found that the signal intensities of dicarboxylic acids and sulfur-containing compounds increased during the two more intense haze episodes, while the relative contributions of wood-burning markers and other aromatic compounds were enhanced during the cleaner periods. We further assessed the optical properties of oxygenated OA components by combining detailed chemical composition measurements with collocated particle light absorption measurements. We show that light absorption enhancement (Eabs) of black carbon (BC) was mostly related to more oxygenated OA (e.g., dicarboxylic acids), likely formed in aqueous-phase reactions during the intense haze periods with higher relative humidity, and speculate that they might contribute to lensing effects. Aromatics and nitro-aromatics (e.g., nitrocatechol and its derivatives) were mostly related to a high light absorption coefficient (babs) consistent with light-absorbing (brown) carbon (BrC). Our results provide information on oxygenated OA components at the molecular level associated with BrC and BC particle light absorption and can serve as a basis for further studies on the effects of anthropogenic OA on radiative forcing in the urban environment.

Atmospheric aerosols have significant effects on the climate and on human health. New particle formation (NPF) is globally an important source of aerosols but its relevance especially towards aerosol mass loadings in highly polluted regions is still controversial. In addition, uncertainties remain regarding the processes leading to severe pollution episodes, concerning e.g. the role of atmospheric transport. In this study, we utilize air mass history analysis in combination with different fields related to the intensity of anthropogenic emissions in order to calculate air mass exposure to anthropogenic emissions (AME) prior to their arrival at Beijing, China. The AME is used as a semi-quantitative metric for describing the effect of air mass history on the potential for aerosol formation. We show that NPF events occur in clean air masses, described by low AME. However, increasing AME seems to be required for substantial growth of nucleation mode (diameter < 30 nm) particles, originating either from NPF or direct emissions, into larger mass-relevant sizes. This finding assists in establishing and understanding the connection between small nucleation mode particles, secondary aerosol formation and the development of pollution episodes. We further use the AME, in combination with basic meteorological variables, for developing a simple and easy-to-apply regression model to predict aerosol volume and mass concentrations. Since the model directly only accounts for changes in meteorological conditions, it can also be used to estimate the influence of emission changes on pollution levels. We apply the developed model to briefly investigate the effects of the COVID-19 lockdown on PM2.5 concentrations in Beijing. While no clear influence directly attributable to the lockdown measures is found, the results are in line with other studies utilizing more widely applied approaches.

Boreal forests are an important source of trace gases and atmospheric aerosols, as well as a crucial carbon sink. As such, they form a strongly interconnected coupled system with the atmosphere. The SMEAR II station is located in a boreal Scots pine forest in Hyytiala, Finland, and has over 25 years of continuous measurements of atmospheric and ecosystem variables. In this study, we analyse the seasonal variations of trace gases, atmospheric aerosols, greenhouse gases, and meteorological variables, measured at the SMEAR II sta-tion during the past two and a half decades. Several ecosystem and atmospheric variables show seasonal correlations with each other, which suggests seasonal interactions within the climate system that links together ecosystem processes, greenhouse gases, trace gases and atmospheric aerosols. For instance, increased global radiation in summer increases air temperature and consequently affects the plant phenology, which promotes the ecosystem carbon exchange and biogenic volatile organic compound (biogenic VOC) release. This further affects the ambient concentrations of highly oxygenated organic molecules (HOMs) as well as the formation and growth of atmospheric organic aerosols. Organic aerosols subsequently influence aerosol optical properties and, through increased scattering, have the potential to cool the climate. We also discuss the impacts of the warm and dry summers of 2010 and 2018 on the studied variables. For these years, we find a higher-than-average ecosystem primary production especially in June, leading to an increased VOC flux from the forest. The increased VOC flux in turn leads to higher HOM and secondary aerosol concentration in the atmosphere. The latter increases light scattering by atmospheric aero-sol particles and thus leads to climate cooling. The results obtained in this study improve our understanding of how boreal forests respond to climate change.

Ice-nucleating particles (INPs) trigger the formation of cloud ice crystals in the atmosphere. Therefore, they strongly influence cloud microphysical and optical properties and precipitation and the life cycle of clouds. Improving weather forecasting and climate projection requires an appropriate formulation of atmospheric INP concentrations. This remains challenging as the global INP distribution and variability depend on a variety of aerosol types and sources, and neither their short-term variability nor their long-term seasonal cycles are well covered by continuous measurements. Here, we provide the first year-long set of observations with a pronounced INP seasonal cycle in a boreal forest environment. Besides the observed seasonal cycle in INP concentrations with a minimum in wintertime and maxima in early and late summer, we also provide indications for a seasonal variation in the prevalent INP type. We show that the seasonal dependency of INP concentrations and prevalent INP types is most likely driven by the abundance of biogenic aerosol. As current parameterizations do not reproduce this variability, we suggest a new mechanistic description for boreal forest environments which considers the seasonal variation in INP concentrations. For this, we use the ambient air temperature measured close to the ground at 4.2 m height as a proxy for the season, which appears to affect the source strength of biogenic emissions and, thus, the INP abundance over the boreal forest. Furthermore, we provide new INP parameterizations based on the Ice Nucleation Active Surface Site (INAS) approach, which specifically describes the ice nucleation activity of boreal aerosols particles prevalent in different seasons. Our results characterize the boreal forest as an important but variable INP source and provide new perspectives to describe these new findings in atmospheric models.

The role of new particle formation (NPF) events and their contribution to haze formation through subsequent growth in polluted megacities is still controversial. To improve the understanding of the sources, meteorological conditions, and chemistry behind air pollution, we performed simultaneous measurements of aerosol composition and particle number size distributions at ground level and at 260 m in central Beijing, China, during a total of 4 months in 2015–2017. Our measurements show a pronounced decoupling of gas-to-particle conversion between the two heights, leading to different haze processes in terms of particle size distributions and chemical compositions. The development of haze was initiated by the growth of freshly formed particles at both heights, whereas the more severe haze at ground level was connected directly to local primary particles and gaseous precursors leading to higher particle growth rates. The particle growth creates a feedback loop, in which a further development of haze increases the atmospheric stability, which in turn strengthens the persisting apparent decoupling between the two heights and increases the severity of haze at ground level. Moreover, we complemented our field observations with model analyses, which suggest that the growth of NPF-originated particles accounted up to ∼60% of the accumulation mode particles in the Beijing–Tianjin–Hebei area during haze conditions. The results suggest that a reduction in anthropogenic gaseous precursors, suppressing particle growth, is a critical step for alleviating haze although the number concentration of freshly formed particles (3–40 nm) via NPF does not reduce after emission controls.

Abstract. Organic aerosol (OA) is a major component of fine particulate matter (PM) affecting air quality, human health, and the climate. The absorptive and reflective behavior of OA components contributes to determining particle optical properties and thus their effects on the radiative budget of the troposphere. There is limited knowledge on the influence of the molecular composition of OA on particle optical properties in the polluted urban environment. In this study, we characterized the molecular composition of oxygenated OA collected on filter samples in autumn of 2018 in Beijing, China, with a filter inlet for gases and aerosols coupled to a high-resolution time-of-flight chemical ionization mass spectrometer (FIGAERO-CIMS). Three haze episodes occurred during our sampling period with daily maximum concentrations of OA of 50, 30, and 55 µg m−3, respectively. We found that the signal intensities of dicarboxylic acids and sulfur-containing compounds increased during the two more intense haze episodes, while the relative contributions of wood-burning markers and other aromatic compounds were enhanced during the cleaner periods. We further assessed the optical properties of oxygenated OA components by combining the detailed chemical composition measurements with collocated particle light absorption measurements. We show that light-absorption enhancement (Eabs) of black carbon (BC) was mostly related to more oxygenated OA (e.g. dicarboxylic acids), likely formed in aqueous-phase reactions during the intense haze periods with higher relative humidity, and speculate that they might contribute to lensing effects. Aromatics and nitro-aromatics (e.g. nitrocatechol and its derivatives) were mostly related to a high light absorption coefficient (babs) consistent with light-absorbing (brown) carbon (BrC). Our results provide information on oxygenated OA components at the molecular level associated with BrC and BC particle light-absorption and can serve as a basis for further studies on the effects of anthropogenic OA on radiative forcing in the urban environment.

Despite the numerous studies investigating haze formation mechanism in China, it is still puzzling that intensive haze episodes could form within hours directly following relatively clean periods. Haze has been suggested to be initiated by the variation of meteorological parameters and then to be substantially enhanced by aerosol–radiation–boundary layer feedback. However, knowledge on the detailed chemical processes and the driving factors for extensive aerosol mass accumulation during the feedback is still scarce. Here, the dependency of the aerosol number size distribution, mass concentration and chemical composition on the daytime mixing layer height (MLH) in urban Beijing is investigated. The size distribution and chemical composition-resolved dry aerosol light extinction is also explored. The results indicate that the aerosol mass concentration and fraction of nitrate increased dramatically when the MLH decreased from high to low conditions, corresponding to relatively clean and polluted conditions, respectively. Particles having their dry diameters in the size of ∼400–700 nm, and especially particle-phase ammonium nitrate and liquid water, contributed greatly to visibility degradation during the winter haze periods. The dependency of aerosol composition on the MLH revealed that ammonium nitrate and aerosol water content increased the most during low MLH conditions, which may have further triggered enhanced formation of sulfate and organic aerosol via heterogeneous reactions. As a result, more sulfate, nitrate and water-soluble organics were formed, leading to an enhanced water uptake ability and increased light extinction by the aerosols. The results of this study contribute towards a more detailed understanding of the aerosol–chemistry–radiation–boundary layer feedback that is likely to be responsible for explosive aerosol mass growth events in urban Beijing.

Atmospheric new particle formation (NPF) events have attracted increasing attention for their contribution to the global aerosol number budget and therefore their effects on climate, air quality and human health. NPF events are regarded as a regional phenomenon, occurring over a large area. Most observations of NPF events in Beijing and its vicinity were conducted in populated areas, whereas observations of NPF events on mountaintops with low anthropogenic emissions are still rare in China. The spatial variation of NPF event intensity has not been investigated in detail by incorporating both urban areas and mountain measurements in Beijing. Here, we provide NPF event characteristics in summer 2018 and 2019 at urban Beijing and a comparison of NPF event characteristics – NPF event frequency, formation rate and growth rate – by comparing an urban Beijing site and a background mountain site separated by ∼80 km from 14 June to 14 July 2019, as well as giving insights into the connection between both locations. During parallel measurements at urban Beijing and mountain background areas, although the median condensation sink during the first 2 h of the common NPF events was around 0.01 s−1 at both sites, there were notable differences in formation rates between the two locations (median of 5.42 cm−3 s−1 at the urban site and 1.13 cm−3 s−1 at the mountain site during the first 2 h of common NPF events). In addition, the growth rates in the 7–15 nm range for common NPF events at the urban site (median of 7.6 nm h−1) were slightly higher than those at the mountain site (median of 6.5 nm h−1). To understand whether the observed events were connected, we compared air mass trajectories as well as meteorological conditions at both stations. Favorable conditions for the occurrence of regional NPF events were largely affected by air mass transport. Overall, our results demonstrate a clear inhomogeneity of regional NPF within a distance of ∼100 km, possibly due to the discretely distributed emission sources.

Organic aerosol (OA) is a major component of fine particulate matter (PM), affecting air quality, human health, and the climate. The absorptive and reflective behavior of OA components contributes to determining particle optical properties and thus their effects on the radiative budget of the troposphere. There is limited knowledge on the influence of the molecular composition of OA on particle optical properties in the polluted urban environment. In this study, we characterized the molecular composition of oxygenated OA collected on filter samples in the autumn of 2018 in Beijing, China, with a filter inlet for gases and aerosols coupled to a high-resolution time-of-flight chemical ionization mass spectrometer (FIGAERO–CIMS). Three haze episodes occurred during our sampling period with daily maximum concentrations of OA of 50, 30, and 55 μg m−3. We found that the signal intensities of dicarboxylic acids and sulfur-containing compounds increased during the two more intense haze episodes, while the relative contributions of wood-burning markers and other aromatic compounds were enhanced during the cleaner periods. We further assessed the optical properties of oxygenated OA components by combining detailed chemical composition measurements with collocated particle light absorption measurements. We show that light absorption enhancement (Eabs) of black carbon (BC) was mostly related to Pl ea se no te th e re m ar ks at th e en d of th e m an us cr ip t. Published by Copernicus Publications on behalf of the European Geosciences Union. 2 J. Cai et al.: Organic aerosol composition and their absorptive properties by offline FIGAERO–CIMS more oxygenated OA (e.g., dicarboxylic acids), likely formed in aqueous-phase reactions during the intense haze periods with higher relative humidity, and speculate that they might contribute to lensing effects. Aromatics and nitro-aromatics (e.g., nitrocatechol and its derivatives) were mostly related to a high light absorption coefficient (babs) consistent with light-absorbing (brown) carbon (BrC). Our results provide information on oxygenated OA components at the molecular level associated with BrC and BC particle light absorption and can serve as a basis for further studies on the effects of anthropogenic OA on radiative forcing in the urban environment.