Investigation of hygroscopic growth effect on aerosol scattering coefficient at a rural site in the southern North China Plain

Significant impact of water-soluble organic matter on hygroscopicity of fine particles under low relative humidity condition

Uncertainties in estimating the hygroscopicity of bulk aerosols under conditions of low relative humidity (RH) or below the deliquescent RH (DRH) of aerosols remain to be significant, mainly due to the presence of water-soluble organic matter (WSOM). To quantify the contributions of WSOM to aerosol hygroscopicity and associated uncertainties, a field campaign was conducted to measure the hygroscopic growth curve (f(RH)) of bulk aerosols online, dominant chemical compositions in PM2.5 online and offline, and size distributions of the dominant chemical compositions offline during the dry and wet seasons of 2019-2020 in urban Guangzhou of south China. Based on the measured f(RH), the hygroscopicity parameter (κ) of bulk aerosols (κ-f(RH)) exhibits a logarithmic increase with increasing RH until RH reaches 69 %. Beyond this threshold, κ-f(RH) increases very slowly with further increase of RH, reaching 0.32 ± 0.04 during the dry season and 0.31 ± 0.05 during the wet season. The κ of WSOM (κ-WSOM) was further estimated to be 0.22 ± 0.03 and 0.13 ± 0.04 in the dry and wet seasons, respectively, when RH > 69 %. WSOM significantly affects κ-f(RH) by retarding the deliquescence process of aerosols and altering the mass ratio of water-soluble inorganic salts (WSIS) to WSOM within the size range of 0.4-0.9 μm, especially under low RH conditions (<60 %). The κ-f(RH) under low RH conditions was revised based on the logarithmic regression equation between RH and the ratio of measured κ-f(RH) to estimated κ-f(RH>69%). f(RH) of WSIS and WSOM were then corrected using the revised κ-f(RH) under low RH conditions, which showed 22-31 % lower values than those produced by the IMPROVE formulas.

Deposition of secondary organic aerosol in human lung model: Effect of photochemically aged aerosol on human respiratory system

Ultrafine particles (UFP) of Secondary Organic Aerosol (SOA) penetrate deep into the human respiratory system and exert fatal effects on human health. However, there is little data on the potential deposited doses of UFP-generated SOA in the human respiratory tract. This study is to estimate the fraction of aerosol deposition using a multiple-path-particle-dosimetry (MPPD) model. For relevancy of real life, the model employed measured concentrations of toluene-derived fresh and aged SOA produced within serially connected smog chamber and PAM-OFR (Potential Aerosol Mass-Oxidation Flow Reactor) under atmospheric environmental conditions (NOx and relative humidity). The number concentrations and chemical composition of fresh and aged aerosols produced within the chambers were measured using Scanning Mobility Particle Sizer (SMPS) and High-Resolution Time-of-Flight Aerosol Mass Spectrometer (HR-ToF-AMS), while the morphology of individual particles was analyzed using Scanning Electron Microscopy (SEM). The number concentration of aged SOA-w/s was more than double compared to that of fresh SOA-w/s (maximum reached after 10 h) with its size less than 100 nm. The O:C ratio for aged SOA-w/s were 0.96 and 1.15 depending on RH (0.96 at 3% RH and 1.15 at 50% RH), and individual spherical particles containing water were present in agglomerates with its size of less than 1 µm. In all inhalable fresh and aged SOA produced in the two chambers, 5-22% of aerosol is deposited in the Head airways, 4-8% in the tracheobronchial, and 8-34% in the alveolar regions. The predominant deposition of the aged aerosol occurred in the alveoli (in the generation 20th lobe), and the deposition faction in the alveoli was 2-3 times higher in the children group than the adults group. This study presented a quantitative exposure assessment of SOA generated under a realistic simulation and suggested the possibility of evaluating long-term exposure to SOA and potential health effects by determining the potential inhalable aerosol doses and the fraction of deposition in the human respiratory system.

Effects of hygroscopicity on aerosol optical properties and direct radiative forcing in Beijing: Based on two-year observations

The influence of relative humidity on aerosol properties and the direct radiative forcing of PM₁₀ and PM₁ were investigated in Beijing from January 2018 to December 2019. The annual mean scattering hygroscopic growth factor at RH = 80 % [f(80 %)] of PM₁₀ and PM₁ were 1.60 ± 0.24 and 1.58 ± 0.22, respectively. The variation of aerosol hygroscopic growth factors of PM₁₀ and PM₁ aerosols was similar, which is mainly due to the fact that aerosol scattering in Beijing is dominated by fine particles. The seasonal mean f(80 %) of PM₁₀ from spring to winter were 1.66 ± 0.23, 1.71 ± 0.25, 1.51 ± 0.20, 1.49 ± 0.16, respectively, which were higher in spring and summer, and lower in autumn and winter. The diurnal variation of f(80 %) was relatively higher from 12:00 to 18:00, which could be related to the formation of secondary aerosols by photochemical reactions. f(80 %) shows a strong positive relationship with both the scattering Angström exponent (SAE) and the single scattering albedo (ω₀) under dry conditions; therefore, the scattering hygroscopic growth factor could be estimated using these two parameters. The upscatter fraction (β) and single scattering albedo, which are the key aerosol optical properties for the calculation of direct radiative forcing, are also RH-dependent. As RH increases, the upscatter fraction (backscatter fraction) decreases and ω₀ increases. The aerosol radiative forcing at RH 80 % was 1.48 times as that in the dry state. The sensitivity experiment showed that the variation in the scattering coefficient with relative humidity had the greatest influence on radiation forcing, followed by β and ω₀. The seasonal variation of ΔF(80 %)/ΔF(dry) coincides with that of the aerosol hygroscopic growth factor. Our study suggests that understanding the influence of relative humidity on aerosol properties and direct radiative forcing is important for accurately estimating the radiative forcing of aerosols.

Exploring the Sensitivity of Visibility to PM2.5 Mass Concentration and Relative Humidity for Different Aerosol Types

Fine particle (PM2.5) mass concentration and relative humidity (RH) are the primary factors influencing atmospheric visibility. There are some studies focused on the complex, nonlinear relationships among visibility, PM2.5 concentration, and RH. However, the relative contribution of the two factors to visibility degradation, especially for different aerosol types, is difficult to quantify. In this study, the normalized forward sensitivity index method for identifying the dominant factors of visibility was used on the basis of the sensitivity of visibility to PM2.5 and RH changes. The visibility variation per unit of PM2.5 or RH was parameterized by derivation of the visibility multivariate function. The method was verified and evaluated based on 4453 valid hour data records in Tianjin, and visibility was identified as being in the RH-sensitive regime when RH was above 75%. In addition, the influence of aerosol chemical compositions on sensitivity of visibility to PM2.5 and RH changes was discussed by analyzing the characteristics of extinction components ((NH4)2SO4, NH4NO3, organic matter, and elemental carbon) measured in Tianjin, 2015. The result showed that the fitting equation of visibility, PM2.5, and RH, separately for different aerosol types, further improved the accuracy of the parameterization scheme for visibility in most cases.

Effects of chemical compositions in fine particles and their identified sources on hygroscopic growth factor during dry season in urban Guangzhou of South China

Knowledge of aerosol hygroscopicity is essential to assess visibility improvement and aerosol radiative forcing. Aerosol hygroscopicity is highly dependent on emission sources, while the hygroscopicity of different sources remains largely unexplored. In the current study, the hygroscopic growth factor (i.e., f(RH)) and relevant chemical compositions (e.g., water-soluble inorganic ions, carbonaceous fractions and elements) in fine particles were synchronously measured for nearly 3 months within 2019-2020 in an urban site of Guangzhou. The mean value (± standard deviation) of f(RH) at 70% RH was 1.50 (± 0.11). The diurnal cycle in aerosol hygroscopic growth strongly depended on the mass fraction of hydrophilic chemical compositions (e.g., SO₄^2-, NO₃^- and NH₄⁺) in fine particles and variation in contributions of aerosol sources. A Positive Matrix Factorization model was applied to distinguish the different hygroscopicity of specific source factors in a mixed aerosol. Secondary nitrate and secondary sulfate were more hydrophilic, whereas emissions from primary combustion processes (i.e., ship emission, coal combustion and road traffic) were less hygroscopic. Soil dust was almost insoluble. The hygroscopic growth of each source was parameterized that quantified the emission sources and f(RH) relationship for use of air quality and radiative transfer models either as input or as validation.

Effect of urban underlying surface on PM2.5 vertical distribution based on UAV in Xi'an, China

Fine particulate matter (PM_2.5) has become a significant issue of ecological environment. However, few studies have explored the vertical distribution of PM_2.5 in cities. The objectives of this paper are to reveal the vertical distribution regular pattern of PM_2.5 over urban underlying surfaces near the ground with a hexacopter-type unmanned aerial vehicle (UAV) in winter. Results showed that the maximum vertical gradient of PM_2.5 near the ground was typically the greatest in the morning as the stable atmospheric conditions. Moreover, regression model illustrated that relative humidity had the greatest impact on the vertical profile of PM_2.5 compared to air temperature and altitude as hygroscopic of PM_2.5 aerosols. Curve model shown that vertical profile of PM_2.5 over the surfaces of water and green space first increased slowly and then declined, besides, the highest concentration inflection of PM_2.5 above the water body (23.7 m) is higher than the green space (14.3 m). Thus, suggesting residents living vertical of 10-30 m from the ground around large water bodies and green spaces should not open windows for ventilation in the morning. Therefore, this study provides insights into the vertical distributions of PM_2.5 over different underlying surfaces and should be of reference value to urban planners for designing urban spaces to optimize atmosphere environment to provide a healthy living environment.

Variation Characteristics and Transportation of Aerosol, NO2, SO2, and HCHO in Coastal Cities of Eastern China: Dalian, Qingdao, and Shanghai

This paper studied the method for converting the aerosol extinction to the mass concentration of particulate matter (PM) and obtained the spatio-temporal distribution and transportation of aerosol, nitrogen dioxide (NO2), sulfur dioxide (SO2), and formaldehyde (HCHO) based on multi-axis differential optical absorption spectroscopy (MAX-DOAS) observations in Dalian (38.85°N, 121.36°E), Qingdao (36.35°N, 120.69°E), and Shanghai (31.60°N, 121.80°E) from 2019 to 2020. The PM2.5 measured by the in situ instrument and the PM2.5 simulated by the conversion formula showed a good correlation. The correlation coefficients R were 0.93 (Dalian), 0.90 (Qingdao), and 0.88 (Shanghai). A regular seasonality of the three trace gases is found, but not for aerosols. Considerable amplitudes in the weekly cycles were determined for NO2 and aerosols, but not for SO2 and HCHO. The aerosol profiles were nearly Gaussian, and the shapes of the trace gas profiles were nearly exponential, except for SO2 in Shanghai and HCHO in Qingdao. PM2.5 presented the largest transport flux, followed by NO2 and SO2. The main transport flux was the output flux from inland to sea in spring and winter. The MAX-DOAS and the Copernicus Atmosphere Monitoring Service (CAMS) models’ results were compared. The overestimation of NO2 and SO2 by CAMS is due to its overestimation of near-surface gas volume mixing ratios.

Triple charge-coupled device cameras combined backscatter lidar for retrieving PM2.5 from aerosol extinction coefficient

The vertical aerosol extinction coefficient profile is the key to evaluating the aerosol radiation effect. In addition, it is also the premise of indirect inversion of PM_2.5 concentration from the lidar signal. A novel lidar system combining tripe charge-coupled device (CCD) cameras and backscatter lidar is proposed to retrieve accurately the aerosol extinction coefficient. The relative humidity segmentation method is used to convert the aerosol extinction coefficient into PM_2.5 concentrations. The results show that the combination of triple CCDs and backscatter lidar can obtain a wider-range extinction coefficient profile. The variation trend of fitted PM_2.5 concentrations and PM_2.5 concentrations measured by the in situ instrument show a high correlation. The correlation coefficient reaches 0.850.

Measuring the Vertical Profiles of Aerosol Extinction in the Lower Troposphere by MAX-DOAS at a Rural Site in the North China Plain

Ground-based multi-axis differential optical absorption spectroscopy (MAX-DOAS) measurements were performed during the summer (13 June–20 August) of 2014 at a rural site in North China Plain. The vertical profiles of aerosol extinction (AE) in the lower troposphere were retrieved to analyze the temporal variations of AE profiles, near-surface AE, and aerosol optical depth (AOD). The average AOD and near-surface AE over the period of study were 0.51 ± 0.26 and 0.33 ± 0.18 km−1 during the effective observation period, respectively. High AE events and elevated AE layers were identified based on the time series of hourly AE profiles, near-surface AEs and AODs. It is found that in addition to the planetary boundary layer height (PBLH) and relative humidity (RH), the variations in the wind field have large impacts on the near-surface AE, AOD, and AE profile. Among 16 wind sectors, higher AOD or AE occur mostly in the directions of the cities upstream. The diurnal variations of the AE profiles, AODs and near-surface AEs are significant and influenced mainly by the source emissions, PBLH, and RH. The AE profile shape from MAX-DOAS measurement is generally in agreement with that from light detection and ranging (lidar) observations, although the AE absolute levels are different. Overall, ground-based MAX-DOAS can serve as a supplement to measure the AE vertical profiles in the lower troposphere.

Three-years monitoring of PM2.5 and scattering coefficients in Shanghai, China

The absorption and scattering of aerosols are critical factors that influence in global climate and visibility degradation. From January 2013 to December 2015, aerosol scattering coefficients, PM_2.5, and meteorological parameters were continuously measured at a monitoring site in Shanghai, China. The annual means of scattering coefficients were 312.3, 232.1, and 261.9 Mm^-1 for the years 2013, 2014, and 2015, respectively. The corresponding values for PM_2.5 were 61.6, 51.6, and 52.9 μg/m³. Compared with the average scattering coefficient of the year 2013, those of 2014 and 2015decreased by 26% and 16%, respectively. Furthermore, the annual average PM_2.5 decreased by 16% and 14% in 2014 and 2015, respectively. Although this study concluded that PM_2.5 was generally correlated with scattering coefficients during the entire measurement period, the decrease in the former was much less than the latter. On this basis, ultrafine particles may decrease significantly because they cause aerosol scattering. This finding should be investigated further in the future. The inter-annual meteorological changes affected PM_2.5 and scattering coefficient inter-annual variations. In the northwest and southwest direction, the seasonal and diurnal variations of aerosol scattering coefficients showed larger values when the wind speeds were about 3-5 m/s. The serious pollution in the northwest direction were mainly due to long-distance transport of pollutants during winter, whereas those in the southwest direction were attributed to local emission. The westerly wind frequency is the crucial factor influencing local pollution transport significantly. Backward trajectory analysis indicated that the air pollution in Shanghai in 2013-2015 is attributed to long-distance transport and primarily affected by the air mass from northwest direction. Observations on long-term aerosol optical properties on the basis of in-situ measurements can help thoroughly understand the radiative forcing characteristics of aerosol.

Pollution severity-dependent aerosol light scattering enhanced by inorganic species formation in Beijing haze

The dependence of aerosol optical properties on the chemical composition and size of particles in haze in Beijing was studied. We measured the scattering coefficient of dehydrated PM_2.5 aerosols (σ_{sp_dry}) and analyzed the chemical composition of PM_2.5. We also monitored the size distribution of particles in the range of ~10-700 nm to observe the particle growth (PG_size). Results showed that the concentrations of secondary inorganic aerosols (SIAs) and the mean size of PM_2.5 particles (scattering Ångström exponent decreasing) increased with the deterioration of the air quality and increase in relative humidity (RH) which enhanced mass scattering efficiency and increased PM_2.5. Thus, the increase in σ_{sp_dry} was particularly dramatic and highly sensitive to the ambient RH in severe haze stages. When the ratio of SIAs to PM_2.5 (M_SIAs) exceeded 0.35 during the polluted environment, the water content, PG_size, and σ_{sp_dry} showed distinct increases, indicating that the formation of SIAs enhanced water vapor condensation and particle growth. This finding revealed the existence of a critical value for M_SIAs in terms of describing the correlation of σ_{sp_dry} variation with pollution severity. The estimation of the respective contributions of individual components to σ_{sp_dry} with the IMPROVE formula revealed that ammonium nitrate and ammonium sulfate were the two largest contributors. These results indicate that the rapid formation of SIAs and PG_size under humid conditions are the key factors contributing to the increased σ_{sp_dry} via enhanced mass scattering efficiency and increased PM_2.5 in the severe haze observed in this study.

Aerosol optical properties under different pollution levels in the Pearl River Delta (PRD) region of China

To clarify the aerosol hygroscopic growth and optical properties of the Pearl River Delta (PRD) region, integrated observations were conducted in Heshan City of Guangdong Province from October 19 to November 17, 2014. The concentrations and chemical compositions of PM_2.5, aerosol optical properties and meteorological parameters were measured. The mean value of PM_2.5 increased from less than 35 (excellent) to 35-75 μg/m³ (good) and then to greater than 75 μg/m³ (pollution), corresponding to mean PM_2.5 values of 24.9, 51.2, and 93.3 μg/m³, respectively. The aerosol scattering hygroscopic growth factor (f(RH = 80%)) values were 2.0, 2.12, and 2.18 for the excellent, good, and pollution levels, respectively. The atmospheric extinction coefficient (σ_ext) and the absorption coefficient of aerosols (σ_ap) increased, and the single scattering albedo (SSA) decreased from the excellent to the pollution levels. For different air mass sources, under excellent and good levels, the land air mass from northern Heshan had lower f(RH) and σ_sp values. In addition, the mixed aerosol from the sea and coastal cities had lower f(RH) and showed that the local sources of coastal cities have higher scattering characteristics in pollution periods.

Effects of relative humidity and PM2.5 chemical compositions on visibility impairment in Chengdu, China

To better understand the potential causes of visibility impairment in autumn and winter in Chengdu, relative humidity (RH), visibility, the concentrations of PM_2.5 and its chemical components were on-line measured continuously in Chengdu from Nov. 2016 to Jan. 2017. Six obvious haze episodes occurred in Chengdu, with the total time of haze episodes accounted for more than 90% of the total observation period, and higher NO₂ concentrations and RH were related to the high particle concentrations in haze episodes. The visibility decreased in a non-linear tendency under different RH conditions with the increase of PM_2.5 concentrations, which was more sensitive to RH under lower PM_2.5 concentrations. The threshold concentration of PM_2.5 got more smaller with the increase of RH. During the entire observation period, organic matter (OM) was the largest contributor (31.12% to extinction coefficient (b_ext)), followed by NH₄NO₃ and (NH₄)₂SO₄ with 28.03% and 23.01%, respectively. However, with the visibility impairment from Type I (visibility > 10 km) to Type IV (visibility ≤2 km), the contribution of OM to b_ext decreased from 38.12% to 26.77%, while the contribution of NH₄NO₃ and (NH₄)₂SO₄ to b_ext increased from 19.09% and 20.20% to 34.29% and 24.35%, respectively, and NH₄NO₃ became the largest contributor to b_ext at Type IV. The results showed that OM and NH₄NO₃ were the key components of PM_2.5 for visibility impairment in Chengdu, indicating that the control of precursors emissions of carbonaceous species and NH₄NO₃ could effectively improve the visibility in Chengdu.

Roles of Relative Humidity in Aerosol Pollution Aggravation over Central China during Wintertime

Aerosol pollution elicits considerable public concern due to the adverse influence on air quality, climate change, and human health. Outside of emissions, haze formation is closely related to meteorological conditions, especially relative humidity (RH). Partly due to insufficient investigations on the aerosol hygroscopicity, the accuracy of pollution prediction in Central China is limited. In this study, taking Wuhan as a sample city, we investigated the response of aerosol pollution to RH during wintertime based on in-situ measurements. The results show that, aerosol pollution in Wuhan is dominated by PM_2.5 (aerodynamic particle size not larger than 2.5 μm) on wet days (RH ≥ 60%), with the averaged mass fraction of 0.62 for PM₁₀. Based on the RH dependence of aerosol light scattering (f (RH)), aerosol hygroscopicity was evaluated and shows the high dependence on the particle size distribution and chemical compositions. f (RH = 80%) in Wuhan was 2.18 (±0.73), which is comparable to that measured in the Pearl River Delta and Yangtze River Delta regions for urban aerosols, and generally greater than values in Beijing. Ammonium (NH₄⁺), sulfate (SO₄²⁻), and nitrate (NO₃⁻) were enhanced by approximately 2.5-, 2-, and 1.5-fold respectively under wet conditions, and the ammonia-rich conditions in wintertime efficiently promoted the formation of SO₄²⁻ and NO₃⁻, especially at high RH. These secondary ions play an important role in aggravating the pollution level and aerosol light scattering. This study has important implications for understanding the roles of RH in aerosol pollution aggravation over Central China, and the fitted equation between f (RH) and RH may be helpful for pollution forecasting in this region.

Observational study of aerosol hygroscopic growth on scattering coefficient in Beijing: A case study in March of 2018

A humidified nephelometer system was deployed to measure the aerosol scattering coefficients at RH < 30% and RH in the range of 40 to 85% simultaneously in megacity Beijing in March 2018. The aerosol optical properties and aerosol hygroscopicity of two sizes (PM₁₀ and PM₁) during the pollution period, dust period and a new particle formation event (NPF) were analyzed. During the pollution period, the scattering and absorption coefficients increased dramatically with the accumulation of pollutants, while scattering Ångström exponent (SAE), submicron scattering fraction (Rsp), submicron absorption fraction (Rap) decreased, as well as single scattering albedo (SSA) rose slightly, which indicated the increasing contribution of larger particle to scattering and absorption, and enhanced the scattering ability of aerosols. The average PM₁₀ mass scattering efficiency is 3.86 ± 1.19 m² g^-1 with a range of 2.05-5.74 m² g^-1 during the pollution period, and 0.40 ± 0.05 m² g^-1 during the dust period. Rsp at wavelength of 550 nm varied from 55.8% to 89.3% during the measurement period, with the average of 64.8% ± 5.2% and 73.1% ± 6.8% during the pollution period and dust period, respectively, which suggests that the aerosol scattering coefficient is mainly affected by fine particles. The average PM₁₀ and PM₁ aerosol scattering hygroscopic growth factors f(80%) are 1.75 ± 0.05 and 1.75 ± 0.04 during the pollution period, 1.14 ± 0.09 and 1.15 ± 0.06 during the dust period, 1.59 ± 0.05 and 1.60 ± 0.06 during the NPF event period, respectively. Aerosol scattering hygroscopic growth factors showed a strong correlation with the scattering Ångström exponent which suggests the hygroscopicity is much stronger for fine particles (SAE > 1.5) than the coarse particles (SAE < 1.0).

Inorganic ion chemistry of local particulate matter in a populated city of North China at light, medium, and severe pollution levels

Twenty-six pairs of PM_2.5 and PM₁₀ samples were collected during haze episodes in Zhengzhou (113°28' E, 34°37' N), a highly populated city in North China. The samples were used to examine the inorganic ion chemistry of particulate matter (PM) of local origin at light (PM_2.5 < 60 μg m^-3 and PM₁₀ < 135 μg m^-3), medium (PM_2.5: 60-170 μg m^-3 and PM₁₀: 135-325 μg m^-3), and severe (PM_2.5 > 170 μg m^-3 and PM₁₀ > 325 μg m^-3) pollution levels. At the light and severe pollution levels, the increase of PM₁₀ was accounted for by the increase of PM_2.5, and the variation of PM_10-2.5 was small. In contrast, the increase of PM₁₀ at the medium pollution level was caused by the increase in both PM_2.5 and PM_10-2.5. Sulfate (SO₄^2-), nitrate (NO₃^-), ammonium (NH₄⁺), and chloride in the form of ammonium chloride (Cl^-_S) accounted for 47.8% and 60.3% of the PM_2.5 mass at the light and severe levels, respectively. These values indicate a large contribution of secondary inorganic species to the PM_2.5 growth. As the pollution level changed from light to medium, the contribution of SO₄^2- to the growth of PM_2.5 decreased from 49.0% to 15.1%, while those of NO₃^- and Cl^-_S increased from 25.1% and 0.6% to 32.5% and 2.8%, respectively, indicating the substantial production of nitrate and chloride. At the severe level, the contribution of SO₄^2- was 30.1%, while those of NO₃^- and Cl^-_S were 5.9% and 0.5%, respectively, suggesting a hindering effect of sulfate on the production of nitrate and chloride. These results indicate that the production of secondary species with the increase of PM_2.5 was dominated by sulfate-associated conversions at the light and severe pollution levels and was substantially influenced by nitrate- and chloride-associated conversions at the medium pollution level. The estimation of carbonate presence in the PM indicates that part of the carbonate in coarse particles (PM_10-2.5) of crustal origin enhanced sulfate production via heterogeneous surface reactions. Quantification of the contribution of primary and secondary species to PM_2.5 showed that it was dominated by both primary and secondary particles at the light pollution level, and it was mainly composed of secondary species at the severe pollution level.