Estimation and Uncertainty Analysis of Secondary Organic Carbon Using 1 Year of Hourly Organic and Elemental Carbon Data

Abstract

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.

Amplification of black carbon light absorption induced by atmospheric aging: temporal variation at seasonal and diel scales in urban Guangzhou

“…Average EC concentrations (with 1 standard deviation, hereafter) were 1.94±0.93 and 2.81±2.01 μgC m -3 in wet and dry seasons, respectively. The EC level was comparable to the measurements made in 2012 at a Guangzhou suburban site (1.67±1.35 μgC m -3 in wet season, 3.47±2.75 μgC m -3 in dry season)(Wu et al, 2019). Back trajectories analysis showed that in wet season most air masses were from South China Sea, with only 31.67% Author(s) 2019.…”

“…A filed carbon analyzer (Model RT-4, Sunset Laboratory Inc, Tigard, Oregon, USA) was used for OC and EC determination. The detailed sampling procedures can be found in our previous study (Wu et al, 2019), and only a brief description is given here. The sample was collected in the first 45 minutes of each hour at a flow 210 rate of 8 L min -1 .…”

“…Although the quantitative contribution estimation of sulfate and SOA to Eabs is not possible in this study, a rough estimation can be projected. Considering the typical annual average SOC concentration (3 μgC m -3 ) (Wu et al, 2019), typical SOA/SOC mass ratio (1.8) (Li et al, 2017), and sulfate concentration (8 μg m -3 ) (Liu et al, 2017b) in the PRD region, SOA and sulfate would likely have comparable contributions to 570 the Eabs, according to the Eabs dependency on sulfate-to-SOA mass ratio results by Zhang et al (2018a).…”

“…In the present study, due to the lack of quantitative chemical speciation data, quantification of contributions from different chemical species on Eabs is not possible. A recent study in Guangzhou (Wu et al, 2019) found that traffic-derived SOC could be a significant source of SOC in the urban area, which can account for half of the total SOC. In that sense, traffic emissions are 550 expected to have a considerable contribution to BC light absorption enhancement in both wet and dry seasons.…”

Amplification of black carbon light absorption induced by atmospheric aging: temporal variation at seasonal and diel scales in urban Guangzhou

“…These pieces of evidence imply the potential role of semivolatile coatings on BC in regulating the diurnal dynamics of E abs . In China, the sulfate problem has been effectively mitigated by the reduction measures implemented in recent years (Xia et al, 2016;Wang et al, 2017b). In contrast, nitrate increased substantially in recent years (Xu et al, 2019;Wang et al, 2019).…”