Figure 1a and b display meteorological data (relative humidity, temperature and precipitation) and mass concentrations of non-refractory PM1 (NR-PM1) species and OA components, respectively, between September and December 2013. Total NR-PM1 concentrations vary from 2.1 to 76.4 µg m-3 with an average of 25.9 ± 13.0 µg m-3. ACSM NR-PM1 concentrations co-vary with that of PM2.5 measured by TEOM (R2= 0.64, slope = 0.59; Fig. S1); the low slope value may be caused by the different size cuts of ACSM and TEOM and the presence of refractory materials such as elemental carbon (and to a lesser extent mineral dust and sea salt) which the ACSM cannot detect. Overall, daily PM2.5 concentrations range from 3.7 to 106.0 µg m-3 and are largely (90.0 %) within the 24 h air quality standard of 75 µg m-3 set by the Hong Kong Air Quality Objectives. Days with better air quality (PM2.5 < 35 µg m-3) are mainly observed in the month of September and in rainy periods of other months. The prevailing winds from the ocean in September not only bring in less polluted air mass but also dilute the local air pollutants compared with other seasons (Yuan et al., 2006; Li et al., 2015). Precipitation has an obvious impact on total NR-PM1 concentrations but, as we will discuss, has a lesser effect on primary organics.

Overall, NR-PM1 is dominated by organics and sulfate with relative contributions of 58.2 and 23.3 % and average concentrations of 15.1 ± 8.1 and 6.0 ± 3.5 µg m-3, respectively (Fig. 2a). Other inorganic species (ammonium, nitrate and chloride) amount to approximately 20 % of NR-PM1. The dominance of organics and sulfate is consistent with previous online studies in urban areas (e.g., Salcedo et al., 2006; Aiken et al., 2009; Sun et al., 2012, 2013b) as well as previous filter-based studies in MK (e.g., Louie et al., 2005; Cheng et al., 2010; Huang et al., 2014). The measured composition is consistent with earlier HR-AMS measurements carried out at the same site in spring and summer 2013 (Lee et al., 2015) with very similar overall species distribution but slightly lower measured concentrations as compared to the ACSM, likely due to the fact that sampling took place in different time periods (spring–summer 2013 for the AMS campaign, fall–winter 2013 for the ACSM campaign). In the AMS study, PMF aerosol factors were identified (one additional OOA factor and one additional COA factor). A marked difference is observed in the distribution of primary OA (POA) and SOA; whereas in spring and summer (AMS) POA makes up 65 % of total organics, the reverse is observed for fall and winter (ACSM) when POA only amounts to 42 % overall. A possible reason for this discrepancy is the fact that impacts of regional pollution and long-range transport are usually higher during fall and winter (Yuan et al., 2013; Li et al., 2015), thus contributing more SOA.

Elemental carbon (EC) concentrations are significant at the Mong Kok site but not measurable by ACSM due to its high refractory temperature. EC has been discussed extensively in the previously mentioned filter-based studies and a brief comparison of online elemental carbon/organic carbon (ECOC) measurements to the results of HR-AMS measurements has been presented in an HR-AMS study (Lee et al., 2015). We therefore do not discuss EC in detail in this work.

PMF-resolved four factors, including two primary OA factors (HOA from traffic emissions and COA) and two OOAs: LV-OOA and the less-oxidized SV-OOA (Aiken et al., 2008; Jimenez et al., 2009; Tiitta et al., 2014). The mass spectra are depicted in Fig. 3. The mass concentration of primary OA factors (HOA and COA), a surrogate of local emissions, constitutes 42 % of total organics and is slightly higher than that of LV-OOA (38 %; Fig. 2b). SV-OOA contributes approximately 20 % to total OA and is associated with both the primary organic aerosol sources and LV-OOA (see Sect. 3.2).

The diurnal profiles of NR-PM1 species and OA components are depicted in Fig. 4. Total organics display a diurnal pattern with two pronounced peaks during 12:00–14:00 and 19:00–21:00, corresponding to the peaks of COA at lunch and dinner time, respectively. In addition, organics increase at about 10:00, which may be related to the increase of local emissions of HOA and COA by 2.3 and 1.1 µg m-3, respectively, from 06:00 to 10:00.

The mass concentration of sulfate (Fig. 4b) does not show any diel variation. It is likely that sulfate, as a regional pollutant, is mainly formed during long-range transport, leading to the lack of a specific diurnal pattern at MK; a similar flat diurnal pattern for sulfate has also been found at the HKUST supersite in Hong Kong (Lee et al., 2013; Li et al., 2015). These results differ significantly from observations in Beijing and Lanzhou in China and Welgegund in South Africa (Sun et al., 2012, 2013b; Xu et al., 2014; Tiitta et al., 2014) where sulfate displays an obvious increase at noontime in summer and wet seasons due to either photochemical reaction or aqueous oxidation of SO2. The difference may result from the much lower level of sulfur dioxide (SO2) with an average of 4.6 ppb in MK compared to, for example, ∼ 32 ppb in Beijing, where coal combustion leads to a much higher SO2 concentration (Lin et al., 2011); sulfate and relative humidity (RH) have almost no correlation (R2= 0.06) in MK, suggesting little importance of local aqueous processing for the formation of sulfate.

Nitrate shows a slight dip around noontime, corresponding to the increase of the ambient temperature (Fig. 4j); evaporative loss of particulate nitrate might outweigh the secondary production of nitrate during this time. The diurnal pattern of ammonium (Fig. 4d) is very similar to that of sulfate, as expected based on their commonly observed association in atmospheric particles. Chloride (Fig. 4e) has rather low concentrations and shows a similar diurnal variation to that of nitrate, likely due to its volatility.

Figure 8a shows the average concentration trends on individual days of the week for NR-PM1 species. Figure 8b describes the diurnal patterns of the OA components for weekdays, Saturdays and Sundays, respectively. Because of the small data sets on Saturdays and Sundays, data beyond 1 standard deviation from the mean (25.9 ± 13.0 µg m-3) were removed from the whole data set to remove the influence of episodic events in this analysis. Overall, total NR-PM1 concentrations have no obvious variation (average variation less than 5 %) from Monday to Saturday, but they drop by 16 % on Sundays compared to Saturdays. This weekend difference is opposite to the result found in Beijing where higher concentrations occurred on Sundays than Saturdays (Sun et al., 2013b). However, some others such as Lough et al. (2006) and Rattigan et al. (2010) reported that both Saturdays and Sundays had obvious traffic emissions reduction due to less human activities on weekends in Los Angeles and New York, respectively.

Organics and secondary inorganics (SO4, NH4 and NO3) contributed 54 and 46 %, respectively, to the concentration difference between Sundays and Saturdays in MK. The difference in organics is mainly attributed to the variation of HOA, which shows very similar diurnal variations on Saturdays and weekdays, but has an average decrease of 23 % after 07:00 on Sundays. A 37 % reduction of traffic-related carbonaceous aerosol on Sundays compared with weekdays in MK has been reported (Huang et al., 2014). In Hong Kong many people work on Saturday, which leads to a traffic pattern similar to normal weekdays. COA shows nearly the same diurnal patterns on all days, and LV-OOA and SV-OOA do not show obvious variations. Overall, local emissions from traffic contribute most to the day-of-week variations in organics.

Figure 9a, b and c show the variation in hourly mass concentration of NR-PM1 species and OA components and their mass fractions as a function of hourly total NR-PM1 mass loading, respectively. Below 50 µg m-3, all aerosol species display a nearly linear increase with PM1 mass loading, with slopes of about 0.5 for organics, 0.25 for sulfate and LV-OOA and around 0.1 for nitrate, ammonium, COA, HOA and SV-OOA (Fig. 9a). While the fractions of NH4 and organics remain relatively stable, sulfate exhibits a decrease and then an increase, and NO3 and chloride show a gradual increase and then a decrease, respectively, as NR-PM1 increased to 50 µg m-3 (Fig. 9b, c). Although the mass concentrations of all organic factors increase as NR-PM1 increases, SV-OOA is the only factor that increased in mass fraction. Primary OA components (HOA and COA) and transported OA (LV-OOA) show a decrease in fraction and stable contributions, respectively, as NR-PM1 increases to 50 µg m-3, while the contribution of SV-OOA increases sharply from around 5 to 25 % of total organic mass. It suggests that SV-OOA plays an important role as NR-PM1 increases to 50 µg m-3 in MK. However, beyond 50 µg m-3, the mass loadings of SO4 and organics increase, while those of NH4, NO3 and LV-OOA remain almost constant, which differs from the observations in Beijing, where NH4 and NO3 kept a linear increase from 50 to about 200 µg m-3 (Sun et al., 2013b; Zhang et al., 2014). In terms of fractions, only COA and, to a lesser extent, SV-OOA increase as NR-PM1 increases further. In fact, over 80 % of the high hourly NR-PM1 concentrations (> 50 µg m-3) are observed during the mealtime periods with enhanced cooking activities.

When the hourly averages in Fig. 9 are replaced by daily averages (Fig. 10), the COA concentration varies little and its fraction does not exhibit an increase but instead decreases significantly with increasing daily NR-PM1. Contrarily, the fractions of SV-OOA and LV-OOA clearly increase. This analysis suggests that while cooking OA is responsible for the hourly high concentrations during mealtime and potential high hourly PM levels, LV-OOA/SV-OOA is responsible for episodic events and high day-to-day PM levels.

To analyze the difference in particle composition and meteorological conditions among episodic periods and clean periods, three heavy polluted episodes (19–22, 23–26 October and 10–13 December) and two clean periods (17–18 September and 14–18 December), highlighted in Fig. 1, were analyzed. The average concentrations of these chosen periods are larger than 1 standard deviation from the average concentration of the campaign (25.9 ± 13.0 µg m-3). The composition, meteorological features (T and RH) and oxidation index (Ox and f44) of these five events are shown in Table 4. Clean period 1 (C1) is characterized by low NR-PM1 concentrations (below 13 µg m-3), prevailing coastal wind (easterly wind), lack of rain, high ambient temperature (∼ 28 ∘C) and high relative humidity (∼ 70 %). Another clean period (C2) features continuous precipitation with the coldest and most humid weather condition in the period studied. Haze period 1 (H1) has similar temperature and humidity as C1 but is marked by mixed continental/oceanic winds. From H1 to the following haze period (H2), the observed wind direction shifts to reflect continental transport, with a significant decrease in RH to 36 %. Haze period 3 (H3), just before C2, is also dominated by continental winds but with lower temperatures (∼ 19 ∘C) than during other haze events.

Although the total NR-PM1 of C1 (12.2 µg m-3) and C2 (11.8 µg m-3) are both only 25–30 % of that during haze periods, they were driven by different mechanisms. The main differences in meteorological conditions between C1 and C2 are the dominance of continental wind rather than coastal wind, much lower temperature and the existence of precipitation in C2. The low concentration of C1 is mainly attributed to easterly wind bringing less air pollutants and diluting local air pollutants. To a lesser extent, it is influenced by both particle evaporation, especially for SV-OOA, and dilution of local emissions during high temperatures, which might be the reason why HOA, COA and SV-OOA in C1 are lower than in C2 despite the lack of rain. The low mass loading of C2 was mainly caused by the wet deposition of precipitation. It dramatically reduces the concentration of secondary species, such as SO4, NH4, NO3, SV-OOA and LV-OOA, but not primary HOA and COA. Compared to the adjacent period H3, the total organic mass reduces by 68 % to an average of 8.1 µg m-3 (Table 4). Precipitation effectively removes secondary particles but is less efficient for primary particles that are continuously generated locally.

With similar continental source region as C2, the most severe pollution event H3 occurred during 10–13 December with an average NR-PM1 of 47.7 µg m-3. The persistent northerly wind continually brought air masses from the PRD region into Hong Kong and lead to a marked mass increase of secondary species of SO4, NH4, NO3, LV-OOA and SV-OOA. Furthermore, H3 is characterized by the highest mass concentration and relative contribution of nitrate and SV-OOA compared with other haze periods. This is likely due to the average temperature of H3 being 5–6 ∘C lower than that of other haze events. In addition, although all three haze events have very similar SO4 mass loading, there is a ∼ 50 % increase in NH4 concentration during the H3 episode, consistent with the increase of nitrate in that period.

The other two haze events are adjacent with influence from both continental and oceanic region in H1 and continental source region in H2. The mixed pattern of source regions during H1 identified as land–sea breeze (Fig. S14) can redistribute PM pollution over the whole PRD region and accumulate air pollutants effectively (Lo et al., 2006; Chan and Yao, 2008; Lee et al., 2013). The pronounced high concentration of LV-OOA and SV-OOA, jointly contributing 70 % of total organics, reflects the oxidation of primary emissions in the PRD under such cycles, which is also observed at the suburban HKUST site (Lee et al., 2013). The periodic nitrate peaks in H1 with low concentration in daytime and high concentration in nighttime coincide with temperature changes. During H2 period, the prevailing wind is northwesterly and there is a sharp decrease in relative humidity. It is interesting to note that the dip in RH during H2 coincides with the dip in sulfate, ammonium, nitrate and LV-OOA; this might be caused by decreased aqueous-phase processing and by decreased gas-particle partitioning associated with water uptake under low RH for secondary aerosol particles (Sun et al., 2013a, b).

The fractions of f44 during these three haze occasions are all lower than that at HKUST (Li et al., 2013), which reflects a larger abundance of the less oxygenated POA at the urban MK site. In addition, the POA concentration (HOA + COA) does not change much between clean periods and haze periods. However, its relative contribution decreases from about 50 % during clean periods to 30 % during haze events because of the pronounced variation of secondary OA as shown in Fig. 11.