Measurements of submicron aerosols at the California-Mexico border during the Cal-Mex 2010 field campaign
We present measurements of submicron aerosols in Tijuana, Mexico during the Cal-Mex 2010 field campaign. A suite of aerosol instrumentations were deployed, including a hygroscopic-volatility tandem differential mobility analyzer (HV-TDMA), aerosol particle mass analyzer (APM), condensation particle counter (CPC), cavity ring-down spectrometer (CRDS), and nephelometer to measure the aerosol size distributions, effective density, hygroscopic growth factors (HGF), volatility growth factors (VGF), and optical properties. The average mass concentration of PM0.6 is 10.39 ± 7.61 μg m-3, and the derived average black carbon (BC) mass concentration is 2.87 ± 2.65 μg m-3. There is little new particle formation or particle growth during the day, and the mass loading is dominated by organic aerosols and BC, which on average are 37% and 27% of PM1.0, respectively. For four particle sizes of 46, 81, 151, and 240 nm, the measured particle effective density, HGFs, and VGFs exhibit distinct diurnal trends and size-dependence. For smaller particles (46 and 81 nm), the effective density distribution is unimodal during the day and night, signifying an internally mixed aerosol composition. In contrast, larger particles (151 and 240 nm) exhibit a bi-modal effective density distribution during the daytime, indicating an external mixture of fresh BC and organic aerosols, but a unimodal distribution during the night, corresponding to an internal mixture of BC and organic aerosols. The smaller particles show a noticeable diurnal trend in the effective density distribution, with the highest effective density (1.70 g cm-3) occurring shortly after midnight and the lowest value (0.90 g cm-3) occurring during the afternoon, corresponding most likely to primary organic aerosols and BC, respectively. Both HGFs and VGFs measured are strongly size-dependent. HGFs increase with increasing particle size, indicating that the largest particles are more hygroscopic. VGFs decrease with increasing particle size, indicating that larger particles are more volatile. The hygroscopicity distributions of smaller particles (46 and 81 nm) are unimodal, with a HGF value close to unity. Large particles typically exhibit a bi-modal distribution, with a non-hygroscopic mode and a hygroscopic mode. For all particle sizes, the VGF distributions are bimodal, with a primary non-volatile mode and a secondary volatile mode. The average extinction, scattering, and absorption coefficients are 86.04, 63.07, and 22.97 Mm-1, respectively, and the average SSA is 0.75. Our results reveal that gasoline and diesel vehicles produce a significant amount of black carbon particles in this US-Mexico border region, which impacts the regional environment and climate.