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Abstract. During the COVID-19 lockdown, the dramatic reduction of anthropogenicemissions provided a unique opportunity to investigate the effects ofreduced anthropogenic activity and primary emissions on atmospheric chemicalprocesses and the consequent formation of secondary pollutants. Here, weutilize comprehensive observations to examine the response of atmosphericnew particle formation (NPF) to the changes in the atmospheric chemicalcocktail. We find that the main clustering process was unaffected by thedrastically reduced traffic emissions, and the formation rate of 1.5 nmparticles remained unaltered. However, particle survival probability wasenhanced due to an increased particle growth rate (GR) during the lockdownperiod, explaining the enhanced NPF activity in earlier studies. For GR at1.5–3 nm, sulfuric acid (SA) was the main contributor at high temperatures,whilst there were unaccounted contributing vapors at low temperatures. ForGR at 3–7 and 7–15 nm, oxygenated organic molecules (OOMs) played amajor role. Surprisingly, OOM composition and volatility were insensitive tothe large change of atmospheric NOx concentration; instead theassociated high particle growth rates and high OOM concentration during thelockdown period were mostly caused by the enhanced atmospheric oxidativecapacity. Overall, our findings suggest a limited role of traffic emissionsin NPF. 

Abstract. Air quality models have not been able to reproduce the magnitude of theobserved concentrations of fine particulate matter (PM2.5) duringwintertime Chinese haze events. The discrepancy has been at least partlyattributed to low biases in modeled sulfate production rates, due to the lackof heterogeneous sulfate production on aerosolsin the models. In this study, we explicitly implement four heterogeneous sulfate formationmechanisms into a regional chemical transport model, in addition togas-phase and in-cloud sulfate production. We compare the model results withobservations of sulfate concentrations and oxygen isotopes, Δ17O(SO42-), in the winter of 2014–2015, the latter of whichis highly sensitive to the relative importance of different sulfateproduction mechanisms. Model results suggest that heterogeneous sulfateproduction on aerosols accounts for about 20 % of sulfate production inclean and polluted conditions, partially reducing the modeled low bias insulfate concentrations. Model sensitivity studies in comparison with theΔ17O(SO42-) observations suggest that heterogeneoussulfate formation is dominated by transition metal ion-catalyzed oxidation of SO2. 

Abstract. pH is an important property of aerosol particles but is difficult to measure directly. Several studies have estimated the pH values for fine particles in northern China winter haze using thermodynamic models (i.e., E-AIM and ISORROPIA) and ambient measurements. The reported pH values differ widely, ranging from close to 0 (highly acidic) to as high as 7 (neutral). In order to understand the reason for this discrepancy, we calculated pH values using these models with different assumptions with regard to model inputs and particle phase states. We find that the large discrepancy is due primarily to differences in the model assumptions adopted in previous studies. Calculations using only aerosol-phase composition as inputs (i.e., reverse mode) are sensitive to the measurement errors of ionic species, and inferred pH values exhibit a bimodal distribution, with peaks between −2 and 2 and between 7 and 10, depending on whether anions or cations are in excess. Calculations using total (gas plus aerosol phase) measurements as inputs (i.e., forward mode) are affected much less by these measurement errors. In future studies, the reverse mode should be avoided whereas the forward mode should be used. Forward-mode calculations in this and previous studies collectively indicate a moderately acidic condition (pH from about 4 to about 5) for fine particles in northern China winter haze, indicating further that ammonia plays an important role in determining this property. The assumed particle phase state, either stable (solid plus liquid) or metastable (only liquid), does not significantly impact pH predictions. The unrealistic pH values of about 7 in a few previous studies (using the standard ISORROPIA model and stable state assumption) resulted from coding errors in the model, which have been identified and fixed in this study.

 

Abstract. Isoprene-derived secondary organic aerosol (iSOA) is a significantcontributor to organic carbon (OC) in some forested regions, such astropical rainforests and the Southeastern US. However, its contribution toorganic aerosol in urban areas that have high levels of anthropogenicpollutants is poorly understood. In this study, we examined the formation ofanthropogenically influenced iSOA during summer in Beijing, China. Localisoprene emissions and high levels of anthropogenic pollutants, inparticular NOx and particulate SO42-, led to the formation ofiSOA under both high- and low-NO oxidation conditions, with significantheterogeneous transformations of isoprene-derived oxidation products toparticulate organosulfates (OSs) and nitrooxy-organosulfates (NOSs).Ultra-high-performance liquid chromatography coupled to high-resolution massspectrometry was combined with a rapid automated data processing techniqueto quantify 31 proposed iSOA tracers in offline PM2.5 filterextracts. The co-elution of the inorganic ions in the extracts caused matrixeffects that impacted two authentic standards differently. The averageconcentration of iSOA OSs and NOSs was 82.5 ng m−3, which was around 3 timeshigher than the observed concentrations of their oxygenated precursors(2-methyltetrols and 2-methylglyceric acid). OS formation was dependant onboth photochemistry and the sulfate available for reactive uptake, as shown by astrong correlation with the product of ozone (O3) and particulatesulfate (SO42-). A greater proportion of high-NO OS products wereobserved in Beijing compared with previous studies in less pollutedenvironments. The iSOA-derived OSs and NOSs represented 0.62 %of the oxidized organic aerosol measured by aerosol mass spectrometry on average, butthis increased to ∼3 % on certain days. These resultsindicate for the first time that iSOA formation in urban Beijing is stronglycontrolled by anthropogenic emissions and results in extensive conversion toOS products from heterogenous reactions. 

Aerosols have significant and complex impacts on regional climate in East Asia. Cloud‐aerosol‐precipitation interactions (CAPI) remain most challenging in climate studies. The quantitative understanding of CAPI requires good knowledge of aerosols, ranging from their formation, composition, transport, and their radiative, hygroscopic, and microphysical properties. A comprehensive review is presented here centered on the CAPI based chiefly, but not limited to, publications in the special section named EAST‐AIRcpc concerning (1) observations of aerosol loading and properties, (2) relationships between aerosols and meteorological variables affecting CAPI, (3) mechanisms behind CAPI, and (4) quantification of CAPI and their impact on climate. Heavy aerosol loading in East Asia has significant radiative effects by reducing surface radiation, increasing the air temperature, and lowering the boundary layer height. A key factor is aerosol absorption, which is particularly strong in central China. This absorption can have a wide range of impacts such as creating an imbalance of aerosol radiative forcing at the top and bottom of the atmosphere, leading to inconsistent retrievals of cloud variables from space‐borne and ground‐based instruments. Aerosol radiative forcing can delay or suppress the initiation and development of convective clouds whose microphysics can be further altered by the microphysical effect of aerosols. For the same cloud thickness, the likelihood of precipitation is influenced by aerosols: suppressing light rain and enhancing heavy rain, delaying but intensifying thunderstorms, and reducing the onset of isolated showers in most parts of China. Rainfall has become more inhomogeneous and more extreme in the heavily polluted urban regions.