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The San Joaquin Valley (SJV) in California experiences persistent air-quality problems associated with elevated particulate matter (PM)concentrations due to anthropogenic emissions, topography, andmeteorological conditions. Thus it is important to unravel the varioussources and processes that affect the physicochemical properties of PM inorder to better inform pollution abatement strategies and improveparameterizations in air-quality models. During January and February 2013, a ground supersite was installed at theFresno–Garland California Air Resources Board (CARB) monitoring station,where comprehensive, real-time measurements of PM and trace gases wereperformed using instruments including an Aerodyne high-resolutiontime-of-flight aerosol mass spectrometer (HR-ToF-AMS) and an Ionicon protontransfer reaction time-of-flight mass spectrometer (PTR-TOF-MS) as part ofthe NASA Deriving Information on Surface Conditions from Column andVertically Resolved Observations Relevant to Air Quality (DISCOVER-AQ)campaign. The average submicron aerosol (PM1) concentration was 31.0 µg m−3 and the total mass was dominated by organic aerosols (OA,55 %), followed by ammonium nitrate (35 %). High PM pollution eventswere commonly associated with elevated OA concentrations, mostly fromprimary sources. Organic aerosols had average atomic oxygen-to-carbon (O / C),hydrogen-to-carbon (H / C), and nitrogen-to-carbon (N / C) ratios of 0.42, 1.70,and 0.017, respectively. Six distinct sources of organic aerosol wereidentified from positive matrix factorization (PMF) analysis of the AMSdata: hydrocarbon-like OA (HOA; 9 % of total OA, O / C  =  0.09) associatedwith local traffic, cooking OA (COA; 18 % of total OA, O / C  =  0.19)associated with food cooking activities, two biomass burning OA (BBOA1:13 % of total OA, O / C  =  0.33; BBOA2: 20 % of total OA, O / C  =  0.60) most likely associated with residential space heating from woodcombustion, and semivolatile oxygenated OA (SV-OOA; 16 % of total OA, O / C  =  0.63) and low-volatilityoxygenated OA (LV-OOA; 24 % of total OA, O / C  =  0.90) formed via chemical reactions in the atmosphere. Large differences in aerosol chemistry at Fresno were observed between thecurrent campaign (winter 2013) and a previous campaign in winter 2010, mostnotably that PM1 concentrations were nearly 3 times higher in 2013than in 2010. These variations were attributed to differences in themeteorological conditions, which influenced primary emissions and secondaryaerosol formation. In particular, COA and BBOA concentrations were greaterin 2013 than 2010, where colder temperatures in 2013 likely resulted inincreased biomass burning activities. The influence from a nighttime formedresidual layer that mixed down in the morning was found to be much moreintense in 2013 than 2010, leading to sharp increases in ground-levelconcentrations of secondary aerosol species including nitrate, sulfate, andOOA, in the morning between 08:00 and 12:00 PST. This is an indication thatnighttime chemical reactions may have played a more important role in 2013.As solar radiation was stronger in 2013 the higher nitrate and OOAconcentrations in 2013 could also be partly due to greater photochemicalproduction of secondary aerosol species. The greater solar radiation andlarger range in temperature in 2013 also likely led to both SV-OOA andLV-OOA being observed in 2013 whereas only a single OOA factor wasidentified in 2010

Influences of emission sources and meteorology on aerosol chemistry in a polluted urban environment: results from DISCOVER-AQ California