We investigate the effects of sulfate and nitrate on the formation and evolution of secondary organic aerosol formed in the aqueous phase (aqSOA) from photooxidation of two phenolic carbonyls emitted from wood burning. AqSOA was formed efficiently from the photooxidation of both syringaldehyde (CHO) and acetosyringone (CHO) in ammonium sulfate and ammonium nitrate solutions, with mass yields ranging from 30% to 120%. Positive matrix factorization on the organic mass spectra acquired by an Aerosol Mass Spectrometer revealed a combination of functionalization, oligomerization, and fragmentation processes in the chemical evolution of aqSOA. Functionalization and oligomerization dominated in the first 4 h of reaction, with phenolic oligomers and their derivatives significantly contributing to aqSOA formation; and oxidation of the first-generation products led to an abundance of oxygenated ring-opening products. Degradation rates of syringaldehyde and acetosyringone in nitrate solutions were 1.5 and 3.5 times faster than rates in sulfate solutions, and aqSOA yields in nitrate experiments are twice as high as in sulfate experiments. Nitrate likely promoted the reactions because it is a photolytic source of OH radicals, while sulfate is not, highlighting the importance of aerosol-phase nitrate in the formation of aqSOA by facilitating the photooxidation of organic precursors.

The hygroscopic tandem differential mobility analyzer (HTDMA) has been frequently used to measure the hygroscopic properties of atmospheric aerosols at a fixed high relative humidity (RH) of about 90%. To evaluate if such measurements could be used to determine the hygroscopicity of aerosols at lower RH, simultaneous hygroscopic growth factor (GF) and sizeresolved composition measurements were made with an HTDMA and a high-resolution aerosol mass spectrometer (HR-AMS), respectively, at a coastal site in Hong Kong from January to June and in August 2012. A total of 58 cycles of dehydration (decreasing RH) and hydration (increasing RH) of 100 nm and 200 nm particles with organic-to-inorganic mass ratio ranging from 0.19 to 1.97 were measured at RH D 10-93%. The Kappa (k) equation developed by Petters and Kreidenweis in the year 2007 was used to determine (i) k at individual RHs (k RH ) and (ii) best-fit k covering the range of RHs measured (k f ) for the morehygroscopic (MH) mode, which describes more than 80% of the particles in each cycle, during dehydration. Overall, k at 90% RH or above (k >90 ) fell between 0.18 and 0.48, and was within 15% of k f in 83% of the datasets. Regression analysis between k >90 or k f and AMS mass fractions showed that k was positively correlated with sulfate but negatively correlated with organic and nitrate. In most cases, k RH increased as RH decreased and the average increase in k was 45% from 90% RH to 40% RH, but these differences yielded insignificant changes in the GF-RH curves. The Zdanovskii-Stokes-Robinson (ZSR) estimated k were mostly within 20% of k >90 and k f . GF predictions using the empirical correlation of k with AMS mass fractions or the ZSR estimated k were within 10% of additional measurements and hence k >90 is useful for predicting GF at lower RHs.

Abstract. Simultaneous measurements of aerosol volatility and carbonaceous matters were conducted at a suburban site in Guangzhou, China, in February and March 2014 using a volatility tandem differential mobility analyzer (VTDMA) and an organic carbon/elemental carbon (OC ∕ EC) analyzer. Low volatility (LV) particles, with a volatility shrink factor (VSF) at 300 °C exceeding 0.9, contributed 5 % of number concentrations of the 40 nm particles and 11–15 % of the 80–300 nm particles. They were composed of non-volatile material externally mixed with volatile material, and therefore did not evaporate significantly at 300 °C. Non-volatile material mixed internally with the volatile material was referred to as medium volatility (MV, 0.4  <  VSF  <  0.9) and high volatility (HV, VSF  <  0.4) particles. The MV and HV particles contributed 57–71 % of number concentration for the particles between 40 and 300 nm in size. The average EC and OC concentrations measured by the OC ∕ EC analyzer were 3.4 ± 3.0 and 9.0 ± 6.0 µg m−3, respectively. Non-volatile OC evaporating at 475 °C or above, together with EC, contributed 67 % of the total carbon mass. In spite of the daily maximum and minimum, the diurnal variations in the volume fractions of the volatile material, HV, MV and LV residuals were less than 15 % for the 80–300 nm particles. Back trajectory analysis also suggests that over 90 % of the air masses influencing the sampling site were well aged as they were transported at low altitudes (below 1500 m) for over 40 h before arrival. Further comparison with the diurnal variations in the mass fractions of EC and the non-volatile OC in PM2.5 suggests that the non-volatile residuals may be related to both EC and non-volatile OC in the afternoon, during which the concentration of aged organics increased. A closure analysis of the total mass of LV and MV residuals and the mass of EC or the sum of EC and non-volatile OC was conducted. It suggests that non-volatile OC, in addition to EC, was one of the components of the non-volatile residuals measured by the VTDMA in this study.