2.1 Sampling site and campaigns

Instrumentation was deployed in the University of Houston–Rice University Mobile Air Quality Laboratory (MAQL), as described in Leong et al. (2017) and Wallace et al. (2018). The winter campaign was conducted from 3 to 17 February 2014, and the summer campaign was conducted from 1 to 31 May 2014. The measurement site was located on the campus of the University of Houston at Sugar Land (UHSL) (29.5740^∘ N, 95.6518^∘ W). The campus is situated southwest of downtown and the Houston Ship Channel (HSC). The map of the measurement site is presented in Fig. S1 in the Supplement. The nearby interstate highway (I-69) extends to the west of downtown and serves as a major traffic emission source. The WA Parish Generating Station, a coal-fired power plant that is the largest electricity-generating facility in Texas, is ∼ 10 km south of the site (Fig. S1). The data collected in the winter campaign are limited in duration; thus, the following discussion focuses primarily on the summer campaign.

2.2 Measurements

The data used in this paper are reported in local time, which is 6 and 5 h behind Universal Coordinated Time in winter and summer, respectively. The details regarding the instrumental setup and data processing of these measurements were the same as described in Wallace et al. (2018). The NR-PM₁ composition was measured using an Aerodyne HR-ToF-AMS (DeCarlo et al., 2006; Canagaratna et al., 2007). A PM_2.5 Teflon^®-coated cyclone inlet was installed above the MAQL at a height of 6 m above ground to remove coarse particles and to introduce air into the sampling line at a rate of 16.7 SLPM. A Nafion dryer (Perma Pure, LLC) was mounted upstream of the HR-ToF-AMS to dry the sample to below 45 % relative humidity (RH). Particles are focused into a narrow beam via an aerodynamic lens and accelerated under high vacuum into the particle sizing measurement chamber. After passing the particle sizing chamber, the nonrefractory components are flash vaporized at near 600 ^∘C and ionized using electron impact at 70 eV. Ionized mass fragments are then transmitted directly into the time-of-flight region so that the mass spectra can be obtained. In this study, the HR-ToF-AMS was operated in V mode to obtain the nonrefractory chemical components with a higher sensitivity and lower mass spectral resolution compared to the W mode. Ionization efficiency (IE) calibration was performed using monodisperse ammonium nitrate (NH₄NO₃) at the beginning and end of each campaign. Filtered ambient air was sampled every 2 days for approximately 20 to 30 min to provide a baseline of signal for the HR-ToF-AMS during campaigns. The detection limits (Table S1) were calculated by multiplying the standard deviations of the filter periods by 3.

Trace gas mixing ratios and meteorological parameters also were measured on the MAQL during the campaigns. Carbon monoxide (CO) was measured with high-resolution cavity enhanced direct-absorption spectroscopy (Los Gatos Research, Inc.), and sulfur dioxide (SO₂) was quantified using a pulsed fluorescence analyzer (ThermoFischer Scientific, model 43i-TLE). Nitric oxide (NO) and nitrogen dioxide (NO₂) were measured with a chemiluminescence monitor with an ultraviolet-light-emitting-diode NO₂ photolytic converter on the NO₂ channel (AQD, Inc.) The total reactive nitrogen (NO_y) was measured with a Thermo 49c-TL with a heated molybdenum inlet converter. Ozone (O₃) mixing ratio was measured with ultraviolet absorption (2BTech, Inc., model 205). Meteorological parameters including ambient temperature, solar radiation, RH, wind speed (WS) and wind direction (WD) were measured using an RM Young meteorological station. Precipitation totals from a nearby Texas Commission on Environmental Quality (TCEQ) monitoring site (EPA Site: 48_157_0696) were downloaded from the TCEQ website. The total cloud cover data were downloaded from the READY Archived Meteorology website of the National Oceanic and Atmospheric Administration.

2.3 Data processing

The HR-ToF-AMS data analysis was performed using SQUIRREL version 1.56A and PIKA version 1.19D in Igor Pro 6.37 (Wave Metrics Inc.). The relative IEs were applied to OA (1.4), ${\mathrm{SO}}_{\mathrm{4}}^{\mathrm{2}-}$ (1.2), ${\mathrm{NO}}_{\mathrm{3}}^{-}$ (1.1), ${\mathrm{NH}}_{\mathrm{4}}^{+}$ (4.0) and chloride (Cl⁻, 1.3) following the standard data analysis procedures. The composition-dependent collection efficiency was applied to the data based on Middlebrook et al. (2012). Elemental ratios (H ∕ C, O ∕ C and N ∕ C, where H is hydrogen, C is carbon, and N is nitrogen) and the ratio of organic mass to organic carbon (OM ∕ OC) were generated using the procedures described by Canagaratna et al. (2015). Example data are shown in Fig. S2.

2.3.3 Estimation of aerosol liquid water content (LWC)

Aerosol LWC includes water associated with inorganic aerosol and OA, which were calculated using a thermodynamic model and an empirical method, respectively. Inorganic LWC (W_i) in moles per liter (mol L⁻¹) was predicted by ISORROPIA-II in forward mode (Fountoukis and Nenes, 2007). Inputs for ISORROPIA-II include inorganic aerosol mass concentrations (${\mathrm{SO}}_{\mathrm{4}}^{\mathrm{2}-}$, inorganic ${\mathrm{NO}}_{\mathrm{3}}^{-}$ and ${\mathrm{NH}}_{\mathrm{4}}^{+}$) and meteorological parameters (temperature and RH). Calculation of empirical organic LWC (W_O) follows (Petters and Kreidenweis, 2007; Guo et al., 2015)

$$\begin{array}{}\text{(8)}& W_{\text{O}}={\displaystyle \frac{m_{\text{org}}{\mathit{\rho }}_{\text{w}}}{{\mathit{\rho }}_{\text{org}}}}{\displaystyle \frac{{\mathit{\kappa }}_{\text{org}}}{\mathrm{1}/{\text{RH}}^{-\mathrm{1}}}},\end{array}$$

where m_org is the organic mass concentration (µg m⁻³) and ρ_w is the density of water (1 g cm⁻³). The organic density (ρ_org, g cm⁻³) was estimated using an empirical equation based on elemental ratios (Kuwata et al., 2012; Guo et al., 2015):

$$\begin{array}{}\text{(9)}& {\mathit{\rho }}_{\text{org}}=\mathrm{1000}\times \left[{\displaystyle \frac{\mathrm{12}+\frac{\text{H}}{\text{C}}+\mathrm{16}\times \frac{\text{O}}{\text{C}}}{\mathrm{7.0}+\mathrm{5}\times \frac{\text{H}}{\text{C}}+\mathrm{4.15}\times \frac{\text{O}}{\text{C}}}}\right].\end{array}$$

The hygroscopicity of SOA generated during chamber studies under subsaturated regimes depends on the OA degree of oxidation (Prenni et al., 2007; Jimenez et al., 2009; Petters et al., 2009; Chang et al., 2010). A simple linear relationship between the OA degree of oxidation (defined as the fraction of the total signal at m∕z 44, f₄₄) and hygroscopicity (κ_org) is used (Duplissy et al., 2011):

$$\begin{array}{}\text{(10)}& {\mathit{\kappa }}_{\text{org}}=\mathrm{2.2}\times f_{\mathrm{44}}-\mathrm{0.13}.\end{array}$$

The total LWC is then found by summing the water content associated with each mass fraction:

$$\begin{array}{}\text{(11)}& \text{LWC}=W_{\text{i}}+W_{\text{O}}.\end{array}$$

Figure 1Time series of data collected at UHSL in Houston during the sampling periods in winter and summer 2014. Time series of 5 min average campaign data for (a, f) ambient temperature (T), relative humidity (RH) and total cloud cover (%, with 3 h interval); (b, g) precipitation and wind direction (WD), with colors showing different wind speeds (WS); (c, h) CO, SO₂ and NO₂; (d, i) O₃, O_x (NO₂+O₃) and solar radiometer; (e, j) NR-PM₁ species, including OA, ${\mathrm{NO}}_{\mathrm{3}}^{-}$, ${\mathrm{SO}}_{\mathrm{4}}^{\mathrm{2}-}$, ${\mathrm{NH}}_{\mathrm{4}}^{+}$ and Cl⁻.

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Table 1Statistics of meteorological parameters, gas-phase pollutants, NR-PM₁ species and PMF OA factors for the winter and summer campaigns at UHSL.

^a BDL: below detection limit. ^b Statistically determined factor concentrations with values below $\mathrm{1.0}\times {\mathrm{10}}^{-\mathrm{3}}$ are listed as 0.

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3.1 Temporal dependences of submicron aerosol composition

Campaign overview data for winter and summer are shown in Table 1 and Fig. 1. This includes meteorological parameters (e.g., temperature, RH, radiometer, precipitation, wind direction and speed), trace gases (e.g., CO, SO₂, NO₂ and O₃) and chemically resolved NR-PM₁ concentrations.

Data indicate that the average concentration of NR-PM₁ during the winter campaign was 6.0 ± 3.7 µg m⁻³, ranging from 0.5 to 14.8 µg m⁻³. Mass loadings of NR-PM₁ at this measurement site are relatively smaller than at a site near the HSC in winter 2015 (10.8 µg m⁻³, Wallace et al., 2018), perhaps suggesting a weaker industrial influence at the UHSL site.

The average concentration of NR-PM₁ during summer was 3.6 ± 1.7 µg m⁻³, ranging from 0.3 to 13.7 µg m⁻³. For comparison, a summer campaign in 2006 on an elevated building near downtown Houston showed an average NR-PM₁ concentration of approximately 11 µg m⁻³ (Cleveland et al., 2012). An elevated NR-PM₁ episode was observed from 28 to 31 May (Fig. 1j), with high solar radiation and O_x (O_x = NO₂+O₃) levels during the daytime, and high RH at night, resulting in OA becoming the largest fractional species, likely due to gas-phase photochemical production of SOA together with the nighttime increase in SOA associated with high RH, lowered boundary layer and cooler temperatures.

In winter, OA was the largest component of NR-PM₁, accounting for 38 % ± 13 % on average of the total mass, followed by ${\mathrm{SO}}_{\mathrm{4}}^{\mathrm{2}-}$ (23 % ± 11 %), ${\mathrm{NO}}_{\mathrm{3}}^{-}$ (23 % ± 11 %), ${\mathrm{NH}}_{\mathrm{4}}^{+}$ (15 % ± 5 %) and Cl⁻ (1 % ± 0.2 %) (Fig. 2). Primary OA (POA = HOA + BBOA + COA) was responsible for 61 % ± 19 % of OA mass. Secondary species (${\mathrm{SO}}_{\mathrm{4}}^{\mathrm{2}-}+{\mathrm{NO}}_{\mathrm{3}}^{-}+{\mathrm{NH}}_{\mathrm{4}}^{+}+\mathrm{LO}-\mathrm{OOA}+\mathrm{MO}-\mathrm{OOA}$) accounted for ∼ 76 % ± 21 % of NR-PM₁ mass, which is higher than that in winter in Seoul (Kim et al., 2017) and Beijing (Hu et al., 2016).

In contrast to winter, OA during the summer campaign constituted on average 47 % ± 18 % of NR-PM₁ mass, and ${\mathrm{SO}}_{\mathrm{4}}^{\mathrm{2}-}$ was the second largest component (36 % ± 15 %), followed by ${\mathrm{NH}}_{\mathrm{4}}^{+}$ (14 % ± 5 %). ${\mathrm{NO}}_{\mathrm{3}}^{-}$ only accounted for 2 % ± 1 % of NR-PM₁ mass in the summer, and Cl⁻ contributed 1 % ± 0.5 % of NR-PM₁ mass. The increased planetary boundary layer (PBL) height in summer (Haman et al., 2012) likely contributed to relatively lower trace gas and NR-PM₁ levels in the summer. Secondary species contributed ∼ 88 % ± 15 % of NR-PM₁ mass, indicating that the relative importance of secondary aerosol formation increased during summer as compared to winter, especially for species such as ${\mathrm{SO}}_{\mathrm{4}}^{\mathrm{2}-}$ and MO-OOA.

Figure 2Average composition of NR-PM₁ species and OA factors during the winter (a, c) and summer campaign (b, d) at UHSL.

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The total OA displayed high values during the nighttime hours in both winter and summer, maintaining a high level until morning rush hour and then decreasing to a minimum value after 09:00 (Fig. 3). The summertime OA presented a small peak at noon, suggesting that photochemical formation of OA played a more important role in summer than in winter. Increasing ambient temperature and PBL height after sunrise causes repartitioning to the gas phase, likely contributing to the decrease in OA, LO-OOA and ON during daytime.

Contributions of PMF factors to wintertime and summertime OA show significant differences. For wintertime OA, on average, BBOA contributed to 26 % of OA mass; MO-OOA and COA made the same contributions of 22 % to total OA mass. The LO-OOA accounted for 17 % of OA mass, followed by HOA (13 %). The POA constituted more than half of OA mass (61 %), with the remainder being OOA (39 %). In the summer, LO-OOA represented the largest fraction of the OA mass (54 % on average), followed by MO-OOA (23 %), HOA (15 %) and BBOA (8 %). In the case of summer, OOA constituted 77 % of OA and 36 % of total NR-PM₁ mass, which are almost 2 times their relative contributions in winter. The time series of mass concentrations of NR-PM₁ species (Fig. 1) and OA factors (Fig. 4) in summer were relatively stable and repeatable, while they varied dramatically in winter due to the different meteorological conditions.

Figure 3Diurnal profiles of the five NR-PM₁ species (OA, ${\mathrm{SO}}_{\mathrm{4}}^{\mathrm{2}-}$, ${\mathrm{NO}}_{\mathrm{3}}^{-}$, ${\mathrm{NH}}_{\mathrm{4}}^{+}$ and Cl⁻), PMF-resolved factors (HOA, BBOA, COA, LO-OOA and MO-OOA), radiometer, O_x, SO₂, $f_{{\mathrm{SO}}_{\mathrm{4}}}$ and elemental ratios (H ∕ C, O ∕ C, OM ∕ OC and N ∕ C). Lines denote the mean value, and bars represent the 5 % and 95 % confidence interval in the mean (blue for winter, green for summer).

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3.2 Seasonal variation of the formation of sulfate and nitrate

During the summer campaign, the prevailing southerly winds from the Gulf of Mexico carry marine aerosols to Houston (Schulze et al., 2018), resulting in a relatively high fraction of ${\mathrm{SO}}_{\mathrm{4}}^{\mathrm{2}-}$. As shown in Fig. 1g and j, the increased contribution of ${\mathrm{SO}}_{\mathrm{4}}^{\mathrm{2}-}$ occurred when winds originated from the south at a high speed (e.g., 16–27 May), while the contribution of ${\mathrm{SO}}_{\mathrm{4}}^{\mathrm{2}-}$ decreased significantly when winds originated from the north (e.g., 10 May and 13–15 May). During periods of southerly winds, O ∕ C and OM ∕ OC were relatively higher (Fig. S2c). In addition, elevated SO₂ plumes were recorded during periods of southerly winds (Fig. 1g, h), potentially as a result of emissions from the Parish coal-fired power plant. In contrast to ${\mathrm{SO}}_{\mathrm{4}}^{\mathrm{2}-}$, the fractional contribution of ${\mathrm{NO}}_{\mathrm{3}}^{-}$ and OA increased greatly when the winds were not southerly. Primary pollutants such as CO and NO₂ were elevated when winds were northerly (Fig. 1h), accompanied by lower O ∕ C and higher H ∕ C ratios during the corresponding periods (Fig. S2c, e.g., 1, 2, 10, 15 May).

Figure 4Time series of each OA factor and associated correlated species for the winter and summer campaign at UHSL.

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Figure 5Diurnal profiles of the estimated range of nitrate functionality from organic nitrate (a) and inorganic nitrate (b) for the winter and summer campaigns. Estimated water associated with inorganic and organic aerosol for the winter (c) and summer campaigns (d). Solid lines denote the mean value (blue for winter, green for summer), and bars represent the 5 % and 95 % confidence interval in the mean.

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Diurnal patterns of NR-PM₁ and other species in the winter and summer (Fig. 3) suggest significant seasonal dependence of sources and formation processes of NR-PM₁ species in Houston. In the case of ${\mathrm{SO}}_{\mathrm{4}}^{\mathrm{2}-}$, the diurnal pattern displayed a daytime peak in both winter and summer, with the peak much more pronounced in summer midday. In winter, the $f_{{\mathrm{SO}}_{\mathrm{4}}}$ (mole ratio of [${\mathrm{SO}}_{\mathrm{4}}^{\mathrm{2}-}$] to the sum of [SO₂] and [${\mathrm{SO}}_{\mathrm{4}}^{\mathrm{2}-}$]) and LWC have concurrent peak values during the night (Fig. 5). However, there is no obvious correlation between $f_{{\mathrm{SO}}_{\mathrm{4}}}$ and LWC in summer, though a moderate correlation (r=0.44) was found in winter. By comparing the diurnal plots of sulfate in winter and summer, it appears that sulfate generated from aqueous chemistry accounted for more mass and a greater fraction of total sulfate production in winter than in summer.

The total nitrate concentration was higher in winter than in summer. The ${\mathrm{NO}}_{\mathrm{3},\mathrm{AN}}^{-}$ was very low in summer due to its thermal instability under high temperature, while it was relatively enhanced in winter. According to the ${\mathrm{NO}}_x^{+}$ ratio method described in Sect. 2.3.1, the mass fraction of ${\mathrm{NO}}_{\mathrm{3},\mathrm{AN}}^{-}$ in total nitrate was in the range of 65 %–66 % in winter and in the range of 19 %–39 % in summer. The averaged bound concentrations of NO_3,ON ranged from 0.22 to 0.34 µg m⁻³ in winter and 0.05 to 0.06 µg m⁻³ in summer. The seasonal variation of ${\mathrm{NO}}_{\mathrm{3},\mathrm{AN}}^{-}$ is much stronger than that of NO_3,ON. This is in accordance with previous observations in Atlanta, Georgia; and Centreville, Alabama (Xu et al., 2015).

Figure 6Mass spectra of PMF-resolved OA factors (a, c) and correlation coefficients between OA factors and other variables (tracer ions, trace gas, meteorological parameters, etc.) (b, d) for winter and summer campaigns at UHSL.

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The diurnal profiles of NO_3,ON show that it reached peak value before dawn in both seasons (Fig. 5). However, ${\mathrm{NO}}_{\mathrm{3},\mathrm{AN}}^{-}$ presents a bimodal diurnal profile in both seasons. The ${\mathrm{NO}}_{\mathrm{3},\mathrm{AN}}^{-}$, which increased from late afternoon and peaked at 02:00–04:00, was likely formed through nighttime chemistry from dinitrogen pentoxide (N₂O₅) hydrolysis, as the LWC displayed a trend similar to that of ${\mathrm{NO}}_{\mathrm{3},\mathrm{AN}}^{-}$. This was corroborated by the observation of O_x (>25 ppb), which is needed to form N₂O₅ (via ${\mathrm{NO}}_{\mathrm{3}}^{\mathrm{•}}$). The second peak observed during morning rush hour was likely formed through photochemical processing of NO_x emitted from vehicles because the traffic flow and O_x level are elevated during morning rush hour. The decreasing trend of ${\mathrm{NO}}_{\mathrm{3},\mathrm{AN}}^{-}$ after 09:00 is presumed to be a result of enhanced PBL height and evaporation.

The estimated ON accounted for 4 %–8 % of the total NR-PM₁ and 9 %–17 % of the OA in summer and 12 %–27 % of the total NR-PM₁ and 31 %–66 % percent of the OA in winter, comparable to other studies (Fry et al., 2009; Rollins et al., 2010; Xu et al., 2015; Berkemeier et al., 2016). A proxy for the ${\mathrm{NO}}_{\mathrm{3}}^{\mathrm{•}}$ production rate is based on the product of the observations of [NO₂] and [O₃] (Rollins et al., 2012), where brackets represent mixing ratios in parts per billion (ppb). The O_x (> 25 ppb) and elevated NO_x observed at night in summer (Fig. 3) resulted in rapid ${\mathrm{NO}}_{\mathrm{3}}^{\mathrm{•}}$ formation. Thus, the concurrent enhancement in ON and O₃ times NO₂ occurring during nighttime (Fig. S4) likely indicates the nocturnal ${\mathrm{NO}}_{\mathrm{3}}^{\mathrm{•}}$-initiated oxidation of anthropogenic and biogenic VOCs, with the latter probably being larger than the former (Brown et al., 2013). The high N ∕ C ratio of LO-OOA, concurrent peak value in LO-OOA and ON (MW = 231 g mol⁻¹) during nighttime hours (Fig. 3), and appreciable correlation of LO-OOA and ON in summer (r=0.73) (Fig. 4) together suggest that particle-phase ON from ${\mathrm{NO}}_{\mathrm{3}}^{\mathrm{•}}$-initiated chemistry contributed to nighttime LO-OOA in summer.

3.3 Effects of aqueous-phase and photochemical oxidation on OOA formation

On average, OOA accounted for 39 % ± 19 % of OA mass in winter but increased to 77 % ± 16 % in summer. Note that MO-OOA accounted for more than half of OOA in winter (56 %), indicating the more important role of MO-OOA in winter as compared to LO-OOA on a relative basis. In contrast, LO-OOA dominated OOA in summer (70 %). The mass spectra of MO-OOA in winter and summer are similar (Fig. 6, r=0.84) as is the extent of oxidation (O ∕ C = 1.10 vs. 1.07). However, LO-OOA in winter showed a different spectral pattern compared with that in summer. The mass spectrum of LO-OOA in winter was characterized by high m∕z 32 (mainly CH₄O⁺) and 46 (mainly ${\mathrm{CH}}_{\mathrm{2}}{\mathrm{O}}_{\mathrm{2}}^{+}$) peaks, resulting in a relatively high O ∕ C (0.89) in winter that suggest LO-OOA in winter was more aged than that in summer (O ∕ C = 0.74).

Table 2Correlation (r) of OOA mass spectra with the previously published spectra database (http://cires1.colorado.edu/jimenez-group/HRAMSsd/, last access: 9 July 2019).

^* aq-OOA is an aqueous-phase-processed SOA reported by Sun et al. (2016); LV: less volatile; SV: semivolatile.

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Sun et al. (2016) reported a unique OOA in ambient air, termed aq-OOA (aqueous-phase-processed SOA), that strongly correlated with particle LWC, sulfate and S-containing ions. As shown in Table 2, by comparing the mass spectra of OOA in this work with aq-OOA, it is found that the mass spectra of MO-OOA in winter in this study present a much stronger correlation (r=0.96) with aq-OOA than does LO-OOA in winter in this study (r=0.75). Both MO-OOA and LO-OOA in summer highly correlated with aq-OOA. This result indicates that the formation of LO-OOA in summer and MO-OOA in both seasons may involve aqueous-phase chemistry.

Assuming that OOA deduced from PMF analysis can be used as a surrogate of SOA (Wood et al., 2010; Xu et al., 2017), the two OOA were used to investigate the formation mechanisms and evolutionary processes of SOA. Previous studies have found SOA correlated well with odd oxygen (O_x) in many cities (Wood et al., 2010; Sun et al., 2011; Hayes et al., 2013; Zhang et al., 2015; Xu et al., 2017) and that SOA formation is significantly impacted by aqueous-phase processing (Lim et al., 2010; Ervens et al., 2011; Xu et al., 2017). The relationships between OOA factors and O_x ∕ LWC were used as the metrics to characterize SOA formation mechanisms associated with photochemistry/aqueous oxidation chemistry (Xu et al., 2017).

Figure 7OA mass and frequency histograms of data points in each LWC bin for winter (a) and summer (b). Variations of RH, WS, LO-OOA and MO-OOA mass as a function of LWC in winter (c, e, g, i) and summer (d, f, h, j). The data were binned according to the LWC (with different increment values), and mean (circle), median (horizontal line), 25th and 75th percentiles (lower and upper box), and 5th and 95th percentiles (lower and upper whiskers) are displayed for data in each bin.

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Figure 7a and b indicate the LWC frequency distribution. Winter LWC values are binned in 5 µg m⁻³ increments from 0 to 20 µg m⁻³. Data in the ranges of 20 to 30 µg m⁻³, 30 to 50 µg m⁻³, 50 to 80 µg m⁻³, and 80 to 120 µg m⁻³ are shown as 25, 40, 65 and 100 µg m⁻³, respectively. Summer LWC values are binned in 2.5 µg m⁻³ increments from 0 to 15 µg m⁻³. The bins shown as 17.5 and 27.5 µg m⁻³ represent data from 15 to 20 µg m⁻³ and 20 to 35 µg m⁻³. It should be noted that a fit for the binned data likely results in an increase in R² compared to the fit for the original data. For example, the correlation coefficient of the fit for the averaged binned wintertime MO-OOA (increased from 0.57 to 0.98 µg m⁻³) vs. LWC (increased from 2.5 to 40 µg m⁻³) is 0.55, while it is 0.06 for the original data (Fig. 7i).

The data associated with the artificially created bins in both seasons did not pass the normal test and homogeneity test of variances. The statistical significance of differences between bins was then tested using the Kruskal–Wallis analysis of variance (K–W ANOVA). The differences between winter and summer data of the bins were significant. Thus, the Dunn–Bonferroni test was performed for the post hoc pairwise comparisons. It was found that the difference of all measured variables in different bins shown in Fig. 7 were significant (p<0.01). The results can be found in Tables S6–S7. Figure 7c and d present a clear positive trend of RH as a function of LWC in both winter and summer, which implies an increased potential for aqueous-phase processing at high RH level, enhanced by low wind speed that allows accumulation of pollutants (Fig. 7e, f). The patterns of other parameters as LWC increases in winter were different from those in summer.

The variation of binned mean OA mass against LWC presents a significant seasonal difference (Fig. 7a, b). In winter, the OA mass increased when LWC increased from 2.5 to 12.5 µg m⁻³ but decreased as the LWC increased further. In summer, the OA mass slightly decreased when LWC increased from 1.25 to 6.25 µg m⁻³ but slightly increased when LWC increased further, suggesting the production of OA is not as strong as that in winter because of the relatively lower LWC in summer.

The winter LO-OOA mass decreased dramatically when LWC > 12.5 µg m⁻³ (RH > 80 %, Fig. 7c), while MO-OOA continues increasing until LWC > 40 µg m⁻³. This result indicates that wet removal may dominate under an extremely high RH environment coupled with stagnant air (WS < 2 m s⁻¹, Fig. 7e) or that LO-OOA production decreased at extremely high LWC level (Fig. 7a). Specifically, average LO-OOA (Fig. 7g, h) in winter increased from 0.3 to 0.9 µg m⁻³ when LWC increased from 2.5 to 7.5 µg m⁻³ but decreased as the LWC increased further, particularly when LWC > 40 µg m⁻³. The slope of this decrease was approximately −0.008 µg LO-OOA per µg of LWC. Figure 7a shows that 64 % of the data points were observed in the situation of low LWC (< 12.5 µg m⁻³, RH < 80 %), when the increase in LO-OOA was largest.

In contrast, LO-OOA in summer showed a decreasing trend under low LWC level (LWC < 6.25 µg m⁻³, RH < 80 %) but an increasing trend from approximately 0.77 to 1.8 µg m⁻³ as LWC increased from 6.25 to 27.5 µg m⁻³, a slope of 0.053 µg LO-OOA per µg of LWC. The relatively high summer LO-OOA under low LWC level was likely more regional, with contributions from possibly transported nonaqueous OOA, as the wind speed in this case was relatively high and RH was low. The production of LO-OOA under high LWC level may have been enhanced by local aqueous-phase heterogeneous chemistry.

Figure 8Scatter plots of nighttime OOA vs. LWC for the winter and summer campaign. The linear equations are given for fitting only the green dots. Solid dots denote the average value of data in each bin. Bars indicate standard deviations.

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MO-OOA slightly increased during both seasons as LWC increased (Fig. 7i, j). In winter, MO-OOA presented an increasing trend from 0.57 to 0.98 µg m⁻³ when LWC increased from 2.5 to 40 µg m⁻³ but decreased slightly as the LWC increased further. The slope of this increase was approximately 0.008 µg MO-OOA per µg of LWC with a correlation coefficient of 0.55. In summer, MO-OOA appears to increase from 0.49 to 0.64 µg m⁻³ when LWC increased from 2.5 to 27.5 µg m⁻³, with a slope of 0.005 µg MO-OOA per µg of LWC (R²=0.34). In winter, because of the decrease in LO-OOA with LWC, the relative fraction of MO-OOA increases as LWC increases.

The mutual effect of aqueous-phase and photochemistry on OOA formation prevents solely evaluating the role of the two processes. Sullivan et al. (2016) reported multiple lines of evidence for local aq-SOA formation observed in the Po Valley, Italy, during times of increasing RH, which coincided with dark conditions. Thus, the daytime data were separated to examine the variation of OOA against O_x. The relationship between OOA and aqueous-phase chemistry was investigated further by excluding the daytime data, with the aim of diminishing the instantaneous influence of photochemistry on the data. To do so, nighttime and daytime were based on sunrise and sunset in Houston during the two campaigns (https://www.timeanddate.com/sun/usa/houston, last access: 9 July 2019). On average, the day lengths are 11 h 10 min and 13 h 35 min for the campaigns in February and May 2014, respectively.

A potential linear relationship between OOA and LWC for the nighttime data was investigated by fitting the data with a locally weighted scatter plot smoothing algorithm (LOWESS, Cleveland, 1981). According to the LOWESS curves for the original nighttime data and the resampled data obtained by a bootstrap method (Figs. S14–15), there likely exists a linear relationship between LO-OOA and LWC for data points with LWC less than 20 µg m⁻³ and greater than 6 µg m⁻³ for the winter and summer periods, respectively. As for MO-OOA, such a linear relationship likely exists when LWC is less than 50 and 7 µg m⁻³ for the winter and summer periods, respectively.

Figure 8 presents the scatter plots of OOA vs. LWC during nighttime for the two campaigns. The green dots denote the increasing trend of OOA against LWC. It is found that the increase in wintertime LO-OOA under low LWC level (< 20 µg m⁻³) during the night is stronger than that shown in Fig. 7g. The nighttime LO-OOA linearly increased from 0.04 to 0.64 µg m⁻³ when LWC increased from 2.5 to 17.5 µg m⁻³, a slope of 0.033 µg LO-OOA per µg of LWC. This result indicates that the nighttime production of LO-OOA in winter may be more likely formed via aqueous-phase chemistry in aerosol liquid water than that in daytime. The production of LO-OOA under high LWC level (LWC > 6.25 µg m⁻³) in summer during nighttime (0.055 µg LO-OOA per µg of LWC) was comparable to the increase rate of whole dataset (0.053 µg LO-OOA per µg of LWC). The nighttime increasing trends of MO-OOA against LWC in both seasons are stronger than those shown in Fig. 7i and j with respect to the correlation coefficient values. The slope of the nighttime increase in MO-OOA against LWC during the winter campaign was 0.013 µg MO-OOA per µg of LWC, which is 1.7 times the slope for the whole dataset (daytime and nighttime). For the summer campaign, the increase in nighttime MO-OOA is 2.2 times the rate for the whole dataset.

These results suggest that aqueous-phase processing likely has a strong positive impact, particularly at night, on the production of MO-OOA in the two seasons except for instances when LWC exceeds 100 µg m⁻³ in winter. It also appears to facilitate the local production of LO-OOA under low LWC level (< 17.5 µg m⁻³) in winter and under relatively high LWC level (> 6.25 µg m⁻³) in summer.

Figure 9OA mass and frequency histograms of data points in each O_x bin for winter (a) and summer (b). Variations of solar radiation, RH, LO-OOA and MO-OOA mass as a function of LWC in winter (c, e, g, i) and summer (d, f, h, j). The data were binned according to the O_x (10 ppb increment), and mean (circle), median (horizontal line), 25th and 75th percentiles (lower and upper box), and 5th and 95th percentiles (lower and upper whiskers) are displayed for data in each bin.

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As mentioned previously, ON contributes significantly to summertime LO-OOA, and the concurrent enhancement in ON and LO-OOA during night was associated with elevated RH (Fig. 3). A previous study found that the partitioning of organic compounds to the particle phase was significantly increased at elevated RH levels (70 %) in an urban area dominated by biogenic emissions in Atlanta (Hennigan et al., 2008). The correlation of ON and LO-OOA in summer nighttime (r=0.76) was stronger than that during daytime (r=0.53). This is likely due to the higher ON yields from ${\mathrm{NO}}_{\mathrm{3}}^{\mathrm{•}}$-initiated chemistry involving BVOCs during nighttime compared to hydroxyl-radical-initiated chemistry involving BVOCs during daytime. Additionally, the concurrent enhancement of the LWC and nitrate functionality from organic nitrate during nighttime demonstrates that the LWC does not inhibit increases in concentration, as might be expected if hydrolysis occurred rapidly.

Figure 9a and b present the frequency distribution of O_x. Winter O_x are binned in 10 ppb increments from 0 to 60 ppb. The range for summer is 20 to 70 ppb. The data associated with the artificially created O_x bins in both seasons did not pass the normal test and homogeneity test of variances. The K–W ANOVAs for winter and summer data of the bins were significant. The Dunn–Bonferroni test for the post hoc pairwise comparisons shows that the differences of measured variables among different bins shown in Fig. 9 were significant (Tables S8–S9). The clear positive relationship between solar radiation and O_x is shown in Fig. 9c and d, and the negative relationship between RH and O_x is shown in Fig. 9e and f, confirming strong atmospheric photochemical activity associated with high-O_x periods.

The variations of LO-OOA and MO-OOA showed substantially different patterns with increases in O_x in winter and summer. In winter, LO-OOA and MO-OOA showed comparable increasing trends at low O_x level (< 35 ppb), with MO-OOA having a stronger response. The LO-OOA increased from 0.13 to 0.72 µg m⁻³ when O_x increased from 5 to 35 ppb but decreased as the O_x increased further. The slope of this increase was approximately 0.023 µg LO-OOA per ppb of O_x. MO-OOA increased from 0.13 to 0.88 µg m⁻³ when the O_x increased from 5 to 35 ppb, with a slope of 0.027 µg MO-OOA per ppb of O_x. This leads to a maximum in the mass fraction of MO-OOA as O_x approached its highest observed levels.

In summer, there is a clear decreasing trend of RH with increases in O_x. As discussed previously, the high level of summertime LO-OOA likely was associated with high LWC. Therefore, the high mass fraction of LO-OOA at the lowest O_x level (< 20 ppb) associated with the high RH ∕ LWC was likely from aqueous-phase chemistry. After excluding low-O_x data (< 20 ppb), LO-OOA showed a much stronger response to O_x than did MO-OOA. The summer LO-OOA increased from approximately 0.6 to 1.8 µg m⁻³ when O_x increased from 25 to 65 ppb, a slope of 0.03 µg LO-OOA per ppb of O_x. This increase was likely in the case of low-RH conditions (< 80 %, Fig. 7d), when aqueous-phase chemistry was less likely to promote the production of LO-OOA (Fig. 7h). Summer MO-OOA increased from 0.36 to 0.67 µg m⁻³ when O_x increased from 25 to 55 ppb but decreased as the O_x increased further. The slope of this increase was 0.007 µg MO-OOA per ppb of O_x. Contrary to winter, LO-OOA in summer responded more strongly to increases in O_x than MO-OOA did.

The relationship of OOA vs. O_x was examined further by excluding nighttime data. According to the LOWESS curves for the original daytime data and the resampled data obtained using a bootstrap method (Figs. S16–S17), there likely exists a linear relationship between LO-OOA and O_x when O_x is less than 35 ppb and greater than 20 ppb for the winter and summer period, respectively. As for MO-OOA, the linear relationship likely exists for data points with O_x less than 35 ppb for the winter period, but it is less prominent.

Figure 10 presents the scatter plots of daytime OOA vs. O_x for the winter and summer campaign. The daytime responses of LO-OOA and MO-OOA to O_x in winter were ∼ 1.5 times that for the whole dataset (Fig. 9g, i), and the increase rate of MO-OOA was higher than that of LO-OOA. In summer, the slope of the daytime increase in LO-OOA was 1.24 times that for the whole campaign (Fig. 9h). These results suggest that the photochemical enhancement of OOA in winter on a per-O_x basis was more prominent than that in summer. For the summer campaign, the formation of LO-OOA appears to be more strongly linked to photochemistry compared to MO-OOA.

Figure 10Scatter plots of daytime OOA vs. O_x for the winter and summer campaign. The linear equations are given for fitting the green dots. Bars indicate standard deviations.

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Figure 11O_x vs. LWC dependence of the ratio of MO-OOA ∕ LO-OOA in winter (a) and summer (b).

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The combined effects of photochemistry and aqueous-phase chemistry on OOA composition during winter and summer are further demonstrated in Fig. 11. The ratio of MO-OOA ∕ LO-OOA in winter showed the highest values on the top-left corner in Fig. 11a, suggesting photochemical processing was likely responsible for MO-OOA formation, under low LWC levels (< 10 µg m⁻³). Additionally, data with high MO-OOA ∕ LO-OOA on the bottom-right corner in Fig. 11a indicate the important role of aqueous-phase chemistry under low O_x and high LWC levels. Overall, the concentration of MO-OOA in winter increased as O_x ∕ LWC increased, whereas LO-OOA markedly decreased. This result indicates both photochemical and aqueous-phase processing played a more important role in enhancing MO-OOA than LO-OOA in winter.

In summer, data points with a low MO-OOA ∕ LO-OOA value on the top left of Fig. 11b illustrated that LO-OOA was enhanced in high-O_x and low-LWC conditions, though the low MO-OOA ∕ LO-OOA values are not confined to just the top left. In the case of a high LWC level (LWC > 6.5 µg m⁻³), MO-OOA ∕ LO-OOA were much lower (on the right of Fig. 11b, particularly when LWC > 10 µg m⁻³). Although MO-OOA increased with LWC and O_x, the increase in LO-OOA was more significant. The effects of both photochemistry (≥ 25 ppb) and aqueous-phase chemistry (≥ 6.5 µg m⁻³) were more relevant for the formation of LO-OOA than MO-OOA. On average, the mass concentration of LO-OOA was elevated by nearly 1.2 µg m⁻³ as a ∼ 20 µg change in LWC (increased from 6.25 to 27.5 µg m⁻³, Fig. 7h), which is equivalent to a 40 ppb change in O_x (increased from 25 to 65 ppb, Fig. 9h). This result further suggests that the aqueous-phase chemistry is comparable to photochemistry in processing LO-OOA in summer.