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The daily average ambient PM1 mass concentration was 51.2 ± 48.0 µg m−3 (mean ± standard deviation), ranging from 1.5 to 466 µg m−3, shown in Fig. 2a. The biggest contributor of PM1 mass loading during summer 2013 was organic (35.4%), followed by sulfate (31.3%), ammonium (12.8%) and nitrate (11.4%) (Fig. 2b). Figure 2c indicates the variation in relative contributions of different species as a function of the total PM1 mass loading. We can see that organics show a continuously increasing fraction when PM1 was accumulating, implying OA played a more important role in extremely polluted periods. Therefore, we discuss the source apportionment of OA further below.
Figure 2. (a) Time series of PM1 species. (b) Pie chart showing the average chemical compositions. (c) Evolutions of PM1 compositions (left-hand axis) as a function of PM1 mass concentration, and the probability distributions of PM1 mass concentration (white line to the right-hand axis).
PMF analysis for OA resolved four OA factors: HOA, BBOA, LO-OOA and MO-OOA. The MS profiles and the mass contributions of each factor are shown in Figs. 3a and b. The mass fractions of HOA, BBOA, LO-OOA and MO-OOA were 18.4%, 29.4%, 21.4% and 30.8%, respectively. Note that biomass burning is a large contributor to OA due to the summer wheat harvest in the NCP region. BBOA was found to contribute 36%–39% to OA during harvest seasons in China (Zhang et al., 2015). The characteristics of the mass profile for each factor are distinct. HOA had an MS profile characterized by a large fraction of
${\rm{C}}_x{\rm{H}}_y^+ $ fragments and the lowest O:C ratio of 0.11, as well as the best correlation with BC emitted from traffic (BC_tr) (shown in Fig. S4 in the ESM), indicating its primary nature and high relevance to traffic emissions (Aiken et al., 2009; Huang et al., 2012, 2013; Mohr et al., 2012). The BBOA MS profile contained the highest f60 (ratio of the integrated signal at m/z 60 to the total organics signal) value of 0.9% compared to other factors (Schneider et al., 2006; Alfarra et al., 2007; Cubison et al., 2011). BBOA correlated best with acetonitrile (Fig. S4), which is a VOC tracer for biomass burning (Holzinger et al., 1999). However, its O:C ratio reaches up to 0.62. Therefore, BBOA in this case can be regarded as a factor that undergoes chemical aging from fresh emissions, considering the distance between the sampling site and biomass burning fields. Several studies have reported that fresh BBOA could be oxidized rapidly and converted to OOA in less than one day (Bougiatioti et al., 2014; Zhou et al., 2017). The MS profiles of two subtype OOAs were characterized by${\rm{C}}_x{\rm{H}}_y{\rm{O}}_z^+ $ ions, with different fractions of${\rm{CO}}_2^+ $ , and have better correlations with sulfate and nitrate (Fig. S4). The diurnal variations of the four OA factors are shown in Fig. 3c. HOA had two pronounced “rush hour” increases in the morning (starting from 0700 LST) and in the evening (starting from 1800 LST). The diurnal trend of BBOA had several peaks, which may suggest biomass burning events occurred frequently. LO-OOA and MO-OOA had a similar variation pattern, with an increase starting from 0700 LST and a decrease at 1800 LST, mainly driven by photochemical oxidation accumulation. -
We first applied the
${\rm{NO}}_x^ + $ ratio method to estimate the concentration of nitrate functionality of ON (i.e., NO3,org). The concentration of NO3,org and the mass fraction of NO3,org in total measured NO3 (total NO3) are shown in Table 1. NO3,org accounted for 7.8% to 12% in NO3 by using the upper and lower bound values of RON/$R_{\rm{NH_4NO_3}}$ in the${\rm{NO}}_x^ + $ ratio method. Then, we used the PMF method to estimate NO3,org to verify the reliability of the results. The MS of the NIA factor is dominated by${\rm{NO}}_x^ + $ , and the mass fractions of${\rm{NO}}_x^ + $ > in HOA, BBOA, OOA were 11.8%, 85% and 3.2%, respectively (Fig S2). Figure 4a shows the time series of the NO3, org concentration calculated by the two methods and the computed correlation coefficient between them is good (r = 0.76), and the slope of the fitting line is 1.2, indicating that similar results were achieved. We further calculated that ON contributed 8.1%–19% to total OA assuming an average molecular weight of ON of 200–300 g mol−1 (Rollins et al., 2012), which is comparable to the fraction of ON in OA in Shenzhen during summertime (11%–25%) reported in Yu et al. (2019), indicating ON was the significant component of OA in the rural NCP atmosphere in summer.Values (a) NO+/NO2+ ratio method Lower Upper NO3,org (µg m−3)a 0.46 0.70 NO3,org/total NO3 0.078 0.12 (b) PMF method NO3,org (µg m−3)b 0.54 NO3,org/total NO3 0.093 Notes: aNO3, org for upper bound is denoted as NO3_org_ratio_1, and NO3, org for lower bound is denoted as NO3_org_ratio_2; bNO3, org estimated using the PMF method is denoted as NO3_org_PMF. Table 1. Summary of ON estimations using the NO+/NO2+ ratio method and the PMF method.
Figure 4. (a) Time series of NO3, org concentration estimated by the NO+/NO2+ ratio method and PMF method for the study period. (b) Correlations between NO3_org_ratio_1 and NO3_org_PMF. (c) Diurnal trends of NO3_org_ratio_1, BC_bb (left-hand axis), inorganic nitrates (NO3_inorg) and BBOA (right-hand axis).
Figure 5a shows scatterplots of NO3_org_ratio_1 versus OA factors resolved by PMF analysis, BC from biomass burning (BC_bb), and BC from traffic emissions (BC_tr). We find that NO3_org_ratio_1 had good correlations with BBOA (r = 0.71) and BC_bb (r = 0.67), but a poor correlation with LO-OOA (r = 0.20), which is quite different from the results in other regions that show the highest ON correlation with LO-OOA (Xu et al., 2015a, b; Yu et al., 2019). Particulate ON formation is found to be through photooxidation of biogenic VOCs in the presence of NOx in the daytime (Teng et al., 2015, 2017) and NO3 radicals oxidation of biogenic VOCs at night (Fry et al., 2013; Ayres et al., 2015; Boyd et al., 2015; Xu et al., 2015b; Lee et al., 2016; Yu et al., 2019). However, recent studies show that NO3 radicals reacting with typical VOCs in biomass burning plumes could also produce a substantial fraction of particulate ON (Ahern et al., 2019; Joo et al., 2019). The good correlation between ON and biomass burning aerosols in this study indicate the possible existence of different formation mechanisms of ON relevant to biomass burning plumes in the real atmosphere. Here, we further compared the diurnal variation of NO3_org_ratio, inorganic nitrates (NO3_inorg), BBOA and BC_bb in Fig. 4c. First, the result shows that NO3_org_ratio had a quite different diurnal variation from NO3_inorg, implying that ON has been well separated from inorganic nitrates in this study. Furthermore, NO3_org_ratio increased by nearly two times from 1700 LST to 2200 LST and maintained a relative high mass loading level during the nighttime. We note that there were two similar peaks at 2100–2200 LST and 0300–0400 LST in the NO3_org_ratio, BBOA and BC_bb variation trends. A number of studies have proposed that nighttime biomass burning contributes to OA compositions in field campaigns, in particular with some specific ON species formation (Allan et al., 2010; Iinuma et al., 2010, 2016; Mohr et al., 2013). However, these studies did not describe the influence of biomass burning on overall ON. In order to better understand ON in this case, we will attempt to characterize it from the property of volatility in the following section.
Figure 5. (a) Correlations of NO3, org1_ratio with OA factors resolved by PMF, BC from biomass burning (BC_bb), and BC from traffic emissions (BC_tr). (b) Time series of NO3, org concentration estimated by the NO+/
${\rm{NO}}_2^+ $ ratio method (NO3_org), BC_bb (left axis) and BBOA (right-hand axis). -
The mass fraction remaining (MFR) of different OA factors resolved by PMF analysis, ON and inorganic nitrates are shown in Fig. 6. MFRs varied differently among different OA factors. The MFR of HOA was 0.58 at 50°C and decreased by 1.70 % °C−1; then the evaporation rate slowed down from 50°C to 200°C with a nearly constant rate of 0.35% °C−1, and only 4.9% was left at 200°C. BBOA had a similar MFR variation to HOA, with a fast decrease from ambient temperature to 50°C (evaporation rate was 2% °C−1) and a slower decrease from 50°C to 200°C (0.5% °C−1 for 50°C–150°C and 0.006% °C−1 for 150°C–200°C), but much wider standard deviation (SD) areas at temperature stages, suggesting BBOA contained more compounds that have different evaporation, which agrees with the BBOA factor in this case consisting of fresh and aged ones. The MFR of LO-OOA at 50°C was 0.72, lower than that of MO-OOA (0.80) and with increased temperature, and the MFR of MO-OOA decreased much slower than LO-OOA and other OA factors, both implying that MO-OOA was less volatile compared to other OA factors. The volatility sequence of OA factors in this study was HOA > BBOA > LO-OOA > MO-OOA, determined by the MFR at 50°C (Cao et al., 2018, 2019; Xu et al., 2019).
Figure 6. Variation of the average MFR of OA factors (a–d) resolved by PMF, ON (NO3_org) (e), and inorganic nitrates (f) with the TD temperature. The shaded regions indicate the average ± SD.
In order to investigate the volatility of ON, we estimated the mass concentration of NO3,org at different temperatures using the
${\rm{NO}}_x^+ $ ratio and PMF method as well. Table S1 lists the estimated results and comparison coefficients between the two methods. The good correlation coefficients (R = 0.68–0.75) between the two methods validate the ON estimated results. Based on that, the MFR of ON were further calculated, as shown in Fig. 6f. The MFR of ON was 0.54 at 50°C, 0.37 at 100°C, 0.24 at 150°C, and 0.22 at 200°C. Compared to OA factors, ON evaporate faster from ambient temperature to 50°C (1.86% °C−1), implying a volatile feature for primary emissions. However, ON show a much slower decrease than all OA factors except MO-OOA from 50°C–150°C (0.29% °C−1) and remained flat from 150°C−200°C, suggesting about 20% of the fraction of ON were very difficult to volatilize. In addition, we calculated the MFR for inorganic nitrates, which evaporate much faster than ON and only 4.1% of inorganic nitrates were left at 200°C, confirming the semi-volatility property of inorganic nitrates. Our result provides direct evidence that a considerable remaining fraction of nitrate measured by AMS at high temperature is due to the influence of ON contained within (Cao et al., 2018, 2019; Xu et al., 2019). It should be noted that a laboratory study reported that pure ammonium nitrates can completely evaporate at 50°C (Huffman et al., 2009b), but 35.3% of inorganic nitrates were left at 50°C in our study. The discrepancy may be due to the significant influence of the composition differences and mixing state of ambient aerosols, which has been proposed in previous studies (Huffman et al., 2009b; Nie et al., 2017), and the inaccuracy of the estimation of ON. Therefore, more related studies need to be conducted in the future.