Seasonal differences in formation processes of oxidized organic aerosol near Houston, TX

Abstract

Abstract. Submicron aerosol was measured to the southwest of Houston, Texas, during winter and summer 2014 to investigate its seasonal variability. Data from a high-resolution time-of-flight aerosol mass spectrometer (HR-ToF-AMS) indicated that organic aerosol (OA) was the largest component of nonrefractory submicron particulate matter (NR-PM1) (on average, 38 % ± 13 % and 47 % ± 18 % of the NR-PM1 mass loading in winter and summer, respectively). Positive matrix factorization (PMF) analysis of the OA mass spectra demonstrated that two classes of oxygenated OA (less- and more-oxidized OOA, LO and MO) together dominated OA mass in summer (77 %) and accounted for 39 % of OA mass in winter. The fraction of LO-OOA (out of total OOA) is higher in summer (70 %) than in winter (44 %). Secondary aerosols (sulfate + nitrate + ammonium + OOA) accounted for ∼76 % and 88 % of NR-PM1 mass in winter and summer, respectively, indicating NR-PM1 mass was driven mostly by secondary aerosol formation regardless of the season. The mass loadings and diurnal patterns of these secondary aerosols show a clear winter–summer contrast. Organic nitrate (ON) concentrations were estimated using the NOx+ ratio method, with contributions of 31 %–66 % and 9 %–17 % to OA during winter and summer, respectively. The estimated ON in summer strongly correlated with LO-OOA (r=0.73) and was enhanced at nighttime. The relative importance of aqueous-phase chemistry and photochemistry in processing OOA was investigated by examining the relationship of aerosol liquid water content (LWC) and the sum of ozone (O3) and nitrogen dioxide (NO2) (Ox = O3+NO2) with LO-OOA and MO-OOA. The processing mechanism of LO-OOA apparently was related to relative humidity (RH). In periods of RH < 80 %, aqueous-phase chemistry likely played an important role in the formation of wintertime LO-OOA, whereas photochemistry promoted the formation of summertime LO-OOA. For periods of high RH > 80 %, these effects were opposite those of low-RH periods. Both photochemistry and aqueous-phase processing appear to facilitate increases in MO-OOA concentration except during periods of high LWC, which is likely a result of wet removal during periods of light rain or a negative impact on its formation rate. The nighttime increases in MO-OOA during winter and summer were 0.013 and 0.01 µg MO-OOA per µg of LWC, respectively. The increase in LO-OOA was larger than that for MO-OOA, with increase rates of 0.033 and 0.055 µg LO-OOA per µg of LWC at night during winter and summer, respectively. On average, the mass concentration of LO-OOA in summer was elevated by nearly 1.2 µg m−3 for a ∼20 µg change in LWC, which was accompanied by a 40 ppb change in Ox.

Estimation of particulate organic nitrates from thermodenuder–aerosol mass spectrometer measurements in the North China Plain

“…Most previous studies on pON quantification are based on filter measurements using gas chromatography (GC) (O'Brien et al, 1995) and Fourier transformed infrared (FTIR) spectroscopy (Blando et al, 1998). Online measurement techniques -e.g., thermal dissociation-laser-induced fluorescence (TD-LIF) (Day et al, 2002), thermal dissociation-cavity ring-down spectroscopy (TD-CRDS) (Thieser et al, 2016) and thermal dissociation-cavity attenuated phase shift spectroscopy (TD-CAPS) (Sadanaga et al, 2016) -were also used to characterize pON in the atmosphere. The principle is to convert pON to NO 2 first and then to subtract ambient NO 2 from the total NO 2 .…”

“…Recently, pON were also quantified using a highresolution aerosol mass spectrometer (HR-AMS) mainly based on the differences of fragmentation patterns between pON and inorganic nitrate (Xu et al, 2015b;Farmer et al, 2010;Kortelainen et al, 2017;Ng et al, 2017). The "PMF method" and "NO x method" are the two most commonly used methods for pON quantification (Fry et al, 2013;Xu et al, 2015b;Yu et al, 2019;Dai et al, 2019). The PMF method, which combines organic and inorganic fragments in positive matrix factorization (PMF) analysis, was able to identify organic aerosol factors that showed a largely different NO + / NO + 2 ratio from pure ammonium nitrate (AN).…”

Increase in secondary organic aerosol in an urban environment

“…Relatedly, Zhao et al (2017) revealed the non-linear increasing relation of SOA production as a function of the NMOG-to-NO x precursors ratio (NMOG being the non-methane organic gases emitted by vehicles, which include VOCs), entailing a coupled reduction of both pollutants has to be addressed in order to reduce SOA concentrations. As well as this, Dai et al (2019) highlighted the differences in conditions of liquid water content (LWC), meteorological variables, and OX concentrations for the SOA to take the aqueous-phase chemistry or photochemistry pathways depending on the season. Despite that, there is a need for further investigation about the oxidation state of OA in consideration of different seasons, years, meteorological scenarios and precursor levels to characterize these findings in an urban background site in the WMB.…”

“…Black carbon (BC) concentrations were measured with 1 min time resolution by a multi-angle absorption photometer (MAAP, Thermofisher Model 5012). PM 1 concentrations were measured by summing up the particle size number distribution (ranging 12-470 nm) measured by a scanning mobility particle sizer (SMPS, TSI 3080) with 5 min time res-olution and converted to mass concentrations by means of the composition-dependent aerosol density (DeCarlo et al, 2004). SO 2− 4 , NO − 3 and NH + 4 concentrations were determined in off-line PM 1 filters samples by ion chromatography and selective electrode methods.…”

Estimation of particulate organic nitrates from thermodenuder-aerosol mass spectrometer measurements in North China Plain

“…Recently, pON were also quantified using high-resolution aerosol mass spectrometer (HR-AMS) mainly based on the differences of fragmentation patterns between pON and inorganic nitrate (Xu et al, 2015b;Farmer et al, 2010;Kortelainen et al, 2017;Ng et al, 2017). "PMF method" and "NO x method" are two most commonly used method for pON quantification (Fry et al, 2013;Xu et al, 2015b;Yu et al, 2019;Dai et al, 2019). The "PMF method" by combining organic and inorganic fragments in positive matrix factorization (PMF) analysis was able to identify organic aerosol factors that showed largely different NO + /NO 2 + ratio from pure ammonium nitrate (AN).…”