Evaluation and application of multi-decadal visibility data for trend analysis of atmospheric haze

Abstract. There are few multi-decadal observations of atmospheric aerosols worldwide. This study applies global hourly visibility (Vis) observations at more than 3000 stations to investigate historical trends in atmospheric haze over 1945–1996 for the US, and over 1973–2013 for Europe and eastern Asia. A comprehensive data screening and processing framework is developed and applied to minimize uncertainties and construct monthly statistics of inverse visibility (1/Vis). This data processing includes removal of relatively clean cases with high uncertainty, and change point detection to identify and separate methodological discontinuities such as the introduction of instrumentation. Although the relation between 1/Vis and atmospheric extinction coefficient (b_ext) varies across different stations, spatially coherent trends of the screened 1/Vis data exhibit consistency with the temporal evolution of collocated aerosol measurements, including the b_ext trend of −2.4 % yr⁻¹ (95 % CI: −3.7, −1.1 % yr⁻¹) vs. 1/Vis trend of −1.6 % yr⁻¹ (95 % CI: −2.4, −0.8 % yr⁻¹) over the US for 1989–1996, and the fine aerosol mass (PM_2.5) trend of −5.8 % yr⁻¹ (95 % CI: −7.8, −4.2 % yr⁻¹) vs. 1/Vis trend of −3.4 % yr⁻¹ (95 % CI: −4.4, −2.4 % yr⁻¹) over Europe for 2006–2013. Regional 1/Vis and Emissions Database for Global Atmospheric Research (EDGAR) sulfur dioxide (SO₂) emissions are significantly correlated over the eastern US for 1970–1995 (r = 0.73), over Europe for 1973–2008 (r ∼ 0.9) and over China for 1973–2008 (r ∼ 0.9). Consistent "reversal points" from increasing to decreasing in SO₂ emission data are also captured by the regional 1/Vis time series (e.g., late 1970s for the eastern US, early 1980s for western Europe, late 1980s for eastern Europe, and mid 2000s for China). The consistency of 1/Vis trends with other in situ measurements and emission data demonstrates promise in applying these quality assured 1/Vis data for historical air quality studies.