The effects of accumulated refractory particles and the peak inert mode temperature on semi-continuous organic carbon and elemental carbon measurements during the CAREBeijing 2006 campaign

Abstract

Two semi-continuous Sunset carbon analyzers, with different peak inert mode temperatures (615 and 740℃), were simultaneously operated to measure fine particulate organic carbon (OC) and elemental carbon (EC) using a thermal optical transmittance method at an urban site in Beijing, China, from 16 August to 3 September 2006 during the CAREBeijing 2006 campaign. Excellent agreements were obtained between the collocated semi-continuous carbon analyzers, with slopes of 1.02 (R2 = 0.91) for OC and 1.06 (R2 = 0.93) for EC, resulting in very similar average EC/total carbon (TC) ratios of ∼0.36. These results imply that the different peak inert mode (100% helium) temperatures did not cause significant biases on the semi-continuous OC and EC measurements. However, it was found that the EC/TC ratio was greatly influenced by the accumulated refractory particles remaining on a quartz filter (PM refractory). Fresh quartz filters, with low PM refractory loadings, which is defined as a laser correction factor ≥0.94, gave ∼8-10% lower EC/TC ratios than aged quartz filters with high PM refractory loadings. The linear regression slope between EC and optically measured EC (OPT-EC) was much higher with fresh quartz filters (slope = 1.03, R2 = 0.96) than aged quartz filters (slope = 0.89, R2 = 0.95), suggesting the underestimation of EC on fresh quartz filters by ∼15% compared to those measured on aged quartz filters. Authentic standard humic-like substances (HULIS) on the clean quartz filter showed the highest extent of pyrolyzed organic carbon (POC) formation (47.4% in total detected carbon mass), followed by those on the Asian dust loaded quartz filter (37%) and the refractory urban pollutant loaded quartz filter (34.1%), indicating that the Asian dust and refractory urban pollutant reduced the POC formation from the HULIS. Thus, this study suggested that the PM refractory loading plays an important role in the semi-continuous OC and EC measurements by altering the degree of POC formation in the inert atmosphere

Sources of PM<sub>2.5</sub> carbonaceous aerosol in Riyadh, Saudi Arabia

“…This phenomenon has been pointed out previously, and versions RT-Calc 114 and newer of the Sunset instrument analysis software introduced a laser correction factor to counteract the influence of temperature on the laser signal. This correction factor is calculated in each cycle from the variation in the laser signal when the analysis enters the methane calculation stage (Jung et al, 2011). However, it was obvious that this correction approach did not work well for the Riyadh samples, since many returned EC / TC = 0 (where TC is total carbon), the case when the initial reflectance is not recovered in the analysis.…”

“…However, unusual EC and OC splits for a large number of samples were observed during the study period: (a) split points jumped to the end of the analysis because the laser response did not rebound to its initial value before the CH 4 calibration phase; or (b) split points were located in the pre-oxygen position. These split-point deviations were ascribed to refractory residue on the filters: the laser correction factor supplied in the standard manufacturer software may not be applicable to the dusty environment of Riyadh (Polidori et al, 2006;Jung et al, 2011;Wang et al, 2012). Therefore, observed relationships between laser response and temperature in the CH 4 + O 2 injection calibration phase were used to develop a corrected split point.…”

Detailed comparison of OC/EC aerosol at an urban and a rural Czech background site during summer and winter

“…The OC1 fraction is elevated at the urban site but all other fractions have similar concentrations. The difference in PC may be an artefact caused by, for example, higher mineral dust concentrations at the urban site, causing the oxidation of part of PC in the He phase (Jung et al, 2011).…”

Sources of PM<sub>2.5</sub> carbonaceous aerosol in Riyadh, Saudi Arabia

“…This phenomenon has been pointed out previously, and Versions RT-Calc 114 and newer of the Sunset instrument analysis software introduced 476 a laser correction factor to counteract the influence of temperature on the laser signal. This correction factor is calculated in each cycle from the variation in the laser signal when 478 the analysis enters the methane calculation stage (Jung et al, 2011). However, it was obvious that this correction approach did not work well for the Riyadh samples, since 480 many returned EC/TC=0, the case when the initial reflectance is not recovered in the analysis.…”

“…However, unusual EC and OC splits for a large 134 number of samples were observed during the study period: (a) split points jumped to the end of the analysis because the laser response did not rebound to its initial value before 136 the CH4 calibration phase; or (b) split-points were located in the pre-oxygen position. These split point deviations may be ascribed to refractory residue on the filters: the laser 138 correction factor supplied in the standard manufacturer software may not be applicable to the dusty environment of Riyadh (Polidori et al, 2006;Jung et al, 2011;Wang et al, 140 2012). Therefore, observed relationships between laser response and temperature in the CH4 + O2 injection calibration phase were used to develop a corrected split point.…”