Comparisons of Upper Tropospheric Ozone at a Coastal and Urban Site and the Impact of Local Surface Emissions

Abstract

This study quantified the impact of urban emission sources on the enhancement of ozone (O3) in the troposphere using data from simultaneous ozonesonde launches at Half Moon Bay (HMB), CA and San Jose (SJ), CA on eleven dates in July and August 2018. The urban O3 enhancement in the SJ vertical profile was derived by subtracting out HMB, which represents baseline O3, from the SJ profile. This enhancement was averaged into 1 km layers and statistically analyzed with surface emission concentration data from three locations in SJ, however this did not reveal any discernable trends between the surface and O3 profile. Within the planetary boundary layer (PBL), SJ O3 was 20-30 ppb higher than HMB for all dates. Above the PBL, most enhancement profiles remained close to zero, indicating few differences aloft between HMB and SJ. The two sites also had strong CCs and a least squares regression for all vertical levels between SJ and HMB revealed a slope of 0.79 and intercept of 0.02. The percent contribution of mixing layer (ML) O3 to tropospheric O3 and tropospheric O3 to total column O3 revealed similar results. The contribution of ML O3 was low for all dates, between 2-6%, whereas the tropospheric contribution was 11-18%. These findings emphasize the importance of baseline O3 in regions with reduced vertical mixing; polluted urban air near the surface has minimal impacts on O3 concentrations above the PBL.