Urban haze is a multifaceted threat. Foremost a major health hazard, it also affects the passage of light through the lower atmosphere. In this paper, we present a study addressing the impact of haze on the performance pf photovoltaic installations in cities. Using long-term, high resolution field data from Delhi and Singapore we derive an empirical relation between reduction in insolation and fine particulate matter (PM2.5) concentration. This approach enables a straightforward way to estimate air pollution related losses to photovoltaic power generation anywhere on the planet. For Delhi, we find that insolation received by silicon PV panels was reduced by 11.5% ± 1.5% or 200 kWh/m 2 per year between 2016 and 2017 due to air pollution. We extended this analysis to 16 more cities around the planet and estimated insolation reductions ranging from 2.0% (Singapore) to 9.1% (Beijing). Using spectrum data from Singapore, we projected how other photovoltaic technologies would be affected and found an additional reduction compared to silicon of between 23% relative for GaAs and 42% for a 1.64 eV perovskite material. Considering current installation targets and local prices for electricity, we project that annual losses in revenue from photovoltaic installations could exceed 20 million USD for Delhi alone, indicating that annual economic damage from air pollution to photovoltaic site operators and investors worldwide could be billions of dollars. Motivation:In June 2013, three of the authors of this paper, I. M. Peters, L. Haohui and A. Nobre, lived in Singapore and were witnesses of the most severe haze event to have occurred in the city to date. For a couple of days, the pollutant standard index (PSI) jumped from its usual value of about 25 to over 200. The normally clear view from our office windows on the sixth floor became filled with an impenetrable fog that swallowed up neighboring buildings. Face masks were sold out in a matter of hours. People were panicking. The event, in many ways, served as a wake-up call. Given our research focus on photovoltaic installations, we wanted to investigate the impact of haze on solar cell performance. From the reduced visibility it was evident that haze must have an effect, and we set out to quantify it. This paper summarizes our understanding so far. We have since learned about the devastating effects of urban air pollution on human health. This paper adds another aspect -the detrimental effect on photovoltaic power generation due to the reduction of light received. We hope that, in a small way, we can help raise awareness and make progress to improve the quality of life in what more and more people call home in the 21 st century -cities.

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