Statistical results of carbonaceous components, i.e., OC and EC, and the ratios of SOC to OC in PM2.5 collected at Lin’an site during summer 2015 are listed in Table 1, and their temporal variation patterns are shown in Fig. 2. In this study, the average ratio of OC/EC was 4.75, varying from 1.80 to 12.0, of which most values were between 2.0 and 6.0 (84%) (Liang et al., 2017). If the OC/EC ratio exceeds 2.0, the presence of SOA is implied (Chow et al., 1996); and this indicates an obvious contribution of SOA to the ambient aerosol at the background site. In addition, secondary organic carbon (SOC) was estimated by the EC-tracer method (Lim and Turpin, 2002). Briefly, SOC was estimated by the following formulae:
Average STD Min Max OC (µgC m−3) 14.30 3.95 8.02 32.80 EC (µgC m−3) 3.33 1.47 0.77 9.97 TC (µgC m−3) 17.60 4.90 10.00 36.00 OC/EC 4.75 1.64 1.80 12.00 POC (µgC m−3) 6.00 2.64 1.39 18.00 SOC (µgC m−3) 8.26 3.31 0.03 27.00 SOC/OC (%) 58.50 12.60 20.80 85.00 O3 (ppb) 35.20 14.30 10.10 81.70 CO (ppm) 0.52 0.11 0.32 0.82 Temperature (°C) 25.40 3.10 17.10 31.80 Relative humidity (%) 83.00 10.00 57.00 99.00 Average wind speed (m s−1) 2.10 0.90 1.10 7.00 Visibility (km) 10.30 7.77 1.37 30.60 Total solar radiation (W m−2) 289.00 150.00 37.90 56.40 Total cloud cover (%) 77.90 30.40 0.00 100.00 Precipitation (mm) 8.52 16.70 0.00 87.80
Table 1. Average concentrations and range of carbonaceous components in PM2.5, ratios of OC/EC and SOC/OC, gas concentrations of O3 and CO, and values of meteorological parameters during summer 2015 at Lin’an site
Figure 2. Temporal variations of carbonaceous components (POC, SOC, and EC) and the ratios of SOC to OC in PM2.5 collected at Lin’an site during summer 2015
Primary OC (POC) = EC × (OC/EC)min,
SOC = OC − POC,
where (OC/EC)min indicates the minimum ratio of OC/EC. The mean concentrations of POC and SOC in PM2.5 at Lin’an during the sampling time were 6.00 ± 2.64 and 8.26 ± 3.31 µgC m–3, respectively (Table 1). The ratios of SOC to OC varied from 20.8% to 85.0% with an average of 58.5 ± 12.6%. The frequency distribution of the SOC/OC ratios is shown in Fig. 3. The daily SOC/OC ratios had a primarily skewed distribution, and more than 60% of SOC/OC ratios were distributed in the range of 50%−70%. The SOC/OC ratios in our study were similar to those reported at different sites around the world (Zhang et al., 2007; Ge et al., 2017; Chen et al., 2018; Wu et al., 2018), illustrating that secondary organic aerosol is ubiquitous and presents a dominance of oxygenated species in organic aerosols in the atmosphere.
Figure 3. The mass frequency distribution for the ratios of SOC/OC in PM2.5 collected at Lin’an site during summer 2015
During the sampling time, almost all RH values were higher than 60%, except on one day, i.e., 4 August 2016 (RH = 57%), which was excluded in the following discussion. The average RH value was 84%. Based on meteorological data analysis, increases in clouds and precipitation induced higher RH and decreased total solar radiation (Fig. 4). Statistical analysis of RH and total solar radiation showed an obvious negative correlation (r = 0.89, p < 0.01, Fig. 5). In the case of low solar radiation and high RH, the surface temperature fluctuation was inhibited, causing the boundary layer height to decrease; as a consequence, aerosols accumulated, enhancing light scattering and thus decreasing visibility (Fig. 5). In the meantime, photochemical activity was suppressed under low solar radiation, and the formation of secondary aerosol from this pathway became less significant.
Figure 4. Correlations between RH and precipitation and total cloud cover during summer 2015 at Lin’an site. The k indicates regression slope
In order to explore the impact of higher RH on SOA concentration, the relationships of POC and SOC with RH are shown in Fig. 6. The concentration of POC showed no relationship with RH (p > 0.05, Fig. 6a). On the other hand, SOC exhibited an obvious negative relationship when RH was higher than 60% (r = 0.56, p < 0.01, Fig. 6b). This indicates that chemical processing of SOC appeared to be inhibited under high RH (> 60%) at Lin’an. This observation was similar to that reported by Wu et al. (2018), i.e., the two less oxygenated SOA factors (LSOA and SVOOA) responded negatively to RH. These authors concluded that the two less oxygenated SOA products were mainly driven by photochemical process.
Figure 6. Relationships between (a) POC and RH, and (b) SOC and RH. Also shown is color-coded temperature.
In addition, high SOC values were observed at high temperatures (red color, Fig. 6), demonstrating that higher temperatures can elevate SOA concentrations in the ambient environment, especially on days of more than 25°C. High temperatures in summer are typically associated with stronger solar radiation, and the concentrations of photochemical oxidants, i.e., O3 and OH, will increase and the photolysis rates will be larger accordingly. Since SOC is, to a large extent, a product of photochemical reactions, enhanced SOC production rates are expected on days with high temperatures and intense photochemistry. Furthermore, numerous studies have reported that high temperatures can increase the emission of biogenic SOA precursors, e.g., isoprene and monoterpenes (Monson et al., 1994; Dominguez-Taylor et al., 2007). Thus, under high temperatures, SOA formation rates will be higher due to increased biogenic precursor emission and enhanced photolysis rates in the ambient environment.
The variations of average concentrations of POC, SOC, EC, OC, CO, and aerosol pH as a function of RH throughout the study are shown in Fig. 7. SOC exhibited a decreasing trend when RH was higher than 60%. SOC showed the largest mass decreasing rate when RH increased from 60% to 80% (−2.98 μgC m−3/10% RH), whereas the decreasing rate of those species was reduced to −0.46 μgC m−3/10% RH at RH between 70% and 90% (Fig. 7, Table 2). Our explanation for this phenomenon is that the rate of SOA production via the photochemical oxidation reaction pathway was strongly decreased at RH between 60% and 80%; if the rate of SOA production via the heterogeneous reactions was not increased enough, the net value of SOA production rate was significantly decreased. However, with the increasing RH, heterogeneous reactions under the higher RH condition enhanced SOA yields, and compensated for the influence from the photochemical oxidation pathway at RH between 70% and 90%. Thus, the net decreasing rate at RH between 70% and 90% was lower than that at RH between 60% and 80%. It should be noted that POC and EC were expected to be non-hygroscopic, while they were observed still slightly elevated as RH increased from 60% to 80%, probably because of their continuous yet weak accumulation. Moreover, likely due to wet deposition, all carbonaceous components appeared to have similar decreasing rates when RH increased from the range between 80% and 90% to the range higher than 90%. Because of both concentrations of POC and SOC decreasing at the range of RH higher than 90%, OC concentrations exhibited a significant decreasing rate of −3.63 μgC m−3/10% RH (Fig. 7, Table 2). Compared to primary aerosol components, i.e., POC and EC, the primary gaseous species CO exhibited much more stable behavior with increasing RH (Fig. 7).
CO 0.10 0.04 −0.01 OC −1.45 −0.01 −3.63 SOC −2.98 −0.46 −2.05 POC 1.69 0.45 −1.59 EC 0.93 0.25 −0.88 pH 0.03 0.20 0.34 * means the average concentration of the parameter in latter RH range minus that in the former RH range.
Table 2. The increase rate of carbonaceous components (μgC m−3/10% RH), CO (ppm/10% RH), and aerosol pH (/10% RH) when RH was higher than 60%
Previous chamber studies have found that heterogeneous reactions under the high RH condition can significantly enhance SOA yields. Taking isoprene SOA chamber studies as an example, earlier work has demonstrated that under lower NOx conditions, the isoprene SOA yield was enhanced in the presence of wet and acidic sulfate aerosol, especially resulting in higher concentrations of isoprene-derived epoxydiols (IEPOX) SOA (Surratt et al., 2010; Zhang et al., 2011, 2014; Lin et al., 2012). To evaluate the effect of chemical production, SOC concentrations were normalized to EC (derived from only primary emissions and being rather inert to chemical reactions), in order to exclude accumulation and/or dilution effects. The EC-scaled method will largely eliminate the variations due to physical reactions, i.e., mixing and dilution, and better represent the contribution from chemical reactions (Zheng et al., 2015).
Compared to previous chamber studies, the results obtained in our ambient study show contrary patterns: SOC appeared to be inhibited under high RH (> 60%), as indicated by an obvious downward trend of SOC/EC with increasing RH (Figs. 8, 9). Apparently, SOA at our location had no heterogeneous formation pathway under the high RH condition compared to those reported in other studies (Kaul et al., 2011; Ge et al., 2012). On the contrary, there was a negative effect on the formation of SOA observed in our study. This result was similar to that reported by Zheng et al. (2015), i.e., SOC/EC ratios exhibiting constant behavior during winter in Beijing when RH increased from 50% to 80%, although the SOA concentrations were increasing rapidly with rising RH. By using HOA (hydrocarbon-like organic aerosol) instead of EC, a similar trend was also observed by Sun et al. (2013): although the OOA concentration was observed obviously elevated when RH rose from 50% to 90%, the HOA-normalized OOA was fairly constant across the high RH levels.
Figure 8. Change of SOC/EC with RH variation. In the box-and-whisker plots, the boxes and whiskers indicate the maximum, 75th percentiles, 50th percentiles (median), 25th percentiles, and minimum, respectively.
Figure 9. Variations of concentrations of SOC, POC, and O3; total solar radiation (TSR); and the ratio of SOC/EC, as a function of RH (each point indicates the mean of the parameter between the former and latter RH conditions).
There might be several reasons for the results observed in our study, different from chamber simulations. First, our study was conducted at high RH conditions, which was typically accompanied by increased clouds and precipitation, resulting in reduced solar radiation at the ground level, while photochemical activity is weakened under low solar radiation. Accordingly, the total solar radiation was observed to sharply decrease with RH rise, and O3 was also detected to decrease when RH > 70%, indicating secondary aerosol formation via the photochemical oxidation reaction pathway became less important (Fig. 9). On the other hand, Zheng et al. (2015) suggested that aerosol may be accumulated at the high RH stable synoptic condition and increased to support aqueous-phase production, which is not sufficient to compensate for the weakened photochemical activity influence and may lead to a net decrease in the formation of ambient SOA.
Second, based on concentrations of aerosol chemical species, temperature, and RH, the ISORROPIA-II model is able to offer a more rigorous approach to calculate particle pH (Guo et al., 2015). The average pH in this study was 1.86 ± 0.54, ranging from 0.38 to 4.17, predicted by the thermodynamic ISORROPIA-II model. These systematically low pH levels observed in this study may be significantly influencing acid-catalyzed reactions as part of SOA production. Prior studies have found that acidic sulfate particles can significantly enhance the isoprene SOA yields in the atmosphere (Gaston et al., 2014; Riedel et al., 2015). However, due to more water uptake under the higher RH condition, the aerosol droplet acidity decreased and the enhancement of SOA formation by acidity was accordingly reduced. Therefore, the variation of average aerosol pH in our study was observed to be elevated with the enhanced RH as well, increasing from 1.63 to 2.20 when RH changed from the range of 60%–70% to > 90%, indicating aerosol acidity was obviously weakened under the high RH condition (Fig. 7). It should be noted that the predicted aerosol pH in this study was underestimated by lack of the NH3 data, while it had no impact on the elevated pH trend with increasing RH, which was previously reported (Guo et al., 2015). Moreover, the simulated sulfate was in good agreement with observations (R2 = 0.98, slope 0.95), implying that the ISORROPIA-II model performed well.
In addition, it should be mentioned that recent studies have demonstrated that compared to dry conditions, where SOA is semisolid, high RH conditions can lower the particle’s microscopic viscosity, which affects the particle chemical reactivity, allowing for more rapid diffusion of reactants (Renbaum-Wolff et al., 2013; Grayson et al., 2016). Zhang et al. (2018) found that aerosol viscosity also affects the reactive uptake yield of SOA. The adverse effect of aerosol viscosity on reactive uptake is also a function of RH, which is due to RH affecting the viscosity of pre-existing SOA coatings. Moreover, different portions of SOA may have different predominant mechanisms in the ambient environment. Wu et al. (2018) found highly oxygenated SOA (LVOOA) was more associated with aqueous-phase processes, while the less oxygenated components (LSOA and SVOOA) were governed by photochemical processing in ambient aerosols.
|OC (µgC m−3)||14.30||3.95||8.02||32.80|
|EC (µgC m−3)||3.33||1.47||0.77||9.97|
|TC (µgC m−3)||17.60||4.90||10.00||36.00|
|POC (µgC m−3)||6.00||2.64||1.39||18.00|
|SOC (µgC m−3)||8.26||3.31||0.03||27.00|
|Relative humidity (%)||83.00||10.00||57.00||99.00|
|Average wind speed (m s−1)||2.10||0.90||1.10||7.00|
|Total solar radiation (W m−2)||289.00||150.00||37.90||56.40|
|Total cloud cover (%)||77.90||30.40||0.00||100.00|