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Hourly particulate matter 2.5 organic carbon (OC) and elemental carbon (EC) ambient concentrations were measured from February 2012 to January 2013 at a suburban site in the city of Guangzhou, China. The major primary OC and EC sources at this site are vehicular exhaust and regional combustion sources. The annual average OC and EC concentrations (±1SD) were 7.30 ± 5.68 and 2.67 ± 2.28 μgC/m3, respectively. Taking advantage of this large data set of more than 7,000 measurements, we examined the diurnal variation dynamics of the primary OC‐to‐EC ratio ([OC/EC]pri), derived using the minimum R squared (MRS) method, which deploys an algorithm that calculates (OC/EC)pri by exploring the assumed independency between EC and secondary OC (SOC). The MRS‐derived hour‐by‐hour (OC/EC)pri was the lowest (1.8–2.0) for the morning hours while higher (2.4–3.0) for the afternoon and evening hours. Cumulative evidence from other pollutant measurements suggests that the increase in MRS‐derived (OC/EC)pri in the afternoon and evening was a result of SOC formation from photooxidation of precursors that covary with EC (e.g., vehicular exhaust) and thereby inflate (OC/EC)pri. We subsequently separate SOC into two categories based on their correlations with EC, that is, SOCa that covary with EC and SOCb that is unassociated with EC variations. SOCa and SOCb make comparable contributions to SOC, demonstrating that their separation is essential for a more accurate SOC estimation. Otherwise, SOCa would be mistakenly accounted as primary OC. This approach in estimating SOC may underestimate SOCa and subject to bias stemming from OC emissions from cooking and biomass burning. Key Points: Increase in hour‐by‐hour primary OC‐to‐EC ratio in the afternoon and evening was linked to SOC formed from precursors that covary with ECSOC estimation includes two categories: SOCa that covary with EC and SOCb that is unassociated with EC variationsSOCa is likely linked with traffic‐derived precursors and makes substantial contributions in this area of significant urban influence [ABSTRACT FROM AUTHOR]

The modulation of the El Niño/Southern Oscillation (ENSO) on winter aerosol pollution (WAP) in the eastern region of China (ERC) was investigated from a climatological perspective. The results show that the weakened East Asian winter monsoon during El Niño events has an important influence on increased WAP in ERC, especially over northern China. The enhancement of the East Asian winter monsoon related to La Niña events reduces WAP in northeast, east, and south China but increases it in central and western China. Compared to northern China, anomalies in dynamic and thermodynamic conditions related to ENSO lead to more uncertainties in its effect on aerosol pollution over southern China. El Niño (La Niña) events tend to cause southwesterly (northeasterly) wind anomalies in winter southern China, along with the blocking effect of Nanling Mountains on southward (northward) air pollution, which is not (is) beneficial for the transport outside or dilution in local aerosols over southern China. Meanwhile, positive (negative) water vapor convergence and rainfall anomalies caused by El Niño (La Niña) are (are not) beneficial for the local removal of aerosols. The dominance of both factors determines the ENSO modulation on aerosols over southern China. An atmospheric global climate model largely reproduces the observed findings. Consistent with the observation, El Niño events exacerbate WAP in most of ERC, whereas La Niña ones reduce it except for western China. Plain Language Summary: As a source of large‐scale ocean‐atmosphere interactions originating from the tropical Pacific, ENSO not only leads to anomalies in atmospheric circulation locally but also arouses global climatological anomalies. In this study, from a climatological perspective, the modulation of ENSO on winter aerosol pollution over Eastern China was investigated by diagnosis analysis and CAM5 simulation. Key Points: ENSO tends to cause anomalies of aerosol pollution and distribution by modulating East Asian winter monsoonEl Niño and La Niña events have the roughly opposite impact on winter aerosols in ERC, especially in northern ChinaAnomalies in dynamic and thermodynamic conditions associated with ENSO lead to uncertainties in its effect on aerosols over southern China [ABSTRACT FROM AUTHOR]